https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109849
Bug ID: 109849
Summary: suboptimal code for vector walking loop
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: hubicka at gcc dot gnu.org
Target Milestone: ---
jan@localhost:/tmp> cat t.C
#include <vector>
typedef unsigned int uint32_t;
std::vector<std::pair<uint32_t, uint32_t>> stack;
void
test()
{
while (!stack.empty()) {
std::pair<uint32_t, uint32_t> cur = stack.back();
stack.pop_back();
if (cur.second)
break;
}
}
jan@localhost:/tmp> gcc t.C -O3 -S
yields to:
_Z4testv:
.LFB1264:
.cfi_startproc
movq stack(%rip), %rcx
movq stack+8(%rip), %rax
jmp .L5
.p2align 4,,10
.p2align 3
.L6:
movl -4(%rax), %edx
subq $8, %rax
movq %rax, stack+8(%rip)
testl %edx, %edx
jne .L4
.L5:
cmpq %rax, %rcx
jne .L6
.L4:
ret
We really should order the basic blocks putting cmpq before L6 saving a jump.
Moreover clang does
.p2align 4, 0x90
.LBB1_1: # =>This Inner Loop Header: Depth=1
cmpq %rax, %rcx
je .LBB1_3
# %bb.2: # in Loop: Header=BB1_1 Depth=1
cmpl $0, -4(%rcx)
leaq -8(%rcx), %rcx
movq %rcx, stack+8(%rip)
je .LBB1_1
.LBB1_3:
retq
saving an instruction. Why we do not move stack+8 updating out of the loop?
