[cfe-dev] Does Clang correctly compile inline assembly?

[Alexey Samsonov]

Hi!

I wonder if the following code snippet with assembly is broken, or Clang is
wrong:

$ cat tmp/asm/rsp.c
void cpuid2 ( unsigned int* eax, unsigned int* ebx, unsigned int* ecx,
unsigned int* edx,
    unsigned int index, unsigned int ecx_in )
{
  unsigned int a, b, c, d;
  __asm__ __volatile__ ("push %%ebx\n\tcpuid\n\tmovl %%ebx,
%%esi\n\tpop%%ebx"
: "=a" (a), "=S" (b), "=c" (c), "=d" (d) \
: "0" (index), "2"(ecx_in) );
  *eax = a; *ebx = b; *ecx = c; *edx = d;
}

$ ./bin/clang tmp/asm/rsp.c -c -o tmp/asm/rsp.o
$ objdump -d tmp/asm/rsp.o
<...>
0000000000000000 <cpuid2>:
   0: 48 89 7c 24 f8       mov    %rdi,-0x8(%rsp)    <---- %rdi is saved on
stack
   5: 48 89 74 24 f0       mov    %rsi,-0x10(%rsp)
   a: 48 89 54 24 e8       mov    %rdx,-0x18(%rsp)
   f: 48 89 4c 24 e0       mov    %rcx,-0x20(%rsp)
  14: 44 89 44 24 dc       mov    %r8d,-0x24(%rsp)
  19: 44 89 4c 24 d8       mov    %r9d,-0x28(%rsp)
  1e: 44 8b 44 24 dc       mov    -0x24(%rsp),%r8d
  23: 44 8b 4c 24 d8       mov    -0x28(%rsp),%r9d
  28: 44 89 c0             mov    %r8d,%eax
  2b: 44 89 c9             mov    %r9d,%ecx
  2e: 53                   push   %rbx <--------------------------- % rbx
is pushed on stack and overwrites %rdi
  2f: 0f a2                 cpuid
  31: 89 de                 mov    %ebx,%esi
  33: 5b                   pop    %rbx
  34: 89 44 24 d4           mov    %eax,-0x2c(%rsp)
  38: 89 74 24 d0           mov    %esi,-0x30(%rsp)
  3c: 89 4c 24 cc           mov    %ecx,-0x34(%rsp)
  40: 89 54 24 c8           mov    %edx,-0x38(%rsp)
  44: 8b 44 24 d4           mov    -0x2c(%rsp),%eax
  48: 48 8b 7c 24 f8       mov    -0x8(%rsp),%rdi   <----- %rdi is restored
incorrectly
  4d: 89 07                 mov    %eax,(%rdi)
<....>

Adding %rsp to the clobber-list doesn't help (and doesn't change generated
code).

-- 
Alexey Samsonov, MSK
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