Re: [PATCH][combine][RFC][2/2] PR rtl-optimization/68796: Perfer zero_extract comparison against zero rather than unsupported shorter modes

[Jeff Law <law at redhat dot com>]

On 12/17/2015 09:26 AM, Kyrill Tkachov wrote:
> One could argue that if the target has (or advertises having) a native
> QImode register comparison then it's objectively a simplification to
> transform a comparison in a wider mode
> to a comparison in the shorter mode.
Generally true.

The most commonly cited exception is any port that defines 
WORD_REGISTER_OPERATIONS.  However, I would be comfortable with the idea 
that defining QImode comparisons on a target with 
WORD_REGISTER_OPERATIONS is a pretty explicit indication that it wants 
to try and shorten comparisons for one reason or another.




> If, however, the target doesn't have such an instruction (like aarch64
> doesn't have QImode registers) then
> truncating the wider mode to QImode through a subreg is not less complex
> than a zero_extract, as both will
> involve some form of extracting/masking the desired QImode bits. So
> picking a canonical form there makes sense,
> and the documentation already specifies the zero_extract form as the
> canonical.
>
> Would be nice to get a definite clarification on whether the subreg form
> is indeed the canonical one.
The subreg style "extension" isn't really an extension.  It is a way to 
say that we want to look at the object in a wider mode, but we don't 
actually care about the upper bits.  It's generally expected that the 
subreg won't result in the generation of any code.

A zero extract defines all the bits.

In theory the optimizers can use a SUBREG just like they could a REG, 
which should enable additional optimization.  In practice I don't think 
that's been as true as we'd like.

jeff