https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91348
Bug ID: 91348
Summary: Missed optimization: not passing hidden pointer but
copying memory
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: no...@turm-lahnstein.de
Target Milestone: ---
For the following example:
struct Vec3{
double x, y, z;
};
void vadd_v2(struct Vec3* a, struct Vec3* out);
struct Vec3 use_v1(struct Vec3 *in){
struct Vec3 out;
vadd_v2(in, &out);
return out;
}
the resulting assembler (-O2 -Wall) is:
use_v1:
pushq %r12
movq %rdi, %r12
movq %rsi, %rdi
subq $32, %rsp
movq %rsp, %rsi
call vadd_v2
movq 16(%rsp), %rax
movdqa (%rsp), %xmm0
movq %rax, 16(%r12)
movq %r12, %rax
movups %xmm0, (%r12)
addq $32, %rsp
popq %r12
ret
However, the hidden pointer could be passed directly into vadd_v2, which is
what clang is doing:
use_v1: # @use_v1
pushq %rbx
movq %rdi, %rbx
movq %rsi, %rdi
movq %rbx, %rsi
callq vadd_v2
movq %rbx, %rax
popq %rbx
retq
See also https://godbolt.org/z/rT41Sj
