https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90864
Bug ID: 90864
Summary: Suboptimal codegen of structs in C/C++on x86_64
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jaak at ristioja dot ee
Target Milestone: ---
I found two issues with handling of structures, which might be related.
Given structures like this:
struct X { char v; };
struct X2 { X x; char v; };
struct X3 { X2 x2; char v; };
The code generated for the following function:
X3 id(X3 value) { return value; }
with clang/clang++ (version 8.0.0) and -O2 is:
0: 89 f8 mov %edi,%eax
2: c3
but with gcc/g++ 9.1.0 and -O2 the generated code reads:
0: 89 f8 mov %edi,%eax
2: 0f b7 d7 movzwl %di,%edx
5: c1 e8 10 shr $0x10,%eax
8: 0f b6 c0 movzbl %al,%eax
b: 48 c1 e0 10 shl $0x10,%rax
f: 48 09 d0 or %rdx,%rax
12: c3 retq
and with gcc/g++ 8.3.0 it was even worse:
0: 89 fa mov %edi,%edx
2: c1 ef 10 shr $0x10,%edi
5: 0f b6 c2 movzbl %dl,%eax
8: 40 0f b6 ff movzbl %dil,%edi
c: 0f b6 d6 movzbl %dh,%edx
f: 88 d4 mov %dl,%ah
11: 48 c1 e7 10 shl $0x10,%rdi
15: 48 09 f8 or %rdi,%rax
18: c3 retq
I'm not sure whether this is a related issue or not, but something as simple as
the following also fails to compile, even though the argument should be found
in %rdi on x86_64:
void test(X3 a) {
__asm__ ("" :: "r" (a));
}
With the output:
warning: asm operand 0 probably doesn't match constraints
__asm__ ("" :: "r" (a));
^
error: impossible constraint in 'asm'
