Hi Jakub,
Technically, PR97965 doesn't explicitly mention equality/inequality, but
you're
right, it makes sense to tackle this missed optimization at the same time as
we
fix the wrong-code.
On Thu, Nov 26, 2020, Jakub Jelinek wrote:
>On Thu, Nov 26, 2020 at 01:56:03PM -0000, Roger Sayle wrote:
>> --- a/gcc/match.pd
>> +++ b/gcc/match.pd
>> @@ -3998,7 +3998,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>> (cmp @0 { build_real (TREE_TYPE (@1), dconst0); }))
>> /* x != NaN is always true, other ops are always false. */
>> (if (REAL_VALUE_ISNAN (TREE_REAL_CST (@1))
>> - && ! HONOR_SNANS (@1))
>> + && ! flag_trapping_math)
>
>Shouldn't this one stay as is for cmp == EQ_EXPR || cmp == NE_EXPR?
>I mean, == or != only raise exceptions on sNaNs, while other simple
comparisons raise on both sNaNs and qNaNs.
The case to be careful of here is that (x == qNaN) can still raise/trap if x
is a sNaN.
Hence, the full condition should include:
&& (!flag_trapping_math
|| ((cmp == EQ_EXPR || cmp == NE_EXPR)
&& !tree_expr_maybe_signaling_nan_p (@1)
&& !tree_expr_maybe_signaling_nan_p (@0)))
Note: I repeat tree_expr_maybe_signaling_nan_p here for symmetry, though
because
we know @1 is a NaN, this could have been written as
!tree_expr_signaling_nan_p (@1).
>> --- a/gcc/simplify-rtx.c
>> +++ b/gcc/simplify-rtx.c
>> @@ -5732,12 +5732,13 @@ simplify_const_relational_operation (enum
rtx_code code,
>> if (REAL_VALUE_ISNAN (*d0) || REAL_VALUE_ISNAN (*d1))
>> switch (code)
>> {
>> + case NE:
>> + return flag_trapping_math ? 0 : const_true_rtx;
>
> And here too (for NE and would need moving EQ later too.
Yep/Agreed. These cases become:
case NE:
if (flag_trapping_math
&& (REAL_VALUE_ISSIGNALING_NAN (*d0)
|| REAL_VALUE_ISSIGANLING_NAN (*d1)))
return 0;
return const_true_rtx;
case EQ:
if (flag_trapping_math
&& (REAL_VALUE_ISSIGNALING_NAN (*d0)
|| REAL_VALUE_ISSIGANLING_NAN (*d1)))
return 0;
return const0_rtx;
A revised patch is attached (which I've confirmed compiles but
haven't regression tested).
Many thanks again.
Roger
--
&& (!flag_trapping_math
|| ((cmp == EQ_EXPR || cmp == NE_EXPR)
&& !tree_expr_maybe_signaling_nan_p (@1)
&& !tree_expr_maybe_signaling_nan_p (@0)))
case NE:
if (flag_trapping_math
&& (REAL_VALUE_ISSIGNALING_NAN (*d0)
|| REAL_VALUE_ISSIGANLING_NAN (*d1)))
return 0;
return const_true_rtx;
case EQ:
if (flag_trapping_math
&& (REAL_VALUE_ISSIGNALING_NAN (*d0)
|| REAL_VALUE_ISSIGANLING_NAN (*d1)))
return 0;
return const0_rtx;
