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Re: Assembler code with useless instructions

From: Robert Dewar <dewar at adacore dot com>
To: Diego Caballero <diego dot l dot caballero at gmail dot com>
Cc: gcc at gcc dot gnu dot org
Date: Thu, 19 May 2011 13:21:13 -0400
Subject: Re: Assembler code with useless instructions
References: <BANLkTiku0i7zmmubzL8u7iCGYi7u19VPkg@mail.gmail.com>

On 5/19/2011 1:14 PM, Diego Caballero wrote:

Hello,

sorry, this is my first mail and I don't know if I'm in the right place.
I get 10% lower performance due to an assemble instruction that seems
to be useless.
I have a lot of this blocks (exactly the sames) 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),%xm
Thank you in advance.
Best regards.


you are not in the "right place", this mailing list is for
development of gcc, not helping with its use. Try gcc help


--
Diego Caballero

References:
- Assembler code with useless instructions
  - From: Diego Caballero

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