[cfe-dev] rdrand intrinsic under -O0

[Xin Li]

Hi,

I wrote a small test program that uses the _rdrand64_step intrinsic,
where I think I might have hit a bug with clang.

The core portion of the program is:

static inline int
ivy_rng_store(long *buf)
{
	unsigned long long tmp;
	int retry;

	retry = RETRY_COUNT;
	do {
		if (_rdrand64_step(&tmp) == 1) {
			*buf = (long)tmp;
			break;
		}
		retry--;
	} while (retry > 0);

	return (retry);
}

When compiled with -O0, I got:

00000000004009b0 <ivy_rng_store>:
  4009b0:	55                   	push   %rbp
  4009b1:	48 89 e5             	mov    %rsp,%rbp
  4009b4:	48 89 7d f0          	mov    %rdi,-0x10(%rbp)
  4009b8:	c7 45 e4 0a 00 00 00 	movl   $0xa,-0x1c(%rbp)
  4009bf:	48 8d 45 e8          	lea    -0x18(%rbp),%rax
  4009c3:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
  4009c7:	48 8b 45 f8          	mov    -0x8(%rbp),%rax
  4009cb:	48 0f c7 f1          	rdrand %rcx
  4009cf:	89 ca                	mov    %ecx,%edx  # ???
  4009d1:	be 01 00 00 00       	mov    $0x1,%esi
  4009d6:	0f 42 d6             	cmovb  %esi,%edx
  4009d9:	48 89 08             	mov    %rcx,(%rax)
  4009dc:	81 fa 01 00 00 00    	cmp    $0x1,%edx
  4009e2:	0f 85 10 00 00 00    	jne    4009f8 <ivy_rng_store+0x48>
  4009e8:	48 8b 45 e8          	mov    -0x18(%rbp),%rax
  4009ec:	48 8b 4d f0          	mov    -0x10(%rbp),%rcx
  4009f0:	48 89 01             	mov    %rax,(%rcx)
  4009f3:	e9 18 00 00 00       	jmpq   400a10 <ivy_rng_store+0x60>
  4009f8:	8b 45 e4             	mov    -0x1c(%rbp),%eax
  4009fb:	05 ff ff ff ff       	add    $0xffffffff,%eax
  400a00:	89 45 e4             	mov    %eax,-0x1c(%rbp)
  400a03:	81 7d e4 00 00 00 00 	cmpl   $0x0,-0x1c(%rbp)
  400a0a:	0f 8f af ff ff ff    	jg     4009bf <ivy_rng_store+0xf>
  400a10:	8b 45 e4             	mov    -0x1c(%rbp),%eax
  400a13:	5d                   	pop    %rbp
  400a14:	c3                   	retq

It seems that, as the # ??? line suggests, that the compiler assumes
%ecx after rdrand %rcx would never be 1?

Version:

FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512
Target: x86_64-unknown-freebsd11.0
Thread model: posix

Options used:

-O2 -pipe   -mrdrnd -mmmx -O0 -std=gnu99 -fstack-protector
-Qunused-arguments

Thanks in advance!

Cheers,