[Bug target/41505] GCC choosing poor code sequence for certain stores (x86)

[law at redhat dot com]

------- Comment #7 from law at redhat dot com  2009-09-30 14:47 -------
Subject: Re:  GCC choosing poor code sequence for certain
 stores (x86)

On 09/30/09 03:22, jakub at gcc dot gnu dot org wrote:
> ------- Comment #6 from jakub at gcc dot gnu dot org  2009-09-30 09:22 -------
> For x86-64 we perhaps want further checks for the size optimization - if the
> scratch register is %r8d through %r15d, 3 byte xorl %r8d, %r8d and e.g. 3 byte
> movl %r8d, (%rdx) won't be shorter than movl $0, (%rdx) which is 6 bytes).
> And likely the 2 insns will be slower.
> But if the address already needs rex prefix, it is still a win.
>
>
>    
Do we have any good way to test if the address needs a rex prefix?  I 
see the rex_prefix attribute in i386.md, but that's for testing an 
entire insn and based on my quick reading of i386.md it's not complete 
as many insns set the attribute explicitly.

Jeff


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41505