On Thu, Dec 3, 2015 at 7:01 PM, Steve Ellcey <sell...@imgtec.com> wrote: > Can the instruction scheduler actually rewrite instructions? I didn't > think so but when I compile some code on MIPS with: > > -O2 -fno-ivopts -fno-peephole2 -fno-schedule-insns2 > > I get: > > $L4: > lbu $3,0($4) > addiu $4,$4,1 > lbu $2,0($5) > beq $3,$0,$L7 > addiu $5,$5,1 > > beq $3,$2,$L4 > subu $2,$3,$2 > > When I changed -fno-schedule-insns2 to -fschedule-insns2, I get: > > $L4: > lbu $3,0($4) > addiu $5,$5,1 > lbu $2,-1($5) > beq $3,$0,$L7 > addiu $4,$4,1 > > beq $3,$2,$L4 > subu $2,$3,$2 > > I.e. The addiu of $5 and the load using $5 have been swapped around > and the load uses a different offset to compensate. I can't see > where in the instruction scheduler that this would happen. Any > help? This is on MIPS if that matters, though I didn't see any > MIPS specific code for this. This issue is related to my earlier > question about PR 48814 and ivopts (thus the -fno-ivopts option).
IIRC it's because the scheduler *thinks* it can get a tighter schedule - probably because it thinks it can dual issue the lbu from $4 and the addiu to $5. Can it think so ? This may be related - https://gcc.gnu.org/ml/gcc-patches/2012-08/msg00155.html regards Ramana > > The C code I am looking at is the strcmp function from glibc: > > int > strcmp (const char *p1, const char *p2) > { > const unsigned char *s1 = (const unsigned char *) p1; > const unsigned char *s2 = (const unsigned char *) p2; > unsigned char c1, c2; > > do > { > c1 = (unsigned char) *s1++; > c2 = (unsigned char) *s2++; > if (c1 == '\0') > return c1 - c2; > } > while (c1 == c2); > > return c1 - c2; > } > > > Steve Ellcey > sell...@imgtec.com
