https://bugs.llvm.org/show_bug.cgi?id=47438
Bug ID: 47438
Summary: swap goes through memory when it could stay in
registers
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Godbolt: https://godbolt.org/z/Yez1Mc
$ cat t.cc
#include <utility>
struct foo {
float x, y, z, w;
};
void bar(foo& a, foo& b) { std::swap(a, b); }
$ gcc-11 -O3 -S -o - t.cc
bar(foo&, foo&):
movups (%rdi), %xmm0
movdqu (%rsi), %xmm1
movups %xmm1, (%rdi)
movups %xmm0, (%rsi)
ret
$ clang-11 -O3 -S -o - t.cc
bar(foo&, foo&): # @bar(foo&, foo&)
movups (%rdi), %xmm0
movaps %xmm0, -24(%rsp)
movups (%rsi), %xmm0
movups %xmm0, (%rdi)
movaps -24(%rsp), %xmm0
movups %xmm0, (%rsi)
retq
The std::swap expands into an alloca and 3 memcpy's.
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