{PING] [PATCH] Sign extension elimination

Mark Mitchell mark@codesourcery.com
Thu Apr 20 17:23:00 GMT 2006

H. J. Lu wrote:
> On Thu, Apr 20, 2006 at 09:39:23AM -0700, Mark Mitchell wrote:
>> I'm not finding this discussion very illuminating.
>> I'm trying to understand whether the current SEE pass is ready for
>> inclusion in the compiler.  Roger has approved it on technical merits,
>> but HJ has raised the issue that somehow this pass is a bad thing for
>> x86-64.
> The current SEE implemented isn't designed for x86-64 and doesn't help
> x86-64 at all, if it doesn't make it worse on x86-64. But its
> infrastructure is needed for future x86-64 SEE work.

OK.  So, then, there's no problem with incorporating the SEE
implementation, and we proceed with that.

The x86-64 work is probably going to have to wait for 4.3.

>> What is it about the x86-64 instruction set that is so different from
>> PowerPC in this regard?  Are any of our other x86 experts concerned
>> about this pass?
> What is so different for x86-64 is all 32bit register moves are
> automatically zero extended to 64bit. 

Whereas on other architectures a 32-bit register move is either (a)
sign-extending, or (b) leaves the high bits undefined?


Mark Mitchell
(650) 331-3385 x713

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