[Qemu-devel] possible qemu miscompilation by latest gcc

[Qemu-devel] possible qemu miscompilation by latest gcc

[John Regehr]

Hi folks-

Just wanted to let you know that perhaps the function 
helper_neon_rshl_s8() is being miscompiled by the latest gcc.

I'm using qemu 0.12.2 and gcc rev 156103, which is a pre-version of gcc 
4.5.  This is on an x86 machine running Ubuntu 9.10.

At -O2 or higher this is the resulting object code:

00002060 <helper_neon_rshl_s8>:
     2060:	31 c0                	xor    %eax,%eax
     2062:	c3                   	ret

If this is not the intended result, then either the function has a 
latent bug or else the compiler is doing something bad.

Hope this is helpful,

John Regehr

Re: [Qemu-devel] possible qemu miscompilation by latest gcc

[Richard Henderson]

On 01/20/2010 10:15 PM, John Regehr wrote:
> Hi folks-
>
> Just wanted to let you know that perhaps the function
> helper_neon_rshl_s8() is being miscompiled by the latest gcc.
>
> I'm using qemu 0.12.2 and gcc rev 156103, which is a pre-version of gcc
> 4.5. This is on an x86 machine running Ubuntu 9.10.
>
> At -O2 or higher this is the resulting object code:
>
> 00002060 <helper_neon_rshl_s8>:
> 2060: 31 c0 xor %eax,%eax
> 2062: c3 ret
>
> If this is not the intended result, then either the function has a
> latent bug or else the compiler is doing something bad.

Confirmed.  The compiler is at fault.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42833


r~