Fwd: Assembler code with useless instructions

[Andrew Haley]

On 05/19/2011 06:47 PM, Diego Caballero wrote:
> Hello,
> 
> I get 10% lower performance due to an assemble instruction that seems
> to be useless.
> I have a lot of blocks like the following (exactly the same) in my code:
> 
>  callq     400a50 <expf@plt>
>  movaps %xmm0,%xmm4
>  movss   (%r15),%xmm0
>  movss   %xmm4,0x40(%rsp)
>  callq     400a50 <expf@plt>
>  ...
> 
> and at the end of them:
> 
>  movss   0x40(%rsp),%xmm4
> 
> %xmm4 is not read in the middle, so I don't understand why gcc generates
> the second instruction, instead of something like that:
> 
>  movss  %xmm0,0x40(%rsp)
>  movss  (%r15),%xmm0
>  callq  400a50 <expf@plt>
>  ...
>  movss  0x40(%rsp),%xmm4
> 
> Thank you in advance.

Hi Diego,

Can you make a small test case for us to try?  It should be the smallest
program that demonstrates the problem.

Andrew.