https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86131
Bug ID: 86131
Summary: powerpc: gcc uses costly multiply instead of shift
left
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: christophe.le...@c-s.fr
Target Milestone: ---
unsigned long f1(unsigned long a, unsigned long b)
{
return a >> ((4 - a) * 8);
}
unsigned long f2(unsigned long a, unsigned long b)
{
return a >> ((4 - a) << 3);
}
unsigned long g1(unsigned long a, unsigned long b)
{
return a >> (32 - a * 8);
}
unsigned long g2(unsigned long a, unsigned long b)
{
return a >> (32 - (a << 3));
}
when compiling with GCC 8.1 with -O2 -mcpu=860, the following result is
obtained:
00000000 <f1>:
0: 1d 23 ff f8 mulli r9,r3,-8
4: 39 29 00 20 addi r9,r9,32
8: 7c 63 4c 30 srw r3,r3,r9
c: 4e 80 00 20 blr
00000010 <f2>:
10: 21 23 00 04 subfic r9,r3,4
14: 55 29 18 38 rlwinm r9,r9,3,0,28
18: 7c 63 4c 30 srw r3,r3,r9
1c: 4e 80 00 20 blr
00000020 <g1>:
20: 1d 23 ff f8 mulli r9,r3,-8
24: 39 29 00 20 addi r9,r9,32
28: 7c 63 4c 30 srw r3,r3,r9
2c: 4e 80 00 20 blr
00000030 <g2>:
30: 54 69 18 38 rlwinm r9,r3,3,0,28
34: 21 29 00 20 subfic r9,r9,32
38: 7c 63 4c 30 srw r3,r3,r9
3c: 4e 80 00 20 blr
mulli requires 2 cycles, therefore it shouldn't be used, should it ?
The same code compiled with -mcpu=e300c2 gives:
00000000 <f1>:
0: 54 69 18 38 rlwinm r9,r3,3,0,28
4: 21 29 00 20 subfic r9,r9,32
8: 7c 63 4c 30 srw r3,r3,r9
c: 4e 80 00 20 blr
00000010 <f2>:
10: 21 23 00 04 subfic r9,r3,4
14: 55 29 18 38 rlwinm r9,r9,3,0,28
18: 7c 63 4c 30 srw r3,r3,r9
1c: 4e 80 00 20 blr
00000020 <g1>:
20: 54 69 18 38 rlwinm r9,r3,3,0,28
24: 21 29 00 20 subfic r9,r9,32
28: 7c 63 4c 30 srw r3,r3,r9
2c: 4e 80 00 20 blr
00000030 <g2>:
30: 54 69 18 38 rlwinm r9,r3,3,0,28
34: 21 29 00 20 subfic r9,r9,32
38: 7c 63 4c 30 srw r3,r3,r9
3c: 4e 80 00 20 blr
