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9be47adde4
llvm-svn: 54644
755 lines
19 KiB
Plaintext
755 lines
19 KiB
Plaintext
//===- README.txt - Notes for improving PowerPC-specific code gen ---------===//
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TODO:
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* gpr0 allocation
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* implement do-loop -> bdnz transform
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* lmw/stmw pass a la arm load store optimizer for prolog/epilog
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===-------------------------------------------------------------------------===
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Support 'update' load/store instructions. These are cracked on the G5, but are
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still a codesize win.
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With preinc enabled, this:
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long *%test4(long *%X, long *%dest) {
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%Y = getelementptr long* %X, int 4
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%A = load long* %Y
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store long %A, long* %dest
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ret long* %Y
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}
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compiles to:
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_test4:
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mr r2, r3
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lwzu r5, 32(r2)
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lwz r3, 36(r3)
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stw r5, 0(r4)
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stw r3, 4(r4)
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mr r3, r2
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blr
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with -sched=list-burr, I get:
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_test4:
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lwz r2, 36(r3)
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lwzu r5, 32(r3)
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stw r2, 4(r4)
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stw r5, 0(r4)
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blr
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===-------------------------------------------------------------------------===
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We compile the hottest inner loop of viterbi to:
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li r6, 0
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b LBB1_84 ;bb432.i
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LBB1_83: ;bb420.i
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lbzx r8, r5, r7
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addi r6, r7, 1
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stbx r8, r4, r7
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LBB1_84: ;bb432.i
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mr r7, r6
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cmplwi cr0, r7, 143
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bne cr0, LBB1_83 ;bb420.i
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The CBE manages to produce:
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li r0, 143
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mtctr r0
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loop:
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lbzx r2, r2, r11
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stbx r0, r2, r9
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addi r2, r2, 1
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bdz later
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b loop
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This could be much better (bdnz instead of bdz) but it still beats us. If we
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produced this with bdnz, the loop would be a single dispatch group.
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===-------------------------------------------------------------------------===
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Compile:
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void foo(int *P) {
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if (P) *P = 0;
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}
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into:
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_foo:
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cmpwi cr0,r3,0
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beqlr cr0
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li r0,0
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stw r0,0(r3)
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blr
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This is effectively a simple form of predication.
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===-------------------------------------------------------------------------===
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Lump the constant pool for each function into ONE pic object, and reference
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pieces of it as offsets from the start. For functions like this (contrived
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to have lots of constants obviously):
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double X(double Y) { return (Y*1.23 + 4.512)*2.34 + 14.38; }
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We generate:
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_X:
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lis r2, ha16(.CPI_X_0)
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lfd f0, lo16(.CPI_X_0)(r2)
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lis r2, ha16(.CPI_X_1)
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lfd f2, lo16(.CPI_X_1)(r2)
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fmadd f0, f1, f0, f2
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lis r2, ha16(.CPI_X_2)
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lfd f1, lo16(.CPI_X_2)(r2)
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lis r2, ha16(.CPI_X_3)
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lfd f2, lo16(.CPI_X_3)(r2)
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fmadd f1, f0, f1, f2
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blr
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It would be better to materialize .CPI_X into a register, then use immediates
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off of the register to avoid the lis's. This is even more important in PIC
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mode.
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Note that this (and the static variable version) is discussed here for GCC:
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http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00133.html
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Here's another example (the sgn function):
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double testf(double a) {
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return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0);
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}
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it produces a BB like this:
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LBB1_1: ; cond_true
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lis r2, ha16(LCPI1_0)
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lfs f0, lo16(LCPI1_0)(r2)
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lis r2, ha16(LCPI1_1)
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lis r3, ha16(LCPI1_2)
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lfs f2, lo16(LCPI1_2)(r3)
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lfs f3, lo16(LCPI1_1)(r2)
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fsub f0, f0, f1
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fsel f1, f0, f2, f3
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blr
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===-------------------------------------------------------------------------===
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PIC Code Gen IPO optimization:
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Squish small scalar globals together into a single global struct, allowing the
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address of the struct to be CSE'd, avoiding PIC accesses (also reduces the size
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of the GOT on targets with one).
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Note that this is discussed here for GCC:
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http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00133.html
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===-------------------------------------------------------------------------===
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Implement Newton-Rhapson method for improving estimate instructions to the
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correct accuracy, and implementing divide as multiply by reciprocal when it has
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more than one use. Itanium will want this too.
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===-------------------------------------------------------------------------===
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Compile offsets from allocas:
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int *%test() {
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%X = alloca { int, int }
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%Y = getelementptr {int,int}* %X, int 0, uint 1
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ret int* %Y
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}
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into a single add, not two:
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_test:
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addi r2, r1, -8
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addi r3, r2, 4
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blr
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--> important for C++.
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===-------------------------------------------------------------------------===
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No loads or stores of the constants should be needed:
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struct foo { double X, Y; };
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void xxx(struct foo F);
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void bar() { struct foo R = { 1.0, 2.0 }; xxx(R); }
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===-------------------------------------------------------------------------===
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Darwin Stub LICM optimization:
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Loops like this:
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for (...) bar();
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Have to go through an indirect stub if bar is external or linkonce. It would
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be better to compile it as:
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fp = &bar;
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for (...) fp();
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which only computes the address of bar once (instead of each time through the
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stub). This is Darwin specific and would have to be done in the code generator.
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Probably not a win on x86.
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===-------------------------------------------------------------------------===
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Simple IPO for argument passing, change:
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void foo(int X, double Y, int Z) -> void foo(int X, int Z, double Y)
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the Darwin ABI specifies that any integer arguments in the first 32 bytes worth
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of arguments get assigned to r3 through r10. That is, if you have a function
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foo(int, double, int) you get r3, f1, r6, since the 64 bit double ate up the
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argument bytes for r4 and r5. The trick then would be to shuffle the argument
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order for functions we can internalize so that the maximum number of
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integers/pointers get passed in regs before you see any of the fp arguments.
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Instead of implementing this, it would actually probably be easier to just
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implement a PPC fastcc, where we could do whatever we wanted to the CC,
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including having this work sanely.
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===-------------------------------------------------------------------------===
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Fix Darwin FP-In-Integer Registers ABI
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Darwin passes doubles in structures in integer registers, which is very very
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bad. Add something like a BIT_CONVERT to LLVM, then do an i-p transformation
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that percolates these things out of functions.
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Check out how horrible this is:
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http://gcc.gnu.org/ml/gcc/2005-10/msg01036.html
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This is an extension of "interprocedural CC unmunging" that can't be done with
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just fastcc.
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===-------------------------------------------------------------------------===
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Compile this:
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int foo(int a) {
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int b = (a < 8);
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if (b) {
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return b * 3; // ignore the fact that this is always 3.
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} else {
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return 2;
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}
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}
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into something not this:
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_foo:
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1) cmpwi cr7, r3, 8
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mfcr r2, 1
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rlwinm r2, r2, 29, 31, 31
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1) cmpwi cr0, r3, 7
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bgt cr0, LBB1_2 ; UnifiedReturnBlock
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LBB1_1: ; then
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rlwinm r2, r2, 0, 31, 31
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mulli r3, r2, 3
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blr
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LBB1_2: ; UnifiedReturnBlock
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li r3, 2
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blr
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In particular, the two compares (marked 1) could be shared by reversing one.
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This could be done in the dag combiner, by swapping a BR_CC when a SETCC of the
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same operands (but backwards) exists. In this case, this wouldn't save us
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anything though, because the compares still wouldn't be shared.
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===-------------------------------------------------------------------------===
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We should custom expand setcc instead of pretending that we have it. That
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would allow us to expose the access of the crbit after the mfcr, allowing
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that access to be trivially folded into other ops. A simple example:
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int foo(int a, int b) { return (a < b) << 4; }
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compiles into:
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_foo:
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cmpw cr7, r3, r4
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mfcr r2, 1
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rlwinm r2, r2, 29, 31, 31
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slwi r3, r2, 4
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blr
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===-------------------------------------------------------------------------===
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Fold add and sub with constant into non-extern, non-weak addresses so this:
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static int a;
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void bar(int b) { a = b; }
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void foo(unsigned char *c) {
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*c = a;
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}
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So that
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_foo:
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lis r2, ha16(_a)
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la r2, lo16(_a)(r2)
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lbz r2, 3(r2)
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stb r2, 0(r3)
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blr
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Becomes
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_foo:
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lis r2, ha16(_a+3)
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lbz r2, lo16(_a+3)(r2)
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stb r2, 0(r3)
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blr
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===-------------------------------------------------------------------------===
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We generate really bad code for this:
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int f(signed char *a, _Bool b, _Bool c) {
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signed char t = 0;
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if (b) t = *a;
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if (c) *a = t;
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}
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===-------------------------------------------------------------------------===
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This:
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int test(unsigned *P) { return *P >> 24; }
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Should compile to:
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_test:
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lbz r3,0(r3)
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blr
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not:
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_test:
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lwz r2, 0(r3)
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srwi r3, r2, 24
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blr
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===-------------------------------------------------------------------------===
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On the G5, logical CR operations are more expensive in their three
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address form: ops that read/write the same register are half as expensive as
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those that read from two registers that are different from their destination.
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We should model this with two separate instructions. The isel should generate
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the "two address" form of the instructions. When the register allocator
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detects that it needs to insert a copy due to the two-addresness of the CR
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logical op, it will invoke PPCInstrInfo::convertToThreeAddress. At this point
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we can convert to the "three address" instruction, to save code space.
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This only matters when we start generating cr logical ops.
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===-------------------------------------------------------------------------===
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We should compile these two functions to the same thing:
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#include <stdlib.h>
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void f(int a, int b, int *P) {
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*P = (a-b)>=0?(a-b):(b-a);
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}
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void g(int a, int b, int *P) {
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*P = abs(a-b);
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}
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Further, they should compile to something better than:
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_g:
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subf r2, r4, r3
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subfic r3, r2, 0
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cmpwi cr0, r2, -1
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bgt cr0, LBB2_2 ; entry
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LBB2_1: ; entry
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mr r2, r3
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LBB2_2: ; entry
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stw r2, 0(r5)
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blr
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GCC produces:
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_g:
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subf r4,r4,r3
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srawi r2,r4,31
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xor r0,r2,r4
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subf r0,r2,r0
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stw r0,0(r5)
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blr
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... which is much nicer.
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This theoretically may help improve twolf slightly (used in dimbox.c:142?).
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===-------------------------------------------------------------------------===
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int foo(int N, int ***W, int **TK, int X) {
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int t, i;
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for (t = 0; t < N; ++t)
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for (i = 0; i < 4; ++i)
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W[t / X][i][t % X] = TK[i][t];
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return 5;
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}
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We generate relatively atrocious code for this loop compared to gcc.
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We could also strength reduce the rem and the div:
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http://www.lcs.mit.edu/pubs/pdf/MIT-LCS-TM-600.pdf
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===-------------------------------------------------------------------------===
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float foo(float X) { return (int)(X); }
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Currently produces:
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_foo:
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fctiwz f0, f1
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stfd f0, -8(r1)
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lwz r2, -4(r1)
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extsw r2, r2
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std r2, -16(r1)
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lfd f0, -16(r1)
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fcfid f0, f0
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frsp f1, f0
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blr
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We could use a target dag combine to turn the lwz/extsw into an lwa when the
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lwz has a single use. Since LWA is cracked anyway, this would be a codesize
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win only.
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===-------------------------------------------------------------------------===
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We generate ugly code for this:
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void func(unsigned int *ret, float dx, float dy, float dz, float dw) {
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unsigned code = 0;
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if(dx < -dw) code |= 1;
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if(dx > dw) code |= 2;
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if(dy < -dw) code |= 4;
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if(dy > dw) code |= 8;
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if(dz < -dw) code |= 16;
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if(dz > dw) code |= 32;
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*ret = code;
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}
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===-------------------------------------------------------------------------===
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Complete the signed i32 to FP conversion code using 64-bit registers
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transformation, good for PI. See PPCISelLowering.cpp, this comment:
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// FIXME: disable this lowered code. This generates 64-bit register values,
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// and we don't model the fact that the top part is clobbered by calls. We
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// need to flag these together so that the value isn't live across a call.
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//setOperationAction(ISD::SINT_TO_FP, MVT::i32, Custom);
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Also, if the registers are spilled to the stack, we have to ensure that all
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64-bits of them are save/restored, otherwise we will miscompile the code. It
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sounds like we need to get the 64-bit register classes going.
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===-------------------------------------------------------------------------===
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%struct.B = type { i8, [3 x i8] }
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define void @bar(%struct.B* %b) {
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entry:
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%tmp = bitcast %struct.B* %b to i32* ; <uint*> [#uses=1]
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%tmp = load i32* %tmp ; <uint> [#uses=1]
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%tmp3 = bitcast %struct.B* %b to i32* ; <uint*> [#uses=1]
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%tmp4 = load i32* %tmp3 ; <uint> [#uses=1]
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%tmp8 = bitcast %struct.B* %b to i32* ; <uint*> [#uses=2]
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%tmp9 = load i32* %tmp8 ; <uint> [#uses=1]
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%tmp4.mask17 = shl i32 %tmp4, i8 1 ; <uint> [#uses=1]
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%tmp1415 = and i32 %tmp4.mask17, 2147483648 ; <uint> [#uses=1]
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%tmp.masked = and i32 %tmp, 2147483648 ; <uint> [#uses=1]
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%tmp11 = or i32 %tmp1415, %tmp.masked ; <uint> [#uses=1]
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%tmp12 = and i32 %tmp9, 2147483647 ; <uint> [#uses=1]
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%tmp13 = or i32 %tmp12, %tmp11 ; <uint> [#uses=1]
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store i32 %tmp13, i32* %tmp8
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ret void
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}
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We emit:
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_foo:
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lwz r2, 0(r3)
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slwi r4, r2, 1
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or r4, r4, r2
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rlwimi r2, r4, 0, 0, 0
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stw r2, 0(r3)
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blr
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We could collapse a bunch of those ORs and ANDs and generate the following
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equivalent code:
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_foo:
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lwz r2, 0(r3)
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rlwinm r4, r2, 1, 0, 0
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or r2, r2, r4
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stw r2, 0(r3)
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blr
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===-------------------------------------------------------------------------===
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We compile:
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unsigned test6(unsigned x) {
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return ((x & 0x00FF0000) >> 16) | ((x & 0x000000FF) << 16);
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}
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into:
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_test6:
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lis r2, 255
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rlwinm r3, r3, 16, 0, 31
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ori r2, r2, 255
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and r3, r3, r2
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blr
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GCC gets it down to:
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_test6:
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rlwinm r0,r3,16,8,15
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rlwinm r3,r3,16,24,31
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or r3,r3,r0
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blr
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===-------------------------------------------------------------------------===
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Consider a function like this:
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float foo(float X) { return X + 1234.4123f; }
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The FP constant ends up in the constant pool, so we need to get the LR register.
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This ends up producing code like this:
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_foo:
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.LBB_foo_0: ; entry
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mflr r11
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*** stw r11, 8(r1)
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bl "L00000$pb"
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"L00000$pb":
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mflr r2
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addis r2, r2, ha16(.CPI_foo_0-"L00000$pb")
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lfs f0, lo16(.CPI_foo_0-"L00000$pb")(r2)
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fadds f1, f1, f0
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*** lwz r11, 8(r1)
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mtlr r11
|
|
blr
|
|
|
|
This is functional, but there is no reason to spill the LR register all the way
|
|
to the stack (the two marked instrs): spilling it to a GPR is quite enough.
|
|
|
|
Implementing this will require some codegen improvements. Nate writes:
|
|
|
|
"So basically what we need to support the "no stack frame save and restore" is a
|
|
generalization of the LR optimization to "callee-save regs".
|
|
|
|
Currently, we have LR marked as a callee-save reg. The register allocator sees
|
|
that it's callee save, and spills it directly to the stack.
|
|
|
|
Ideally, something like this would happen:
|
|
|
|
LR would be in a separate register class from the GPRs. The class of LR would be
|
|
marked "unspillable". When the register allocator came across an unspillable
|
|
reg, it would ask "what is the best class to copy this into that I *can* spill"
|
|
If it gets a class back, which it will in this case (the gprs), it grabs a free
|
|
register of that class. If it is then later necessary to spill that reg, so be
|
|
it.
|
|
|
|
===-------------------------------------------------------------------------===
|
|
|
|
We compile this:
|
|
int test(_Bool X) {
|
|
return X ? 524288 : 0;
|
|
}
|
|
|
|
to:
|
|
_test:
|
|
cmplwi cr0, r3, 0
|
|
lis r2, 8
|
|
li r3, 0
|
|
beq cr0, LBB1_2 ;entry
|
|
LBB1_1: ;entry
|
|
mr r3, r2
|
|
LBB1_2: ;entry
|
|
blr
|
|
|
|
instead of:
|
|
_test:
|
|
addic r2,r3,-1
|
|
subfe r0,r2,r3
|
|
slwi r3,r0,19
|
|
blr
|
|
|
|
This sort of thing occurs a lot due to globalopt.
|
|
|
|
===-------------------------------------------------------------------------===
|
|
|
|
We currently compile 32-bit bswap:
|
|
|
|
declare i32 @llvm.bswap.i32(i32 %A)
|
|
define i32 @test(i32 %A) {
|
|
%B = call i32 @llvm.bswap.i32(i32 %A)
|
|
ret i32 %B
|
|
}
|
|
|
|
to:
|
|
|
|
_test:
|
|
rlwinm r2, r3, 24, 16, 23
|
|
slwi r4, r3, 24
|
|
rlwimi r2, r3, 8, 24, 31
|
|
rlwimi r4, r3, 8, 8, 15
|
|
rlwimi r4, r2, 0, 16, 31
|
|
mr r3, r4
|
|
blr
|
|
|
|
it would be more efficient to produce:
|
|
|
|
_foo: mr r0,r3
|
|
rlwinm r3,r3,8,0xffffffff
|
|
rlwimi r3,r0,24,0,7
|
|
rlwimi r3,r0,24,16,23
|
|
blr
|
|
|
|
===-------------------------------------------------------------------------===
|
|
|
|
test/CodeGen/PowerPC/2007-03-24-cntlzd.ll compiles to:
|
|
|
|
__ZNK4llvm5APInt17countLeadingZerosEv:
|
|
ld r2, 0(r3)
|
|
cntlzd r2, r2
|
|
or r2, r2, r2 <<-- silly.
|
|
addi r3, r2, -64
|
|
blr
|
|
|
|
The dead or is a 'truncate' from 64- to 32-bits.
|
|
|
|
===-------------------------------------------------------------------------===
|
|
|
|
We generate horrible ppc code for this:
|
|
|
|
#define N 2000000
|
|
double a[N],c[N];
|
|
void simpleloop() {
|
|
int j;
|
|
for (j=0; j<N; j++)
|
|
c[j] = a[j];
|
|
}
|
|
|
|
LBB1_1: ;bb
|
|
lfdx f0, r3, r4
|
|
addi r5, r5, 1 ;; Extra IV for the exit value compare.
|
|
stfdx f0, r2, r4
|
|
addi r4, r4, 8
|
|
|
|
xoris r6, r5, 30 ;; This is due to a large immediate.
|
|
cmplwi cr0, r6, 33920
|
|
bne cr0, LBB1_1
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
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:
|
|
add r4,r3,r4
|
|
subfc r3,r3,r4
|
|
li r3,0
|
|
adde r3,r3,r3
|
|
blr
|
|
|
|
(or better) not:
|
|
|
|
__Z11no_overflowjj:
|
|
add r2, r4, r3
|
|
cmplw cr7, r2, r3
|
|
mfcr r2
|
|
rlwinm r2, r2, 29, 31, 31
|
|
xori r3, r2, 1
|
|
blr
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We compile some FP comparisons into an mfcr with two rlwinms and an or. For
|
|
example:
|
|
#include <math.h>
|
|
int test(double x, double y) { return islessequal(x, y);}
|
|
int test2(double x, double y) { return islessgreater(x, y);}
|
|
int test3(double x, double y) { return !islessequal(x, y);}
|
|
|
|
Compiles into (all three are similar, but the bits differ):
|
|
|
|
_test:
|
|
fcmpu cr7, f1, f2
|
|
mfcr r2
|
|
rlwinm r3, r2, 29, 31, 31
|
|
rlwinm r2, r2, 31, 31, 31
|
|
or r3, r2, r3
|
|
blr
|
|
|
|
GCC compiles this into:
|
|
|
|
_test:
|
|
fcmpu cr7,f1,f2
|
|
cror 30,28,30
|
|
mfcr r3
|
|
rlwinm r3,r3,31,1
|
|
blr
|
|
|
|
which is more efficient and can use mfocr. See PR642 for some more context.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
void foo(float *data, float d) {
|
|
long i;
|
|
for (i = 0; i < 8000; i++)
|
|
data[i] = d;
|
|
}
|
|
void foo2(float *data, float d) {
|
|
long i;
|
|
data--;
|
|
for (i = 0; i < 8000; i++) {
|
|
data[1] = d;
|
|
data++;
|
|
}
|
|
}
|
|
|
|
These compile to:
|
|
|
|
_foo:
|
|
li r2, 0
|
|
LBB1_1: ; bb
|
|
addi r4, r2, 4
|
|
stfsx f1, r3, r2
|
|
cmplwi cr0, r4, 32000
|
|
mr r2, r4
|
|
bne cr0, LBB1_1 ; bb
|
|
blr
|
|
_foo2:
|
|
li r2, 0
|
|
LBB2_1: ; bb
|
|
addi r4, r2, 4
|
|
stfsx f1, r3, r2
|
|
cmplwi cr0, r4, 32000
|
|
mr r2, r4
|
|
bne cr0, LBB2_1 ; bb
|
|
blr
|
|
|
|
The 'mr' could be eliminated to folding the add into the cmp better.
|
|
|
|
//===---------------------------------------------------------------------===//
|