https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97708
Bug ID: 97708
Summary: Inline asm does not use the local register asm
specified with register ... asm() as input
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: bp at alien8 dot de
Target Milestone: ---
Building the below on x86-64 with gcc9, gcc10 and Segher built with gcc11
20201015:
---
int main(void)
{
register float foo asm ("xmm0") = 0.99f;
asm volatile("movl %0, %%r8d\n\t"
"vmcall\n\t"
:: "g" (foo));
return 0;
}
---
results in gcc not using the xmm0 register directly as the input register to
the asm statement but does something funky:
0000000000001125 <main>:
1125: 55 push %rbp
1126: 48 89 e5 mov %rsp,%rbp
1129: f3 0f 10 05 d3 0e 00 movss 0xed3(%rip),%xmm0 # 2004
<_IO_stdin_used+0x4>
1130: 00
1131: 66 0f 7e c0 movd %xmm0,%eax
1135: 41 89 c0 mov %eax,%r8d
1138: 0f 01 c1 vmcall
by loading through the %eax GPR and producing wrong code without even a
warning. I'm not saying this example is supposed to make sense but this should
fail instead by trying to compile:
$ cat xmm1.s
movl %xmm0,%eax
$ x86_64-linux-as xmm1.s -o xmm1.o
xmm1.s: Assembler messages:
xmm1.s:1: Error: unsupported instruction `mov'
and fail the build because plain MOV cannot use an XMM reg as a source.
Thx.
