mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-25 20:23:11 +01:00
923 lines
24 KiB
Plaintext
923 lines
24 KiB
Plaintext
//===---------------------------------------------------------------------===//
|
|
// Random ideas for the X86 backend.
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Add a MUL2U and MUL2S nodes to represent a multiply that returns both the
|
|
Hi and Lo parts (combination of MUL and MULH[SU] into one node). Add this to
|
|
X86, & make the dag combiner produce it when needed. This will eliminate one
|
|
imul from the code generated for:
|
|
|
|
long long test(long long X, long long Y) { return X*Y; }
|
|
|
|
by using the EAX result from the mul. We should add a similar node for
|
|
DIVREM.
|
|
|
|
another case is:
|
|
|
|
long long test(int X, int Y) { return (long long)X*Y; }
|
|
|
|
... which should only be one imul instruction.
|
|
|
|
This can be done with a custom expander, but it would be nice to move this to
|
|
generic code.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
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
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Count leading zeros and count trailing zeros:
|
|
|
|
int clz(int X) { return __builtin_clz(X); }
|
|
int ctz(int X) { return __builtin_ctz(X); }
|
|
|
|
$ gcc t.c -S -o - -O3 -fomit-frame-pointer -masm=intel
|
|
clz:
|
|
bsr %eax, DWORD PTR [%esp+4]
|
|
xor %eax, 31
|
|
ret
|
|
ctz:
|
|
bsf %eax, DWORD PTR [%esp+4]
|
|
ret
|
|
|
|
however, check that these are defined for 0 and 32. Our intrinsics are, GCC's
|
|
aren't.
|
|
|
|
Another example (use predsimplify to eliminate a select):
|
|
|
|
int foo (unsigned long j) {
|
|
if (j)
|
|
return __builtin_ffs (j) - 1;
|
|
else
|
|
return 0;
|
|
}
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Use push/pop instructions in prolog/epilog sequences instead of stores off
|
|
ESP (certain code size win, perf win on some [which?] processors).
|
|
Also, 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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
How about intrinsics? An example is:
|
|
*res = _mm_mulhi_epu16(*A, _mm_mul_epu32(*B, *C));
|
|
|
|
compiles to
|
|
pmuludq (%eax), %xmm0
|
|
movl 8(%esp), %eax
|
|
movdqa (%eax), %xmm1
|
|
pmulhuw %xmm0, %xmm1
|
|
|
|
The transformation probably requires a X86 specific pass or a DAG combiner
|
|
target specific hook.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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:
|
|
|
|
int %test1(int %X) {
|
|
%Y = div int %X, 8
|
|
ret int %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 :)
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
The first BB of this code:
|
|
|
|
declare bool %foo()
|
|
int %bar() {
|
|
%V = call bool %foo()
|
|
br bool %V, label %T, label %F
|
|
T:
|
|
ret int 1
|
|
F:
|
|
call bool %foo()
|
|
ret int 12
|
|
}
|
|
|
|
compiles to:
|
|
|
|
_bar:
|
|
subl $12, %esp
|
|
call L_foo$stub
|
|
xorb $1, %al
|
|
testb %al, %al
|
|
jne LBB_bar_2 # F
|
|
|
|
It would be better to emit "cmp %al, 1" than a xor and test.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Enable X86InstrInfo::convertToThreeAddress().
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
%X = weak global int 0
|
|
|
|
void %foo(int %N) {
|
|
%N = cast int %N to uint
|
|
%tmp.24 = setgt int %N, 0
|
|
br bool %tmp.24, label %no_exit, label %return
|
|
|
|
no_exit:
|
|
%indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ]
|
|
%i.0.0 = cast uint %indvar to int
|
|
volatile store int %i.0.0, int* %X
|
|
%indvar.next = add uint %indvar, 1
|
|
%exitcond = seteq uint %indvar.next, %N
|
|
br bool %exitcond, label %return, label %no_exit
|
|
|
|
return:
|
|
ret void
|
|
}
|
|
|
|
compiles into:
|
|
|
|
.text
|
|
.align 4
|
|
.globl _foo
|
|
_foo:
|
|
movl 4(%esp), %eax
|
|
cmpl $1, %eax
|
|
jl LBB_foo_4 # return
|
|
LBB_foo_1: # no_exit.preheader
|
|
xorl %ecx, %ecx
|
|
LBB_foo_2: # no_exit
|
|
movl L_X$non_lazy_ptr, %edx
|
|
movl %ecx, (%edx)
|
|
incl %ecx
|
|
cmpl %eax, %ecx
|
|
jne LBB_foo_2 # no_exit
|
|
LBB_foo_3: # return.loopexit
|
|
LBB_foo_4: # return
|
|
ret
|
|
|
|
We should hoist "movl L_X$non_lazy_ptr, %edx" out of the loop after
|
|
remateralization is implemented. This can be accomplished with 1) a target
|
|
dependent LICM pass or 2) makeing SelectDAG represent the whole function.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
The following tests perform worse with LSR:
|
|
|
|
lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Teach the coalescer to coalesce vregs of different register classes. e.g. FR32 /
|
|
FR64 to VR128.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
mov $reg, 48(%esp)
|
|
...
|
|
leal 48(%esp), %eax
|
|
mov %eax, (%esp)
|
|
call _foo
|
|
|
|
Obviously it would have been better for the first mov (or any op) to store
|
|
directly %esp[0] if there are no other uses.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
If shorter, we should use things like:
|
|
movzwl %ax, %eax
|
|
instead of:
|
|
andl $65535, %EAX
|
|
|
|
The former can also be used when the two-addressy nature of the 'and' would
|
|
require a copy to be inserted (in X86InstrInfo::convertToThreeAddress).
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Bad codegen:
|
|
|
|
char foo(int x) { return x; }
|
|
|
|
_foo:
|
|
movl 4(%esp), %eax
|
|
shll $24, %eax
|
|
sarl $24, %eax
|
|
ret
|
|
|
|
SIGN_EXTEND_INREG can be implemented as (sext (trunc)) to take advantage of
|
|
sub-registers.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Consider this:
|
|
|
|
typedef struct pair { float A, B; } pair;
|
|
void pairtest(pair P, float *FP) {
|
|
*FP = P.A+P.B;
|
|
}
|
|
|
|
We currently generate this code with llvmgcc4:
|
|
|
|
_pairtest:
|
|
movl 8(%esp), %eax
|
|
movl 4(%esp), %ecx
|
|
movd %eax, %xmm0
|
|
movd %ecx, %xmm1
|
|
addss %xmm0, %xmm1
|
|
movl 12(%esp), %eax
|
|
movss %xmm1, (%eax)
|
|
ret
|
|
|
|
we should be able to generate:
|
|
_pairtest:
|
|
movss 4(%esp), %xmm0
|
|
movl 12(%esp), %eax
|
|
addss 8(%esp), %xmm0
|
|
movss %xmm0, (%eax)
|
|
ret
|
|
|
|
The issue is that llvmgcc4 is forcing the struct to memory, then passing it as
|
|
integer chunks. It does this so that structs like {short,short} are passed in
|
|
a single 32-bit integer stack slot. We should handle the safe cases above much
|
|
nicer, while still handling the hard cases.
|
|
|
|
While true in general, in this specific case we could do better by promoting
|
|
load int + bitcast to float -> load fload. This basically needs alignment info,
|
|
the code is already implemented (but disabled) in dag combine).
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Another instruction selector deficiency:
|
|
|
|
void %bar() {
|
|
%tmp = load int (int)** %foo
|
|
%tmp = tail call int %tmp( int 3 )
|
|
ret void
|
|
}
|
|
|
|
_bar:
|
|
subl $12, %esp
|
|
movl L_foo$non_lazy_ptr, %eax
|
|
movl (%eax), %eax
|
|
call *%eax
|
|
addl $12, %esp
|
|
ret
|
|
|
|
The current isel scheme will not allow the load to be folded in the call since
|
|
the load's chain result is read by the callseq_start.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Don't forget to find a way to squash noop truncates in the JIT environment.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Implement anyext in the same manner as truncate that would allow them to be
|
|
eliminated.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
How about implementing truncate / anyext as a property of machine instruction
|
|
operand? i.e. Print as 32-bit super-class register / 16-bit sub-class register.
|
|
Do this for the cases where a truncate / anyext is guaranteed to be eliminated.
|
|
For IA32 that is truncate from 32 to 16 and anyext from 16 to 32.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Implement CTTZ, CTLZ with bsf and bsr.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
int %foo(int* %a, int %t) {
|
|
entry:
|
|
br label %cond_true
|
|
|
|
cond_true: ; preds = %cond_true, %entry
|
|
%x.0.0 = phi int [ 0, %entry ], [ %tmp9, %cond_true ]
|
|
%t_addr.0.0 = phi int [ %t, %entry ], [ %tmp7, %cond_true ]
|
|
%tmp2 = getelementptr int* %a, int %x.0.0
|
|
%tmp3 = load int* %tmp2 ; <int> [#uses=1]
|
|
%tmp5 = add int %t_addr.0.0, %x.0.0 ; <int> [#uses=1]
|
|
%tmp7 = add int %tmp5, %tmp3 ; <int> [#uses=2]
|
|
%tmp9 = add int %x.0.0, 1 ; <int> [#uses=2]
|
|
%tmp = setgt int %tmp9, 39 ; <bool> [#uses=1]
|
|
br bool %tmp, label %bb12, label %cond_true
|
|
|
|
bb12: ; preds = %cond_true
|
|
ret int %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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Currently we don't have elimination of redundant stack manipulations. Consider
|
|
the code:
|
|
|
|
int %main() {
|
|
entry:
|
|
call fastcc void %test1( )
|
|
call fastcc void %test2( sbyte* cast (void ()* %test1 to sbyte*) )
|
|
ret int 0
|
|
}
|
|
|
|
declare fastcc void %test1()
|
|
|
|
declare fastcc void %test2(sbyte*)
|
|
|
|
|
|
This currently compiles to:
|
|
|
|
subl $16, %esp
|
|
call _test5
|
|
addl $12, %esp
|
|
subl $16, %esp
|
|
movl $_test5, (%esp)
|
|
call _test6
|
|
addl $12, %esp
|
|
|
|
The add\sub pair is really unneeded here.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We currently compile sign_extend_inreg into two shifts:
|
|
|
|
long foo(long X) {
|
|
return (long)(signed char)X;
|
|
}
|
|
|
|
becomes:
|
|
|
|
_foo:
|
|
movl 4(%esp), %eax
|
|
shll $24, %eax
|
|
sarl $24, %eax
|
|
ret
|
|
|
|
This could be:
|
|
|
|
_foo:
|
|
movsbl 4(%esp),%eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Consider the expansion of:
|
|
|
|
uint %test3(uint %X) {
|
|
%tmp1 = rem uint %X, 255
|
|
ret uint %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.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Start using the flags 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
|
|
|
|
and:
|
|
|
|
int add_zf(int *x, int y, int a, int b) {
|
|
if ((*x + y) < 0)
|
|
return a;
|
|
else
|
|
return b;
|
|
}
|
|
|
|
to:
|
|
|
|
add_zf:
|
|
addl (%rdi), %esi
|
|
movl %edx, %eax
|
|
cmovns %ecx, %eax
|
|
ret
|
|
|
|
instead of:
|
|
|
|
_add_zf:
|
|
addl (%rdi), %esi
|
|
testl %esi, %esi
|
|
cmovs %edx, %ecx
|
|
movl %ecx, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This:
|
|
#include <math.h>
|
|
int foo(double X) { return isnan(X); }
|
|
|
|
compiles to (-m64):
|
|
|
|
_foo:
|
|
pxor %xmm1, %xmm1
|
|
ucomisd %xmm1, %xmm0
|
|
setp %al
|
|
movzbl %al, %eax
|
|
ret
|
|
|
|
the pxor is not needed, we could compare the value against itself.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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.
|
|
|
|
//===---------------------------------------------------------------------===//
|