Re: [patch,avr,committed]: Remove flag_strict_overflow from avr.md

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Richard Biener <rguenther@suse.de> writes:
> On Fri, 5 May 2017, Georg-Johann Lay wrote:
>> On 05.05.2017 13:04, Richard Biener wrote:
>> > On Fri, 5 May 2017, Georg-Johann Lay wrote:
>> > 
>> > > Applied this addendum to r247495 which removed flag_strict_overflow. There
>> > > were remains of the flag in avr.md which broke the avr build.
>> > > 
>> > > Committed as r247632.
>> > 
>> > Whoops - sorry for not grepping besides .[ch] files...
>> > 
>> > But... these patterns very much look like premature optimization
>> > and/or bugs.  combine is supposed to handle this via simplify_rtx.
>> 
>> Well, for now the patch just restores avr BE to be able to be build.
>
> Sure.
>
>> > Also note that on RTL we generally assume overflow wraps as we lose
>> > signedness of operands.  Not sure what 'compare' in your patterns
>> > will end up with.
>> > 
>> > The only flag_wrapv checks in RTL otherwise are in simplify-rtx.c
>> > for ABS which seems to be a singed RTL op.
>> 
>> Which is a bug, IMO.  Letting undefined overflow propagate to RTL
>> renders some RTL as if it has undefined behaviour.  Consequence is
>> that testing the MSB must no more use signed comparisons on
>> less-zero resp. greater-or-equal-to-zero.
>> 
>> Cf. https://gcc.gnu.org/PR75964 for an example:
>> 
>> 
>> typedef __UINT8_TYPE__ uint8_t;
>> 
>> uint8_t abs8 (uint8_t x)
>> {
>>     if (x & 0x80)
>>         x = -x;
>> 
>>     if (x & 0x80)
>>         x = 0x7f;
>> 
>>     return x;
>> }
>> 
>> The first comparison is performed by a signed test against 0 (which
>> is reasonable and the best code in that case) but then we conclude
>> that the second test is always false, which is BUG.
>> 
>> IMO the culprit is to let slip undefined overflow to RTL.
>
> Yes.  I thought in RTL overflow is always well-defined (but then
> as I said your patterns are equally bogus).

Yeah, me too.  I don't see how the simplify-rtx.c code can be right.

Is the following OK, if it passes testing?

Thanks,
Richard


2017-05-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR rtl-optimization/75964
	* simplify-rtx.c (simplify_const_relational_operation): Remove
	invalid handling of comparisons of integer ABS.

Index: gcc/simplify-rtx.c
===================================================================
--- gcc/simplify-rtx.c	2017-05-05 13:44:27.364724260 +0100
+++ gcc/simplify-rtx.c	2017-05-05 13:44:36.580195277 +0100
@@ -5316,34 +5316,14 @@ simplify_const_relational_operation (enu
 	{
 	case LT:
 	  /* Optimize abs(x) < 0.0.  */
-	  if (!HONOR_SNANS (mode)
-	      && (!INTEGRAL_MODE_P (mode)
-		  || (!flag_wrapv && !flag_trapv)))
-	    {
-	      if (INTEGRAL_MODE_P (mode)
-		  && (issue_strict_overflow_warning
-		      (WARN_STRICT_OVERFLOW_CONDITIONAL)))
-		warning (OPT_Wstrict_overflow,
-			 ("assuming signed overflow does not occur when "
-			  "assuming abs (x) < 0 is false"));
-	       return const0_rtx;
-	    }
+	  if (!INTEGRAL_MODE_P (mode) && !HONOR_SNANS (mode))
+	    return const0_rtx;
 	  break;
 
 	case GE:
 	  /* Optimize abs(x) >= 0.0.  */
-	  if (!HONOR_NANS (mode)
-	      && (!INTEGRAL_MODE_P (mode)
-		  || (!flag_wrapv && !flag_trapv)))
-	    {
-	      if (INTEGRAL_MODE_P (mode)
-	          && (issue_strict_overflow_warning
-	    	  (WARN_STRICT_OVERFLOW_CONDITIONAL)))
-	        warning (OPT_Wstrict_overflow,
-			 ("assuming signed overflow does not occur when "
-			  "assuming abs (x) >= 0 is true"));
-	      return const_true_rtx;
-	    }
+	  if (!INTEGRAL_MODE_P (mode) && !HONOR_NANS (mode))
+	    return const_true_rtx;
 	  break;
 
 	case UNGE: