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llvm-mirror/lib/Target/X86/README.txt
Eli Friedman fbfaab45a3 Remove a fixed item, update a couple partially-fixed items. 
llvm-svn: 105375
2010-06-03 01:01:48 +00:00

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//===---------------------------------------------------------------------===//
// Random ideas for the X86 backend.
//===---------------------------------------------------------------------===//
We should add support for the "movbe" instruction, which does a byte-swapping
copy (3-addr bswap + memory support?) This is available on Atom processors.
//===---------------------------------------------------------------------===//
CodeGen/X86/lea-3.ll:test3 should be a single LEA, not a shift/move. The X86
backend knows how to three-addressify this shift, but it appears the register
allocator isn't even asking it to do so in this case. We should investigate
why this isn't happening, it could have significant impact on other important
cases for X86 as well.
//===---------------------------------------------------------------------===//
This should be one DIV/IDIV instruction, not a libcall:
unsigned test(unsigned long long X, unsigned Y) {
return X/Y;
}
This can be done trivially with a custom legalizer. What about overflow
though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
//===---------------------------------------------------------------------===//
Improvements to the multiply -> shift/add algorithm:
http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
//===---------------------------------------------------------------------===//
Improve code like this (occurs fairly frequently, e.g. in LLVM):
long long foo(int x) { return 1LL << x; }
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html
Another useful one would be ~0ULL >> X and ~0ULL << X.
One better solution for 1LL << x is:
xorl %eax, %eax
xorl %edx, %edx
testb $32, %cl
sete %al
setne %dl
sall %cl, %eax
sall %cl, %edx
But that requires good 8-bit subreg support.
Also, this might be better. It's an extra shift, but it's one instruction
shorter, and doesn't stress 8-bit subreg support.
(From http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01148.html,
but without the unnecessary and.)
movl %ecx, %eax
shrl $5, %eax
movl %eax, %edx
xorl $1, %edx
sall %cl, %eax
sall %cl. %edx
64-bit shifts (in general) expand to really bad code. Instead of using
cmovs, we should expand to a conditional branch like GCC produces.
//===---------------------------------------------------------------------===//
Compile this:
_Bool f(_Bool a) { return a!=1; }
into:
movzbl %dil, %eax
xorl $1, %eax
ret
(Although note that this isn't a legal way to express the code that llvm-gcc
currently generates for that function.)
//===---------------------------------------------------------------------===//
Some isel ideas:
1. Dynamic programming based approach when compile time if not an
issue.
2. Code duplication (addressing mode) during isel.
3. Other ideas from "Register-Sensitive Selection, Duplication, and
Sequencing of Instructions".
4. Scheduling for reduced register pressure. E.g. "Minimum Register
Instruction Sequence Problem: Revisiting Optimal Code Generation for DAGs"
and other related papers.
http://citeseer.ist.psu.edu/govindarajan01minimum.html
//===---------------------------------------------------------------------===//
Should we promote i16 to i32 to avoid partial register update stalls?
//===---------------------------------------------------------------------===//
Leave any_extend as pseudo instruction and hint to register
allocator. Delay codegen until post register allocation.
Note. any_extend is now turned into an INSERT_SUBREG. We still need to teach
the coalescer how to deal with it though.
//===---------------------------------------------------------------------===//
It appears icc use push for parameter passing. Need to investigate.
//===---------------------------------------------------------------------===//
Only use inc/neg/not instructions on processors where they are faster than
add/sub/xor. They are slower on the P4 due to only updating some processor
flags.
//===---------------------------------------------------------------------===//
The instruction selector sometimes misses folding a load into a compare. The
pattern is written as (cmp reg, (load p)). Because the compare isn't
commutative, it is not matched with the load on both sides. The dag combiner
should be made smart enough to cannonicalize the load into the RHS of a compare
when it can invert the result of the compare for free.
//===---------------------------------------------------------------------===//
In many cases, LLVM generates code like this:
_test:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
setl %al
movzbl %al, %eax
ret
on some processors (which ones?), it is more efficient to do this:
_test:
movl 8(%esp), %ebx
xor %eax, %eax
cmpl %ebx, 4(%esp)
setl %al
ret
Doing this correctly is tricky though, as the xor clobbers the flags.
//===---------------------------------------------------------------------===//
We should generate bts/btr/etc instructions on targets where they are cheap or
when codesize is important. e.g., for:
void setbit(int *target, int bit) {
*target |= (1 << bit);
}
void clearbit(int *target, int bit) {
*target &= ~(1 << bit);
}
//===---------------------------------------------------------------------===//
Instead of the following for memset char*, 1, 10:
movl $16843009, 4(%edx)
movl $16843009, (%edx)
movw $257, 8(%edx)
It might be better to generate
movl $16843009, %eax
movl %eax, 4(%edx)
movl %eax, (%edx)
movw al, 8(%edx)
when we can spare a register. It reduces code size.
//===---------------------------------------------------------------------===//
Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently
get this:
define i32 @test1(i32 %X) {
%Y = sdiv i32 %X, 8
ret i32 %Y
}
_test1:
movl 4(%esp), %eax
movl %eax, %ecx
sarl $31, %ecx
shrl $29, %ecx
addl %ecx, %eax
sarl $3, %eax
ret
GCC knows several different ways to codegen it, one of which is this:
_test1:
movl 4(%esp), %eax
cmpl $-1, %eax
leal 7(%eax), %ecx
cmovle %ecx, %eax
sarl $3, %eax
ret
which is probably slower, but it's interesting at least :)
//===---------------------------------------------------------------------===//
We are currently lowering large (1MB+) memmove/memcpy to rep/stosl and rep/movsl
We should leave these as libcalls for everything over a much lower threshold,
since libc is hand tuned for medium and large mem ops (avoiding RFO for large
stores, TLB preheating, etc)
//===---------------------------------------------------------------------===//
Optimize this into something reasonable:
x * copysign(1.0, y) * copysign(1.0, z)
//===---------------------------------------------------------------------===//
Optimize copysign(x, *y) to use an integer load from y.
//===---------------------------------------------------------------------===//
The following tests perform worse with LSR:
lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor.
//===---------------------------------------------------------------------===//
Adding to the list of cmp / test poor codegen issues:
int test(__m128 *A, __m128 *B) {
if (_mm_comige_ss(*A, *B))
return 3;
else
return 4;
}
_test:
movl 8(%esp), %eax
movaps (%eax), %xmm0
movl 4(%esp), %eax
movaps (%eax), %xmm1
comiss %xmm0, %xmm1
setae %al
movzbl %al, %ecx
movl $3, %eax
movl $4, %edx
cmpl $0, %ecx
cmove %edx, %eax
ret
Note the setae, movzbl, cmpl, cmove can be replaced with a single cmovae. There
are a number of issues. 1) We are introducing a setcc between the result of the
intrisic call and select. 2) The intrinsic is expected to produce a i32 value
so a any extend (which becomes a zero extend) is added.
We probably need some kind of target DAG combine hook to fix this.
//===---------------------------------------------------------------------===//
We generate significantly worse code for this than GCC:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150
http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701
There is also one case we do worse on PPC.
//===---------------------------------------------------------------------===//
For this:
int test(int a)
{
return a * 3;
}
We currently emits
imull $3, 4(%esp), %eax
Perhaps this is what we really should generate is? Is imull three or four
cycles? Note: ICC generates this:
movl 4(%esp), %eax
leal (%eax,%eax,2), %eax
The current instruction priority is based on pattern complexity. The former is
more "complex" because it folds a load so the latter will not be emitted.
Perhaps we should use AddedComplexity to give LEA32r a higher priority? We
should always try to match LEA first since the LEA matching code does some
estimate to determine whether the match is profitable.
However, if we care more about code size, then imull is better. It's two bytes
shorter than movl + leal.
On a Pentium M, both variants have the same characteristics with regard
to throughput; however, the multiplication has a latency of four cycles, as
opposed to two cycles for the movl+lea variant.
//===---------------------------------------------------------------------===//
__builtin_ffs codegen is messy.
int ffs_(unsigned X) { return __builtin_ffs(X); }
llvm produces:
ffs_:
movl 4(%esp), %ecx
bsfl %ecx, %eax
movl $32, %edx
cmove %edx, %eax
incl %eax
xorl %edx, %edx
testl %ecx, %ecx
cmove %edx, %eax
ret
vs gcc:
_ffs_:
movl $-1, %edx
bsfl 4(%esp), %eax
cmove %edx, %eax
addl $1, %eax
ret
Another example of __builtin_ffs (use predsimplify to eliminate a select):
int foo (unsigned long j) {
if (j)
return __builtin_ffs (j) - 1;
else
return 0;
}
//===---------------------------------------------------------------------===//
It appears gcc place string data with linkonce linkage in
.section __TEXT,__const_coal,coalesced instead of
.section __DATA,__const_coal,coalesced.
Take a look at darwin.h, there are other Darwin assembler directives that we
do not make use of.
//===---------------------------------------------------------------------===//
define i32 @foo(i32* %a, i32 %t) {
entry:
br label %cond_true
cond_true: ; preds = %cond_true, %entry
%x.0.0 = phi i32 [ 0, %entry ], [ %tmp9, %cond_true ] ; <i32> [#uses=3]
%t_addr.0.0 = phi i32 [ %t, %entry ], [ %tmp7, %cond_true ] ; <i32> [#uses=1]
%tmp2 = getelementptr i32* %a, i32 %x.0.0 ; <i32*> [#uses=1]
%tmp3 = load i32* %tmp2 ; <i32> [#uses=1]
%tmp5 = add i32 %t_addr.0.0, %x.0.0 ; <i32> [#uses=1]
%tmp7 = add i32 %tmp5, %tmp3 ; <i32> [#uses=2]
%tmp9 = add i32 %x.0.0, 1 ; <i32> [#uses=2]
%tmp = icmp sgt i32 %tmp9, 39 ; <i1> [#uses=1]
br i1 %tmp, label %bb12, label %cond_true
bb12: ; preds = %cond_true
ret i32 %tmp7
}
is pessimized by -loop-reduce and -indvars
//===---------------------------------------------------------------------===//
u32 to float conversion improvement:
float uint32_2_float( unsigned u ) {
float fl = (int) (u & 0xffff);
float fh = (int) (u >> 16);
fh *= 0x1.0p16f;
return fh + fl;
}
00000000 subl $0x04,%esp
00000003 movl 0x08(%esp,1),%eax
00000007 movl %eax,%ecx
00000009 shrl $0x10,%ecx
0000000c cvtsi2ss %ecx,%xmm0
00000010 andl $0x0000ffff,%eax
00000015 cvtsi2ss %eax,%xmm1
00000019 mulss 0x00000078,%xmm0
00000021 addss %xmm1,%xmm0
00000025 movss %xmm0,(%esp,1)
0000002a flds (%esp,1)
0000002d addl $0x04,%esp
00000030 ret
//===---------------------------------------------------------------------===//
When using fastcc abi, align stack slot of argument of type double on 8 byte
boundary to improve performance.
//===---------------------------------------------------------------------===//
Codegen:
int f(int a, int b) {
if (a == 4 || a == 6)
b++;
return b;
}
as:
or eax, 2
cmp eax, 6
jz label
//===---------------------------------------------------------------------===//
GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting
simplifications for integer "x cmp y ? a : b". For example, instead of:
int G;
void f(int X, int Y) {
G = X < 0 ? 14 : 13;
}
compiling to:
_f:
movl $14, %eax
movl $13, %ecx
movl 4(%esp), %edx
testl %edx, %edx
cmovl %eax, %ecx
movl %ecx, _G
ret
it could be:
_f:
movl 4(%esp), %eax
sarl $31, %eax
notl %eax
addl $14, %eax
movl %eax, _G
ret
etc.
Another is:
int usesbb(unsigned int a, unsigned int b) {
return (a < b ? -1 : 0);
}
to:
_usesbb:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
sbbl %eax, %eax
ret
instead of:
_usesbb:
xorl %eax, %eax
movl 8(%esp), %ecx
cmpl %ecx, 4(%esp)
movl $4294967295, %ecx
cmovb %ecx, %eax
ret
//===---------------------------------------------------------------------===//
Consider the expansion of:
define i32 @test3(i32 %X) {
%tmp1 = urem i32 %X, 255
ret i32 %tmp1
}
Currently it compiles to:
...
movl $2155905153, %ecx
movl 8(%esp), %esi
movl %esi, %eax
mull %ecx
...
This could be "reassociated" into:
movl $2155905153, %eax
movl 8(%esp), %ecx
mull %ecx
to avoid the copy. In fact, the existing two-address stuff would do this
except that mul isn't a commutative 2-addr instruction. I guess this has
to be done at isel time based on the #uses to mul?
//===---------------------------------------------------------------------===//
Make sure the instruction which starts a loop does not cross a cacheline
boundary. This requires knowning the exact length of each machine instruction.
That is somewhat complicated, but doable. Example 256.bzip2:
In the new trace, the hot loop has an instruction which crosses a cacheline
boundary. In addition to potential cache misses, this can't help decoding as I
imagine there has to be some kind of complicated decoder reset and realignment
to grab the bytes from the next cacheline.
532 532 0x3cfc movb (1809(%esp, %esi), %bl <<<--- spans 2 64 byte lines
942 942 0x3d03 movl %dh, (1809(%esp, %esi)
937 937 0x3d0a incl %esi
3 3 0x3d0b cmpb %bl, %dl
27 27 0x3d0d jnz 0x000062db <main+11707>
//===---------------------------------------------------------------------===//
In c99 mode, the preprocessor doesn't like assembly comments like #TRUNCATE.
//===---------------------------------------------------------------------===//
This could be a single 16-bit load.
int f(char *p) {
if ((p[0] == 1) & (p[1] == 2)) return 1;
return 0;
}
//===---------------------------------------------------------------------===//
We should inline lrintf and probably other libc functions.
//===---------------------------------------------------------------------===//
Use the FLAGS values from arithmetic instructions more. For example, compile:
int add_zf(int *x, int y, int a, int b) {
if ((*x += y) == 0)
return a;
else
return b;
}
to:
addl %esi, (%rdi)
movl %edx, %eax
cmovne %ecx, %eax
ret
instead of:
_add_zf:
addl (%rdi), %esi
movl %esi, (%rdi)
testl %esi, %esi
cmove %edx, %ecx
movl %ecx, %eax
ret
As another example, compile function f2 in test/CodeGen/X86/cmp-test.ll
without a test instruction.
//===---------------------------------------------------------------------===//
These two functions have identical effects:
unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return i;}
unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;}
We currently compile them to:
_f:
movl 4(%esp), %eax
movl %eax, %ecx
incl %ecx
movl 8(%esp), %edx
cmpl %edx, %ecx
jne LBB1_2 #UnifiedReturnBlock
LBB1_1: #cond_true
addl $2, %eax
ret
LBB1_2: #UnifiedReturnBlock
movl %ecx, %eax
ret
_f2:
movl 4(%esp), %eax
movl %eax, %ecx
incl %ecx
cmpl 8(%esp), %ecx
sete %cl
movzbl %cl, %ecx
leal 1(%ecx,%eax), %eax
ret
both of which are inferior to GCC's:
_f:
movl 4(%esp), %edx
leal 1(%edx), %eax
addl $2, %edx
cmpl 8(%esp), %eax
cmove %edx, %eax
ret
_f2:
movl 4(%esp), %eax
addl $1, %eax
xorl %edx, %edx
cmpl 8(%esp), %eax
sete %dl
addl %edx, %eax
ret
//===---------------------------------------------------------------------===//
This code:
void test(int X) {
if (X) abort();
}
is currently compiled to:
_test:
subl $12, %esp
cmpl $0, 16(%esp)
jne LBB1_1
addl $12, %esp
ret
LBB1_1:
call L_abort$stub
It would be better to produce:
_test:
subl $12, %esp
cmpl $0, 16(%esp)
jne L_abort$stub
addl $12, %esp
ret
This can be applied to any no-return function call that takes no arguments etc.
Alternatively, the stack save/restore logic could be shrink-wrapped, producing
something like this:
_test:
cmpl $0, 4(%esp)
jne LBB1_1
ret
LBB1_1:
subl $12, %esp
call L_abort$stub
Both are useful in different situations. Finally, it could be shrink-wrapped
and tail called, like this:
_test:
cmpl $0, 4(%esp)
jne LBB1_1
ret
LBB1_1:
pop %eax # realign stack.
call L_abort$stub
Though this probably isn't worth it.
//===---------------------------------------------------------------------===//
Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with
a neg instead of a sub instruction. Consider:
int test(char X) { return 7-X; }
we currently produce:
_test:
movl $7, %eax
movsbl 4(%esp), %ecx
subl %ecx, %eax
ret
We would use one fewer register if codegen'd as:
movsbl 4(%esp), %eax
neg %eax
add $7, %eax
ret
Note that this isn't beneficial if the load can be folded into the sub. In
this case, we want a sub:
int test(int X) { return 7-X; }
_test:
movl $7, %eax
subl 4(%esp), %eax
ret
//===---------------------------------------------------------------------===//
Leaf functions that require one 4-byte spill slot have a prolog like this:
_foo:
pushl %esi
subl $4, %esp
...
and an epilog like this:
addl $4, %esp
popl %esi
ret
It would be smaller, and potentially faster, to push eax on entry and to
pop into a dummy register instead of using addl/subl of esp. Just don't pop
into any return registers :)
//===---------------------------------------------------------------------===//
The X86 backend should fold (branch (or (setcc, setcc))) into multiple
branches. We generate really poor code for:
double testf(double a) {
return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0);
}
For example, the entry BB is:
_testf:
subl $20, %esp
pxor %xmm0, %xmm0
movsd 24(%esp), %xmm1
ucomisd %xmm0, %xmm1
setnp %al
sete %cl
testb %cl, %al
jne LBB1_5 # UnifiedReturnBlock
LBB1_1: # cond_true
it would be better to replace the last four instructions with:
jp LBB1_1
je LBB1_5
LBB1_1:
We also codegen the inner ?: into a diamond:
cvtss2sd LCPI1_0(%rip), %xmm2
cvtss2sd LCPI1_1(%rip), %xmm3
ucomisd %xmm1, %xmm0
ja LBB1_3 # cond_true
LBB1_2: # cond_true
movapd %xmm3, %xmm2
LBB1_3: # cond_true
movapd %xmm2, %xmm0
ret
We should sink the load into xmm3 into the LBB1_2 block. This should
be pretty easy, and will nuke all the copies.
//===---------------------------------------------------------------------===//
This:
#include <algorithm>
inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
{ return std::make_pair(a + b, a + b < a); }
bool no_overflow(unsigned a, unsigned b)
{ return !full_add(a, b).second; }
Should compile to:
_Z11no_overflowjj:
addl %edi, %esi
setae %al
ret
FIXME: That code looks wrong; bool return is normally defined as zext.
on x86-64, not:
__Z11no_overflowjj:
addl %edi, %esi
cmpl %edi, %esi
setae %al
movzbl %al, %eax
ret
//===---------------------------------------------------------------------===//
The following code:
bb114.preheader: ; preds = %cond_next94
%tmp231232 = sext i16 %tmp62 to i32 ; <i32> [#uses=1]
%tmp233 = sub i32 32, %tmp231232 ; <i32> [#uses=1]
%tmp245246 = sext i16 %tmp65 to i32 ; <i32> [#uses=1]
%tmp252253 = sext i16 %tmp68 to i32 ; <i32> [#uses=1]
%tmp254 = sub i32 32, %tmp252253 ; <i32> [#uses=1]
%tmp553554 = bitcast i16* %tmp37 to i8* ; <i8*> [#uses=2]
%tmp583584 = sext i16 %tmp98 to i32 ; <i32> [#uses=1]
%tmp585 = sub i32 32, %tmp583584 ; <i32> [#uses=1]
%tmp614615 = sext i16 %tmp101 to i32 ; <i32> [#uses=1]
%tmp621622 = sext i16 %tmp104 to i32 ; <i32> [#uses=1]
%tmp623 = sub i32 32, %tmp621622 ; <i32> [#uses=1]
br label %bb114
produces:
LBB3_5: # bb114.preheader
movswl -68(%ebp), %eax
movl $32, %ecx
movl %ecx, -80(%ebp)
subl %eax, -80(%ebp)
movswl -52(%ebp), %eax
movl %ecx, -84(%ebp)
subl %eax, -84(%ebp)
movswl -70(%ebp), %eax
movl %ecx, -88(%ebp)
subl %eax, -88(%ebp)
movswl -50(%ebp), %eax
subl %eax, %ecx
movl %ecx, -76(%ebp)
movswl -42(%ebp), %eax
movl %eax, -92(%ebp)
movswl -66(%ebp), %eax
movl %eax, -96(%ebp)
movw $0, -98(%ebp)
This appears to be bad because the RA is not folding the store to the stack
slot into the movl. The above instructions could be:
movl $32, -80(%ebp)
...
movl $32, -84(%ebp)
...
This seems like a cross between remat and spill folding.
This has redundant subtractions of %eax from a stack slot. However, %ecx doesn't
change, so we could simply subtract %eax from %ecx first and then use %ecx (or
vice-versa).
//===---------------------------------------------------------------------===//
This code:
%tmp659 = icmp slt i16 %tmp654, 0 ; <i1> [#uses=1]
br i1 %tmp659, label %cond_true662, label %cond_next715
produces this:
testw %cx, %cx
movswl %cx, %esi
jns LBB4_109 # cond_next715
Shark tells us that using %cx in the testw instruction is sub-optimal. It
suggests using the 32-bit register (which is what ICC uses).
//===---------------------------------------------------------------------===//
We compile this:
void compare (long long foo) {
if (foo < 4294967297LL)
abort();
}
to:
compare:
subl $4, %esp
cmpl $0, 8(%esp)
setne %al
movzbw %al, %ax
cmpl $1, 12(%esp)
setg %cl
movzbw %cl, %cx
cmove %ax, %cx
testb $1, %cl
jne .LBB1_2 # UnifiedReturnBlock
.LBB1_1: # ifthen
call abort
.LBB1_2: # UnifiedReturnBlock
addl $4, %esp
ret
(also really horrible code on ppc). This is due to the expand code for 64-bit
compares. GCC produces multiple branches, which is much nicer:
compare:
subl $12, %esp
movl 20(%esp), %edx
movl 16(%esp), %eax
decl %edx
jle .L7
.L5:
addl $12, %esp
ret
.p2align 4,,7
.L7:
jl .L4
cmpl $0, %eax
.p2align 4,,8
ja .L5
.L4:
.p2align 4,,9
call abort
//===---------------------------------------------------------------------===//
Tail call optimization improvements: Tail call optimization currently
pushes all arguments on the top of the stack (their normal place for
non-tail call optimized calls) that source from the callers arguments
or that source from a virtual register (also possibly sourcing from
callers arguments).
This is done to prevent overwriting of parameters (see example
below) that might be used later.
example:
int callee(int32, int64);
int caller(int32 arg1, int32 arg2) {
int64 local = arg2 * 2;
return callee(arg2, (int64)local);
}
[arg1] [!arg2 no longer valid since we moved local onto it]
[arg2] -> [(int64)
[RETADDR] local ]
Moving arg1 onto the stack slot of callee function would overwrite
arg2 of the caller.
Possible optimizations:
- Analyse the actual parameters of the callee to see which would
overwrite a caller parameter which is used by the callee and only
push them onto the top of the stack.
int callee (int32 arg1, int32 arg2);
int caller (int32 arg1, int32 arg2) {
return callee(arg1,arg2);
}
Here we don't need to write any variables to the top of the stack
since they don't overwrite each other.
int callee (int32 arg1, int32 arg2);
int caller (int32 arg1, int32 arg2) {
return callee(arg2,arg1);
}
Here we need to push the arguments because they overwrite each
other.
//===---------------------------------------------------------------------===//
main ()
{
int i = 0;
unsigned long int z = 0;
do {
z -= 0x00004000;
i++;
if (i > 0x00040000)
abort ();
} while (z > 0);
exit (0);
}
gcc compiles this to:
_main:
subl $28, %esp
xorl %eax, %eax
jmp L2
L3:
cmpl $262144, %eax
je L10
L2:
addl $1, %eax
cmpl $262145, %eax
jne L3
call L_abort$stub
L10:
movl $0, (%esp)
call L_exit$stub
llvm:
_main:
subl $12, %esp
movl $1, %eax
movl $16384, %ecx
LBB1_1: # bb
cmpl $262145, %eax
jge LBB1_4 # cond_true
LBB1_2: # cond_next
incl %eax
addl $4294950912, %ecx
cmpl $16384, %ecx
jne LBB1_1 # bb
LBB1_3: # bb11
xorl %eax, %eax
addl $12, %esp
ret
LBB1_4: # cond_true
call L_abort$stub
1. LSR should rewrite the first cmp with induction variable %ecx.
2. DAG combiner should fold
leal 1(%eax), %edx
cmpl $262145, %edx
=>
cmpl $262144, %eax
//===---------------------------------------------------------------------===//
define i64 @test(double %X) {
%Y = fptosi double %X to i64
ret i64 %Y
}
compiles to:
_test:
subl $20, %esp
movsd 24(%esp), %xmm0
movsd %xmm0, 8(%esp)
fldl 8(%esp)
fisttpll (%esp)
movl 4(%esp), %edx
movl (%esp), %eax
addl $20, %esp
#FP_REG_KILL
ret
This should just fldl directly from the input stack slot.
//===---------------------------------------------------------------------===//
This code:
int foo (int x) { return (x & 65535) | 255; }
Should compile into:
_foo:
movzwl 4(%esp), %eax
orl $255, %eax
ret
instead of:
_foo:
movl $255, %eax
orl 4(%esp), %eax
andl $65535, %eax
ret
//===---------------------------------------------------------------------===//
We're codegen'ing multiply of long longs inefficiently:
unsigned long long LLM(unsigned long long arg1, unsigned long long arg2) {
return arg1 * arg2;
}
We compile to (fomit-frame-pointer):
_LLM:
pushl %esi
movl 8(%esp), %ecx
movl 16(%esp), %esi
movl %esi, %eax
mull %ecx
imull 12(%esp), %esi
addl %edx, %esi
imull 20(%esp), %ecx
movl %esi, %edx
addl %ecx, %edx
popl %esi
ret
This looks like a scheduling deficiency and lack of remat of the load from
the argument area. ICC apparently produces:
movl 8(%esp), %ecx
imull 12(%esp), %ecx
movl 16(%esp), %eax
imull 4(%esp), %eax
addl %eax, %ecx
movl 4(%esp), %eax
mull 12(%esp)
addl %ecx, %edx
ret
Note that it remat'd loads from 4(esp) and 12(esp). See this GCC PR:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17236
//===---------------------------------------------------------------------===//
We can fold a store into "zeroing a reg". Instead of:
xorl %eax, %eax
movl %eax, 124(%esp)
we should get:
movl $0, 124(%esp)
if the flags of the xor are dead.
Likewise, we isel "x<<1" into "add reg,reg". If reg is spilled, this should
be folded into: shl [mem], 1
//===---------------------------------------------------------------------===//
In SSE mode, we turn abs and neg into a load from the constant pool plus a xor
or and instruction, for example:
xorpd LCPI1_0, %xmm2
However, if xmm2 gets spilled, we end up with really ugly code like this:
movsd (%esp), %xmm0
xorpd LCPI1_0, %xmm0
movsd %xmm0, (%esp)
Since we 'know' that this is a 'neg', we can actually "fold" the spill into
the neg/abs instruction, turning it into an *integer* operation, like this:
xorl 2147483648, [mem+4] ## 2147483648 = (1 << 31)
you could also use xorb, but xorl is less likely to lead to a partial register
stall. Here is a contrived testcase:
double a, b, c;
void test(double *P) {
double X = *P;
a = X;
bar();
X = -X;
b = X;
bar();
c = X;
}
//===---------------------------------------------------------------------===//
handling llvm.memory.barrier on pre SSE2 cpus
should generate:
lock ; mov %esp, %esp
//===---------------------------------------------------------------------===//
The generated code on x86 for checking for signed overflow on a multiply the
obvious way is much longer than it needs to be.
int x(int a, int b) {
long long prod = (long long)a*b;
return prod > 0x7FFFFFFF || prod < (-0x7FFFFFFF-1);
}
See PR2053 for more details.
//===---------------------------------------------------------------------===//
We should investigate using cdq/ctld (effect: edx = sar eax, 31)
more aggressively; it should cost the same as a move+shift on any modern
processor, but it's a lot shorter. Downside is that it puts more
pressure on register allocation because it has fixed operands.
Example:
int abs(int x) {return x < 0 ? -x : x;}
gcc compiles this to the following when using march/mtune=pentium2/3/4/m/etc.:
abs:
movl 4(%esp), %eax
cltd
xorl %edx, %eax
subl %edx, %eax
ret
//===---------------------------------------------------------------------===//
Consider:
int test(unsigned long a, unsigned long b) { return -(a < b); }
We currently compile this to:
define i32 @test(i32 %a, i32 %b) nounwind {
%tmp3 = icmp ult i32 %a, %b ; <i1> [#uses=1]
%tmp34 = zext i1 %tmp3 to i32 ; <i32> [#uses=1]
%tmp5 = sub i32 0, %tmp34 ; <i32> [#uses=1]
ret i32 %tmp5
}
and
_test:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
setb %al
movzbl %al, %eax
negl %eax
ret
Several deficiencies here. First, we should instcombine zext+neg into sext:
define i32 @test2(i32 %a, i32 %b) nounwind {
%tmp3 = icmp ult i32 %a, %b ; <i1> [#uses=1]
%tmp34 = sext i1 %tmp3 to i32 ; <i32> [#uses=1]
ret i32 %tmp34
}
However, before we can do that, we have to fix the bad codegen that we get for
sext from bool:
_test2:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
setb %al
movzbl %al, %eax
shll $31, %eax
sarl $31, %eax
ret
This code should be at least as good as the code above. Once this is fixed, we
can optimize this specific case even more to:
movl 8(%esp), %eax
xorl %ecx, %ecx
cmpl %eax, 4(%esp)
sbbl %ecx, %ecx
//===---------------------------------------------------------------------===//
Take the following code (from
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541):
extern unsigned char first_one[65536];
int FirstOnet(unsigned long long arg1)
{
if (arg1 >> 48)
return (first_one[arg1 >> 48]);
return 0;
}
The following code is currently generated:
FirstOnet:
movl 8(%esp), %eax
cmpl $65536, %eax
movl 4(%esp), %ecx
jb .LBB1_2 # UnifiedReturnBlock
.LBB1_1: # ifthen
shrl $16, %eax
movzbl first_one(%eax), %eax
ret
.LBB1_2: # UnifiedReturnBlock
xorl %eax, %eax
ret
We could change the "movl 8(%esp), %eax" into "movzwl 10(%esp), %eax"; this
lets us change the cmpl into a testl, which is shorter, and eliminate the shift.
//===---------------------------------------------------------------------===//
We compile this function:
define i32 @foo(i32 %a, i32 %b, i32 %c, i8 zeroext %d) nounwind {
entry:
%tmp2 = icmp eq i8 %d, 0 ; <i1> [#uses=1]
br i1 %tmp2, label %bb7, label %bb
bb: ; preds = %entry
%tmp6 = add i32 %b, %a ; <i32> [#uses=1]
ret i32 %tmp6
bb7: ; preds = %entry
%tmp10 = sub i32 %a, %c ; <i32> [#uses=1]
ret i32 %tmp10
}
to:
foo: # @foo
# BB#0: # %entry
movl 4(%esp), %ecx
cmpb $0, 16(%esp)
je .LBB0_2
# BB#1: # %bb
movl 8(%esp), %eax
addl %ecx, %eax
ret
.LBB0_2: # %bb7
movl 12(%esp), %edx
movl %ecx, %eax
subl %edx, %eax
ret
There's an obviously unnecessary movl in .LBB0_2, and we could eliminate a
couple more movls by putting 4(%esp) into %eax instead of %ecx.
//===---------------------------------------------------------------------===//
See rdar://4653682.
From flops:
LBB1_15: # bb310
cvtss2sd LCPI1_0, %xmm1
addsd %xmm1, %xmm0
movsd 176(%esp), %xmm2
mulsd %xmm0, %xmm2
movapd %xmm2, %xmm3
mulsd %xmm3, %xmm3
movapd %xmm3, %xmm4
mulsd LCPI1_23, %xmm4
addsd LCPI1_24, %xmm4
mulsd %xmm3, %xmm4
addsd LCPI1_25, %xmm4
mulsd %xmm3, %xmm4
addsd LCPI1_26, %xmm4
mulsd %xmm3, %xmm4
addsd LCPI1_27, %xmm4
mulsd %xmm3, %xmm4
addsd LCPI1_28, %xmm4
mulsd %xmm3, %xmm4
addsd %xmm1, %xmm4
mulsd %xmm2, %xmm4
movsd 152(%esp), %xmm1
addsd %xmm4, %xmm1
movsd %xmm1, 152(%esp)
incl %eax
cmpl %eax, %esi
jge LBB1_15 # bb310
LBB1_16: # bb358.loopexit
movsd 152(%esp), %xmm0
addsd %xmm0, %xmm0
addsd LCPI1_22, %xmm0
movsd %xmm0, 152(%esp)
Rather than spilling the result of the last addsd in the loop, we should have
insert a copy to split the interval (one for the duration of the loop, one
extending to the fall through). The register pressure in the loop isn't high
enough to warrant the spill.
Also check why xmm7 is not used at all in the function.
//===---------------------------------------------------------------------===//
Take the following:
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
target triple = "i386-apple-darwin8"
@in_exit.4870.b = internal global i1 false ; <i1*> [#uses=2]
define fastcc void @abort_gzip() noreturn nounwind {
entry:
%tmp.b.i = load i1* @in_exit.4870.b ; <i1> [#uses=1]
br i1 %tmp.b.i, label %bb.i, label %bb4.i
bb.i: ; preds = %entry
tail call void @exit( i32 1 ) noreturn nounwind
unreachable
bb4.i: ; preds = %entry
store i1 true, i1* @in_exit.4870.b
tail call void @exit( i32 1 ) noreturn nounwind
unreachable
}
declare void @exit(i32) noreturn nounwind
This compiles into:
_abort_gzip: ## @abort_gzip
## BB#0: ## %entry
subl $12, %esp
movb _in_exit.4870.b, %al
cmpb $1, %al
jne LBB0_2
We somehow miss folding the movb into the cmpb.
//===---------------------------------------------------------------------===//
We compile:
int test(int x, int y) {
return x-y-1;
}
into (-m64):
_test:
decl %edi
movl %edi, %eax
subl %esi, %eax
ret
it would be better to codegen as: x+~y (notl+addl)
//===---------------------------------------------------------------------===//
This code:
int foo(const char *str,...)
{
__builtin_va_list a; int x;
__builtin_va_start(a,str); x = __builtin_va_arg(a,int); __builtin_va_end(a);
return x;
}
gets compiled into this on x86-64:
subq $200, %rsp
movaps %xmm7, 160(%rsp)
movaps %xmm6, 144(%rsp)
movaps %xmm5, 128(%rsp)
movaps %xmm4, 112(%rsp)
movaps %xmm3, 96(%rsp)
movaps %xmm2, 80(%rsp)
movaps %xmm1, 64(%rsp)
movaps %xmm0, 48(%rsp)
movq %r9, 40(%rsp)
movq %r8, 32(%rsp)
movq %rcx, 24(%rsp)
movq %rdx, 16(%rsp)
movq %rsi, 8(%rsp)
leaq (%rsp), %rax
movq %rax, 192(%rsp)
leaq 208(%rsp), %rax
movq %rax, 184(%rsp)
movl $48, 180(%rsp)
movl $8, 176(%rsp)
movl 176(%rsp), %eax
cmpl $47, %eax
jbe .LBB1_3 # bb
.LBB1_1: # bb3
movq 184(%rsp), %rcx
leaq 8(%rcx), %rax
movq %rax, 184(%rsp)
.LBB1_2: # bb4
movl (%rcx), %eax
addq $200, %rsp
ret
.LBB1_3: # bb
movl %eax, %ecx
addl $8, %eax
addq 192(%rsp), %rcx
movl %eax, 176(%rsp)
jmp .LBB1_2 # bb4
gcc 4.3 generates:
subq $96, %rsp
.LCFI0:
leaq 104(%rsp), %rax
movq %rsi, -80(%rsp)
movl $8, -120(%rsp)
movq %rax, -112(%rsp)
leaq -88(%rsp), %rax
movq %rax, -104(%rsp)
movl $8, %eax
cmpl $48, %eax
jb .L6
movq -112(%rsp), %rdx
movl (%rdx), %eax
addq $96, %rsp
ret
.p2align 4,,10
.p2align 3
.L6:
mov %eax, %edx
addq -104(%rsp), %rdx
addl $8, %eax
movl %eax, -120(%rsp)
movl (%rdx), %eax
addq $96, %rsp
ret
and it gets compiled into this on x86:
pushl %ebp
movl %esp, %ebp
subl $4, %esp
leal 12(%ebp), %eax
movl %eax, -4(%ebp)
leal 16(%ebp), %eax
movl %eax, -4(%ebp)
movl 12(%ebp), %eax
addl $4, %esp
popl %ebp
ret
gcc 4.3 generates:
pushl %ebp
movl %esp, %ebp
movl 12(%ebp), %eax
popl %ebp
ret
//===---------------------------------------------------------------------===//
Teach tblgen not to check bitconvert source type in some cases. This allows us
to consolidate the following patterns in X86InstrMMX.td:
def : Pat<(v2i32 (bitconvert (i64 (vector_extract (v2i64 VR128:$src),
(iPTR 0))))),
(v2i32 (MMX_MOVDQ2Qrr VR128:$src))>;
def : Pat<(v4i16 (bitconvert (i64 (vector_extract (v2i64 VR128:$src),
(iPTR 0))))),
(v4i16 (MMX_MOVDQ2Qrr VR128:$src))>;
def : Pat<(v8i8 (bitconvert (i64 (vector_extract (v2i64 VR128:$src),
(iPTR 0))))),
(v8i8 (MMX_MOVDQ2Qrr VR128:$src))>;
There are other cases in various td files.
//===---------------------------------------------------------------------===//
Take something like the following on x86-32:
unsigned a(unsigned long long x, unsigned y) {return x % y;}
We currently generate a libcall, but we really shouldn't: the expansion is
shorter and likely faster than the libcall. The expected code is something
like the following:
movl 12(%ebp), %eax
movl 16(%ebp), %ecx
xorl %edx, %edx
divl %ecx
movl 8(%ebp), %eax
divl %ecx
movl %edx, %eax
ret
A similar code sequence works for division.
//===---------------------------------------------------------------------===//
These should compile to the same code, but the later codegen's to useless
instructions on X86. This may be a trivial dag combine (GCC PR7061):
struct s1 { unsigned char a, b; };
unsigned long f1(struct s1 x) {
return x.a + x.b;
}
struct s2 { unsigned a: 8, b: 8; };
unsigned long f2(struct s2 x) {
return x.a + x.b;
}
//===---------------------------------------------------------------------===//
We currently compile this:
define i32 @func1(i32 %v1, i32 %v2) nounwind {
entry:
%t = call {i32, i1} @llvm.sadd.with.overflow.i32(i32 %v1, i32 %v2)
%sum = extractvalue {i32, i1} %t, 0
%obit = extractvalue {i32, i1} %t, 1
br i1 %obit, label %overflow, label %normal
normal:
ret i32 %sum
overflow:
call void @llvm.trap()
unreachable
}
declare {i32, i1} @llvm.sadd.with.overflow.i32(i32, i32)
declare void @llvm.trap()
to:
_func1:
movl 4(%esp), %eax
addl 8(%esp), %eax
jo LBB1_2 ## overflow
LBB1_1: ## normal
ret
LBB1_2: ## overflow
ud2
it would be nice to produce "into" someday.
//===---------------------------------------------------------------------===//
This code:
void vec_mpys1(int y[], const int x[], int scaler) {
int i;
for (i = 0; i < 150; i++)
y[i] += (((long long)scaler * (long long)x[i]) >> 31);
}
Compiles to this loop with GCC 3.x:
.L5:
movl %ebx, %eax
imull (%edi,%ecx,4)
shrdl $31, %edx, %eax
addl %eax, (%esi,%ecx,4)
incl %ecx
cmpl $149, %ecx
jle .L5
llvm-gcc compiles it to the much uglier:
LBB1_1: ## bb1
movl 24(%esp), %eax
movl (%eax,%edi,4), %ebx
movl %ebx, %ebp
imull %esi, %ebp
movl %ebx, %eax
mull %ecx
addl %ebp, %edx
sarl $31, %ebx
imull %ecx, %ebx
addl %edx, %ebx
shldl $1, %eax, %ebx
movl 20(%esp), %eax
addl %ebx, (%eax,%edi,4)
incl %edi
cmpl $150, %edi
jne LBB1_1 ## bb1
The issue is that we hoist the cast of "scaler" to long long outside of the
loop, the value comes into the loop as two values, and
RegsForValue::getCopyFromRegs doesn't know how to put an AssertSext on the
constructed BUILD_PAIR which represents the cast value.
//===---------------------------------------------------------------------===//
Test instructions can be eliminated by using EFLAGS values from arithmetic
instructions. This is currently not done for mul, and, or, xor, neg, shl,
sra, srl, shld, shrd, atomic ops, and others. It is also currently not done
for read-modify-write instructions. It is also current not done if the
OF or CF flags are needed.
The shift operators have the complication that when the shift count is
zero, EFLAGS is not set, so they can only subsume a test instruction if
the shift count is known to be non-zero. Also, using the EFLAGS value
from a shift is apparently very slow on some x86 implementations.
In read-modify-write instructions, the root node in the isel match is
the store, and isel has no way for the use of the EFLAGS result of the
arithmetic to be remapped to the new node.
Add and subtract instructions set OF on signed overflow and CF on unsiged
overflow, while test instructions always clear OF and CF. In order to
replace a test with an add or subtract in a situation where OF or CF is
needed, codegen must be able to prove that the operation cannot see
signed or unsigned overflow, respectively.
//===---------------------------------------------------------------------===//
memcpy/memmove do not lower to SSE copies when possible. A silly example is:
define <16 x float> @foo(<16 x float> %A) nounwind {
%tmp = alloca <16 x float>, align 16
%tmp2 = alloca <16 x float>, align 16
store <16 x float> %A, <16 x float>* %tmp
%s = bitcast <16 x float>* %tmp to i8*
%s2 = bitcast <16 x float>* %tmp2 to i8*
call void @llvm.memcpy.i64(i8* %s, i8* %s2, i64 64, i32 16)
%R = load <16 x float>* %tmp2
ret <16 x float> %R
}
declare void @llvm.memcpy.i64(i8* nocapture, i8* nocapture, i64, i32) nounwind
which compiles to:
_foo:
subl $140, %esp
movaps %xmm3, 112(%esp)
movaps %xmm2, 96(%esp)
movaps %xmm1, 80(%esp)
movaps %xmm0, 64(%esp)
movl 60(%esp), %eax
movl %eax, 124(%esp)
movl 56(%esp), %eax
movl %eax, 120(%esp)
movl 52(%esp), %eax
<many many more 32-bit copies>
movaps (%esp), %xmm0
movaps 16(%esp), %xmm1
movaps 32(%esp), %xmm2
movaps 48(%esp), %xmm3
addl $140, %esp
ret
On Nehalem, it may even be cheaper to just use movups when unaligned than to
fall back to lower-granularity chunks.
//===---------------------------------------------------------------------===//
Implement processor-specific optimizations for parity with GCC on these
processors. GCC does two optimizations:
1. ix86_pad_returns inserts a noop before ret instructions if immediately
preceeded by a conditional branch or is the target of a jump.
2. ix86_avoid_jump_misspredicts inserts noops in cases where a 16-byte block of
code contains more than 3 branches.
The first one is done for all AMDs, Core2, and "Generic"
The second one is done for: Atom, Pentium Pro, all AMDs, Pentium 4, Nocona,
Core 2, and "Generic"
//===---------------------------------------------------------------------===//
Testcase:
int a(int x) { return (x & 127) > 31; }
Current output:
movl 4(%esp), %eax
andl $127, %eax
cmpl $31, %eax
seta %al
movzbl %al, %eax
ret
Ideal output:
xorl %eax, %eax
testl $96, 4(%esp)
setne %al
ret
This should definitely be done in instcombine, canonicalizing the range
condition into a != condition. We get this IR:
define i32 @a(i32 %x) nounwind readnone {
entry:
%0 = and i32 %x, 127 ; <i32> [#uses=1]
%1 = icmp ugt i32 %0, 31 ; <i1> [#uses=1]
%2 = zext i1 %1 to i32 ; <i32> [#uses=1]
ret i32 %2
}
Instcombine prefers to strength reduce relational comparisons to equality
comparisons when possible, this should be another case of that. This could
be handled pretty easily in InstCombiner::visitICmpInstWithInstAndIntCst, but it
looks like InstCombiner::visitICmpInstWithInstAndIntCst should really already
be redesigned to use ComputeMaskedBits and friends.
//===---------------------------------------------------------------------===//
Testcase:
int x(int a) { return (a&0xf0)>>4; }
Current output:
movl 4(%esp), %eax
shrl $4, %eax
andl $15, %eax
ret
Ideal output:
movzbl 4(%esp), %eax
shrl $4, %eax
ret
//===---------------------------------------------------------------------===//
Testcase:
int x(int a) { return (a & 0x80) ? 0x100 : 0; }
int y(int a) { return (a & 0x80) *2; }
Current:
testl $128, 4(%esp)
setne %al
movzbl %al, %eax
shll $8, %eax
ret
Better:
movl 4(%esp), %eax
addl %eax, %eax
andl $256, %eax
ret
This is another general instcombine transformation that is profitable on all
targets. In LLVM IR, these functions look like this:
define i32 @x(i32 %a) nounwind readnone {
entry:
%0 = and i32 %a, 128
%1 = icmp eq i32 %0, 0
%iftmp.0.0 = select i1 %1, i32 0, i32 256
ret i32 %iftmp.0.0
}
define i32 @y(i32 %a) nounwind readnone {
entry:
%0 = shl i32 %a, 1
%1 = and i32 %0, 256
ret i32 %1
}
Replacing an icmp+select with a shift should always be considered profitable in
instcombine.
//===---------------------------------------------------------------------===//
Re-implement atomic builtins __sync_add_and_fetch() and __sync_sub_and_fetch
properly.
When the return value is not used (i.e. only care about the value in the
memory), x86 does not have to use add to implement these. Instead, it can use
add, sub, inc, dec instructions with the "lock" prefix.
This is currently implemented using a bit of instruction selection trick. The
issue is the target independent pattern produces one output and a chain and we
want to map it into one that just output a chain. The current trick is to select
it into a MERGE_VALUES with the first definition being an implicit_def. The
proper solution is to add new ISD opcodes for the no-output variant. DAG
combiner can then transform the node before it gets to target node selection.
Problem #2 is we are adding a whole bunch of x86 atomic instructions when in
fact these instructions are identical to the non-lock versions. We need a way to
add target specific information to target nodes and have this information
carried over to machine instructions. Asm printer (or JIT) can use this
information to add the "lock" prefix.
//===---------------------------------------------------------------------===//
_Bool bar(int *x) { return *x & 1; }
define zeroext i1 @bar(i32* nocapture %x) nounwind readonly {
entry:
%tmp1 = load i32* %x ; <i32> [#uses=1]
%and = and i32 %tmp1, 1 ; <i32> [#uses=1]
%tobool = icmp ne i32 %and, 0 ; <i1> [#uses=1]
ret i1 %tobool
}
bar: # @bar
# BB#0: # %entry
movl 4(%esp), %eax
movb (%eax), %al
andb $1, %al
movzbl %al, %eax
ret
Missed optimization: should be movl+andl.
//===---------------------------------------------------------------------===//
Consider the following two functions compiled with clang:
_Bool foo(int *x) { return !(*x & 4); }
unsigned bar(int *x) { return !(*x & 4); }
foo:
movl 4(%esp), %eax
testb $4, (%eax)
sete %al
movzbl %al, %eax
ret
bar:
movl 4(%esp), %eax
movl (%eax), %eax
shrl $2, %eax
andl $1, %eax
xorl $1, %eax
ret
The second function generates more code even though the two functions are
are functionally identical.
//===---------------------------------------------------------------------===//
Take the following C code:
int x(int y) { return (y & 63) << 14; }
Code produced by gcc:
andl $63, %edi
sall $14, %edi
movl %edi, %eax
ret
Code produced by clang:
shll $14, %edi
movl %edi, %eax
andl $1032192, %eax
ret
The code produced by gcc is 3 bytes shorter. This sort of construct often
shows up with bitfields.
//===---------------------------------------------------------------------===//
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