https://bugs.llvm.org/show_bug.cgi?id=50105
Bug ID: 50105
Summary: Suboptimal runtime unrolling
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: david.bolvan...@gmail.com
CC: llvm-bugs@lists.llvm.org
int p(int x, int *arr) {
int s = 0;
for (int i = 0; i < x && i < 3; ++i) {
s += arr[i];
}
return s;
}
LLVM
p(int, int*): # @p(int, int*)
test edi, edi
jle .LBB0_1
add edi, -1
cmp edi, 2
mov ecx, 2
cmovb ecx, edi
mov eax, dword ptr [rsi]
test ecx, ecx
je .LBB0_5
add eax, dword ptr [rsi + 4]
cmp ecx, 1
je .LBB0_5
add eax, dword ptr [rsi + 8]
.LBB0_5:
ret
.LBB0_1:
xor eax, eax
ret
GCC
p(int, int*):
xor eax, eax
test edi, edi
jle .L1
mov eax, DWORD PTR [rsi]
cmp edi, 1
jle .L1
add eax, DWORD PTR [rsi+4]
cmp edi, 2
jle .L1
add eax, DWORD PTR [rsi+8]
.L1:
ret
We have here some useless code which can be eliminated and then LLVM can match
GCC.
Also with for (int i = 0; i < x && i < 16; ++i) LLVM produces a lot of
(vectorized) asm, GCC just unrolls 16x.
https://godbolt.org/z/sc59Gv3a9
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