https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109667
Bug ID: 109667
Summary: [12/13/14 Regression] Unnecessary temporary storage
used for 32-byte struct
Product: gcc
Version: 12.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: chfast at gmail dot com
Target Milestone: ---
Reduced reproducer:
struct i256 {
long v[4];
};
void assign(struct i256 *v, long z) {
struct i256 r = {};
for (int i = 0; i < 1; ++i)
r.v[i] = z;
*v = r;
}
https://godbolt.org/z/avM74o3r6
The compiler allocates temporary storage on stack for `r`:
assign:
pxor xmm0, xmm0
mov QWORD PTR [rsp-40], rsi
movups XMMWORD PTR [rsp-32], xmm0
movdqa xmm1, XMMWORD PTR [rsp-40]
mov QWORD PTR [rsp-16], 0
movdqa xmm2, XMMWORD PTR [rsp-24]
movups XMMWORD PTR [rdi], xmm1
movups XMMWORD PTR [rdi+16], xmm2
ret
Regression since 12. The 11 compiles nicely to:
assign:
mov QWORD PTR [rdi], rsi
mov QWORD PTR [rdi+8], 0
mov QWORD PTR [rdi+16], 0
mov QWORD PTR [rdi+24], 0
ret
