Re: [PATCH] Canonicalize compares in combine [3/3] ARM backend part

[Chung-Lin Tang <cltang at codesourcery dot com>]

On 2011/6/15 09:12 PM, Richard Earnshaw wrote:
> On 22/04/11 16:21, Chung-Lin Tang wrote:
>> Hi Richard, this part's for you.
>>
>> The ARM backend changes needed are very little after the prior patches,
>> basically just a case in arm_canonicalize_comparison() to detect
>> (zero_extend:SI (subreg:QI (reg:SI ...) 0)), and swap it into (and:SI
>> (reg:SI) #255).
>>
>> Had we not tried the combine modifications, this testcase probably could
>> have also be solved by implementing another version of the corresponding
>> *andsi3_compare0/_scratch patterns, with ZERO_EXTEND in the body, and
>> "ands" in the output assembly. Maybe that's an acceptable solution too...
>>
>> About the (ab)use of CANONICALIZE_COMPARISON, if it really should be
>> another macro/hook, then this ARM patch will need updating, but the code
>> should be similar.
>>
>> Thanks,
>> Chung-Lin
>>
>>
>> 3-arm-parts.diff
>>
>>
>> Index: config/arm/arm.c
>> ===================================================================
>> --- config/arm/arm.c	(revision 172860)
>> +++ config/arm/arm.c	(working copy)
>> @@ -3276,6 +3276,19 @@
>>        return code;
>>      }
>>  
>> +  /* If *op0 is (zero_extend:SI (subreg:QI (reg:SI) 0)) and comparing
>> +     with const0_rtx, change it to (and:SI (reg:SI) (const_int 255)),
>> +     to facilitate possible combining with a cmp into 'ands'.  */
>> +  if (mode == SImode
>> +      && GET_CODE (*op0) == ZERO_EXTEND
>> +      && GET_CODE (XEXP (*op0, 0)) == SUBREG
>> +      && GET_MODE (XEXP (*op0, 0)) == QImode
>> +      && GET_MODE (SUBREG_REG (XEXP (*op0, 0))) == SImode
>> +      && SUBREG_BYTE (XEXP (*op0, 0)) == 0
>> +      && *op1 == const0_rtx)
>> +    *op0 = gen_rtx_AND (SImode, SUBREG_REG (XEXP (*op0, 0)),
>> +			GEN_INT (255));
>> +
> 
> This is wrong for big-endian code.  You should use subreg_lowpart_p to
> check the subreg expression (after you've checked that you do have a
> subreg, of course).
> 
> R.
> 

Hi Richard, thanks for catching that. I've updated the patch, and
cross-tested again under both arm/armeb-Linux.

I changed the testcase to use -march=armv6t2 instead of armv6, as the
latter makes the testcase FAIL when configured as --with-mode=thumb.

Is this now okay?

Thanks,
Chung-Lin

Attachment: uxtb-cmp.diff
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