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Re: [PATCH: PR target/44999] Replace "and r0, r0, #255" with uxtb in thumb2

From: Paul Brook <paul at codesourcery dot com>
To: Carrot Wei <carrot at google dot com>
Cc: gcc-patches at gcc dot gnu dot org, Richard Earnshaw <rearnsha at arm dot com>
Date: Mon, 8 Nov 2010 11:55:09 +0000
Subject: Re: [PATCH: PR target/44999] Replace "and r0, r0, #255" with uxtb in thumb2
References: <AANLkTinA9B9PuMdEvoNG4h7wnRd9egzqBNuUChFMJ+S1@mail.gmail.com> <201011031304.02720.paul@codesourcery.com> <AANLkTimibrqy+5xv+CWAa9S=OZsoK2cvrJgxM_gcuaUX@mail.gmail.com>

> > Even ignoring that, I don't think this is the right fix. Why do we need
> > this for Thumb mode, and not ARM mode? Why extendqi and not extendhi?
> 
> In Thumb mode, and with a constant is 32 bit, and uxtb is 16 bit.
> In ARM mode, both are 32 bit, so it is not beneficial.
> The test case shows that and with 0xFFFF is already converted to uxth,
> so it is not handled here.

I'm still not convinced. Why is HImode different to QImode? Why has this 
suddenly changed? Why does gcc generate uxtb in ARM mode but not Thumb mode? 
i.e. I've little confidence that this is actually fixing the problem rather than 
papering over a bug elsewhere, especially given we have a perfectly good 
zero_extendqisi2 pattern+expander.

Paul

Follow-Ups:
- Re: [PATCH: PR target/44999] Replace "and r0, r0, #255" with uxtb in thumb2
  - From: Carrot Wei

References:
- Re: [PATCH: PR target/44999] Replace "and r0, r0, #255" with uxtb in thumb2
  - From: Paul Brook
- Re: [PATCH: PR target/44999] Replace "and r0, r0, #255" with uxtb in thumb2
  - From: Carrot Wei

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