prevent zero-extension when using a memory load instruction

Jeff Law law@redhat.com
Sat Apr 18 18:45:55 GMT 2020


On Sat, 2020-04-18 at 14:36 -0400, William Tambe via Gcc-help wrote:
> On Sat, Apr 18, 2020 at 11:32 AM Marc Glisse <marc.glisse@inria.fr> wrote:
> > On Sat, 18 Apr 2020, William Tambe via Gcc-help wrote:
> > 
> > > In the machine description file, is there a way to tell GCC that a
> > > memory load instruction already zero-extend such that it does not try
> > > to apply zero-extension ?
> > 
> > I would look at it the other way around: you can tell GCC what asm to
> > generate for a zero_extend with a memory operand. Or did you have a
> 
> Thanks; tried above, but GCC still prefer a memory load followed by
> zero-extension.
> 
> The example code used is as follow:
> 
>     unsigned char var;
>     int main() {
>         return var;
>     }
> 
> Also tried x86 and ARM GCC port to see what they produce, and find
> that only x86 will not generate a zero-extension when -Os is used.
> 
> The version of GCC used is 9.2.0.
> 
> Any other suggestions on how to tell GCC not to zero-extend the result
> of a memory load ?
Many ports have this feature and it's discussed in the developer documentation. 
Please read it.

jeff




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