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

[Chung-Lin Tang <cltang@codesourcery.com>]

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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

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Index: config/arm/arm.c
===================================================================
--- config/arm/arm.c	(revision 176385)
+++ config/arm/arm.c	(working copy)
@@ -3172,6 +3172,19 @@
       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_lowpart_p (XEXP (*op0, 0))
+      && *op1 == const0_rtx)
+    *op0 = gen_rtx_AND (SImode, SUBREG_REG (XEXP (*op0, 0)),
+			GEN_INT (255));
+
   /* Comparisons smaller than DImode.  Only adjust comparisons against
      an out-of-range constant.  */
   if (GET_CODE (*op1) != CONST_INT
Index: testsuite/gcc.target/arm/combine-movs.c
===================================================================
--- testsuite/gcc.target/arm/combine-movs.c	(revision 0)
+++ testsuite/gcc.target/arm/combine-movs.c	(revision 0)
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O" }  */
+
+void foo (unsigned long r[], unsigned int d)
+{
+  int i, n = d / 32;
+  for (i = 0; i < n; ++i)
+    r[i] = 0;
+}
+
+/* { dg-final { scan-assembler "movs\tr\[0-9\]" } } */
Index: testsuite/gcc.target/arm/unsigned-extend-2.c
===================================================================
--- testsuite/gcc.target/arm/unsigned-extend-2.c	(revision 0)
+++ testsuite/gcc.target/arm/unsigned-extend-2.c	(revision 0)
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O -march=armv6t2" } */
+
+unsigned short foo (unsigned short x)
+{
+  unsigned char i = 0;
+  for (i = 0; i < 8; i++)
+    {
+      x >>= 1;
+      x &= 0x7fff;
+    }
+  return x;
+}
+
+/* { dg-final { scan-assembler "ands" } } */
+/* { dg-final { scan-assembler-not "uxtb" } } */
+/* { dg-final { scan-assembler-not "cmp" } } */