https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98337
Bug ID: 98337
Summary: Failure to optimize out on-stack array construction
when unneeded
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(int x) {
int a[] = {0, 1};
return a[x];
}
On AMD64, with -O3, this is generated :
f(int):
mov rax, QWORD PTR .LC0[rip]
movsx rdi, edi
mov QWORD PTR [rsp-8], rax
mov eax, DWORD PTR [rsp-8+rdi*4]
ret
.LC0:
.long 0
.long 1
LLVM generates this :
f(int): # @f(int)
movsxd rax, edi
mov eax, dword ptr [4*rax + .L__const.f(int).a]
ret
.L__const.f(int).a:
.long 0 # 0x0
.long 1 # 0x1
It seems to me like not copying the entire array on the stack makes for faster
code.
