https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93800
Bug ID: 93800
Summary: GCC adds unwanted nops to align loops on powerpc 8xx
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: christophe.le...@c-s.fr
Target Milestone: ---
GCC 9.2 add nops in front of loops. GCC 8.1 didn't when compiled for powerpc
8xx. On the 8xx, a nop is 1 cycle and alignment of loops provide no benefit, so
this is a waste of cycles.
Reproducer:
volatile int g;
int f(int a, int b)
{
int i;
for (i = 0; i < b; i++)
a += g;
return a;
}
Built with -m32 -mcpu=860 -O2
00000000 <f>:
0: 2c 04 00 00 cmpwi r4,0
4: 4c 81 00 20 blelr
8: 3d 40 00 00 lis r10,0
a: R_PPC_ADDR16_HA g
c: 7c 89 03 a6 mtctr r4
10: 39 4a 00 00 addi r10,r10,0
12: R_PPC_ADDR16_LO g
14: 60 00 00 00 nop
18: 60 00 00 00 nop
1c: 60 00 00 00 nop
20: 81 2a 00 00 lwz r9,0(r10)
24: 7c 63 4a 14 add r3,r3,r9
28: 42 00 ff f8 bdnz 20 <f+0x20>
2c: 4e 80 00 20 blr
The same with GCC 8.1:
00000000 <f>:
0: 2c 04 00 00 cmpwi r4,0
4: 4c 81 00 20 blelr
8: 3d 40 00 00 lis r10,0
a: R_PPC_ADDR16_HA g
c: 7c 89 03 a6 mtctr r4
10: 39 4a 00 00 addi r10,r10,0
12: R_PPC_ADDR16_LO g
14: 81 2a 00 00 lwz r9,0(r10)
18: 7c 63 4a 14 add r3,r3,r9
1c: 42 00 ff f8 bdnz 14 <f+0x14>
20: 4e 80 00 20 blr
