https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95019
Bug ID: 95019
Summary: Optimizer produces suboptimal code related to
-ftree-ivopts
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zhongyunde at tom dot com
Target Milestone: ---
For the following code, we can known the variable C000005A1 is only used for
the offset of array Dest and Src, and the unit size of the array is 8 bytes, so
an iv variable with step 8 will be good for targets, whose load/store insns
don't folded the lshift operand.
typedef unsigned long long UINT64;
void C00000ADA(UINT64 len, long long *__restrict Src, long long *__restrict
Dest)
{
UINT64 C00000ADD, index, C00000068, offset, C00000ADF;
UINT64 C000005A1 = 0;
for (index = 0; index < len; index++) {
Dest[C000005A1] = Src[C000005A1] * Src[C000005A1];
C000005A1 += len - index;
}
}
test base on the MIPS64 gcc 5.4 on https://gcc.godbolt.org, as the MIPS64
target doesn't have load/store folded the lshift operand such as 'ldr x3,
[x1, x4, lsl 3]' in ARM64 targets , so use ivtmp with step 8 can eliminate the
dsll insn, which is in the kernel loop.
@@ -2,16 +2,17 @@ C00000ADA(unsigned long long, long long*, long long*):
beq $4,$0,.L10 #, len,,
move $7,$0 # C000005A1,
+ dsll $8,$4,3 # tmp, len << 3
+
.L4:
- dsll $2,$7,3 # D.2019, C000005A1,
- daddu $3,$5,$2 # tmp204, Src, D.2019
+ daddu $3,$5,$7 # tmp204, Src, D.2019
ld $3,0($3) # D.2021, *_10
- daddu $2,$6,$2 # tmp205, Dest, D.2019
+ daddu $2,$6,$7 # tmp205, Dest, D.2019
dmult $3,$3 # D.2021, D.2021
daddu $7,$7,$4 # C000005A1, C000005A1, ivtmp.6
- daddiu $4,$4,-1 # ivtmp.6, ivtmp.6,
+ daddiu $4,$4,-8 # ivtmp.6, ivtmp.6,
mflo $3 # D.2021
- bne $4,$0,.L4 #, ivtmp.6,,
+ bne $8,$0,.L4 #, ivtmp.6,,
sd $3,0($2) # D.2021, *_8
.L10:
