Richard Biener <rguent...@suse.de> writes:
> On Fri, 5 May 2017, Richard Sandiford wrote:
>> Richard Biener <rguent...@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?
>
> Yes. Can you add the testcase?
OK, here's what I installed after testing on aarch64-linux-gnu and
x86_64-linux-gnu.
Thanks,
Richard
2017-05-06 Richard Sandiford <richard.sandif...@linaro.org>
gcc/
PR rtl-optimization/75964
* simplify-rtx.c (simplify_const_relational_operation): Remove
invalid handling of comparisons of integer ABS.
gcc/testsuite/
PR rtl-optimization/75964
* gcc.dg/torture/pr75964.c: New test.
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:
Index: gcc/testsuite/gcc.dg/torture/pr75964.c
===================================================================
--- /dev/null 2017-05-05 17:46:05.071578834 +0100
+++ gcc/testsuite/gcc.dg/torture/pr75964.c 2017-05-06 08:35:54.794072285
+0100
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+
+typedef __UINT8_TYPE__ uint8_t;
+
+uint8_t __attribute__ ((noinline, noclone))
+abs8 (uint8_t x)
+{
+ if (x & 0x80)
+ x = -x;
+
+ if (x & 0x80)
+ x = 0x7f;
+
+ return x;
+}
+
+int
+main (void)
+{
+ if (abs8 (0) != 0
+ || abs8 (1) != 1
+ || abs8 (127) != 127
+ || abs8 (128) != 127
+ || abs8 (129) != 127
+ || abs8 (255) != 1)
+ __builtin_abort ();
+ return 0;
+}
