https://bugs.llvm.org/show_bug.cgi?id=39654
Bug ID: 39654
Summary: On x86-64, RAX is unnecessarily preserved when
returning member of struct containing couple of floats
Product: clang
Version: trunk
Hardware: PC
OS: Windows XP
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: s.h...@oisyn.nl
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org,
neeil...@live.com, richard-l...@metafoo.co.uk
Compiler Explorer link: https://godbolt.org/z/9-TCNN
Consider the following code:
====================
using type = float;
struct Data
{
type a, b;
};
type _a, _b;
__attribute__((noinline)) Data GetData() { return {_a, _b}; }
auto FirstMember()
{
return GetData().a;
}
====================
For FirstMember(), this will generate (with -O):
push rax
call GetData()
pop rax
ret
If you change 'type' to 'double', this will just turn into a tail call:
jmp GetData() # TAILCALL
When changing 'type' to 'double' AND returning 'b' rather than 'a', we see the
preservation of RAX again:
push rax
call GetData()
movaps xmm0, xmm1
pop rax
ret
I suspect it has to do with the rule that a function should return a pointer to
the returned struct according to the x86-64 ABI, but I don't think that applies
here as the struct is returned fully in SSE registers.
Note that it does NOT try to preserve RAX when compiling without optimizations.
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