[PATCH] Fix floating point handling in dom (PR tree-optimization/84235)
Jakub Jelinek
jakub@redhat.com
Tue Feb 6 20:40:00 GMT 2018
Hi!
As the following testcase shows, dom2 miscompiles floating point x - x
into 0.0 even when x could be infinity and x - x then a NaN.
The corresponding match.pd optimization is:
/* Simplify x - x.
This is unsafe for certain floats even in non-IEEE formats.
In IEEE, it is unsafe because it does wrong for NaNs.
Also note that operand_equal_p is always false if an operand
is volatile. */
(simplify
(minus @0 @0)
(if (!FLOAT_TYPE_P (type) || !HONOR_NANS (type))
{ build_zero_cst (type); }))
The patch makes it match what match.pd does.
We also have:
/* X / X is one. */
(simplify
(div @0 @0)
/* But not for 0 / 0 so that we can get the proper warnings and errors.
And not for _Fract types where we can't build 1. */
(if (!integer_zerop (@0) && !ALL_FRACT_MODE_P (TYPE_MODE (type)))
{ build_one_cst (type); }))
We can ignore the 0 / 0 case, we have both operands SSA_NAMEs and match.pd
only avoids optimizing away literal 0 / 0, but the rest is valid, some fract
types don't have a way to express 1.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2018-02-06 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/84235
* tree-ssa-scopedtables.c
(avail_exprs_stack::simplify_binary_operation): Fir MINUS_EXPR, punt
if the subtraction is performed in floating point type where NaNs are
honored. For *DIV_EXPR, punt for ALL_FRACT_MODE_Ps where we can't
build 1. Formatting fix.
* gcc.c-torture/execute/ieee/pr84235.c: New test.
--- gcc/tree-ssa-scopedtables.c.jj 2018-01-03 10:19:54.528533857 +0100
+++ gcc/tree-ssa-scopedtables.c 2018-02-06 14:58:08.944673984 +0100
@@ -182,8 +182,15 @@ avail_exprs_stack::simplify_binary_opera
case BIT_AND_EXPR:
return gimple_assign_rhs1 (stmt);
- case BIT_XOR_EXPR:
case MINUS_EXPR:
+ /* This is unsafe for certain floats even in non-IEEE
+ formats. In IEEE, it is unsafe because it does
+ wrong for NaNs. */
+ if (FLOAT_TYPE_P (result_type)
+ && HONOR_NANS (result_type))
+ break;
+ /* FALLTHRU */
+ case BIT_XOR_EXPR:
case TRUNC_MOD_EXPR:
case CEIL_MOD_EXPR:
case FLOOR_MOD_EXPR:
@@ -195,6 +202,9 @@ avail_exprs_stack::simplify_binary_opera
case FLOOR_DIV_EXPR:
case ROUND_DIV_EXPR:
case EXACT_DIV_EXPR:
+ /* Avoid _Fract types where we can't build 1. */
+ if (ALL_FRACT_MODE_P (TYPE_MODE (result_type)))
+ break;
return build_one_cst (result_type);
default:
@@ -204,8 +214,8 @@ avail_exprs_stack::simplify_binary_opera
break;
}
- default:
- break;
+ default:
+ break;
}
}
}
--- gcc/testsuite/gcc.c-torture/execute/ieee/pr84235.c.jj 2018-02-06 15:04:26.528454766 +0100
+++ gcc/testsuite/gcc.c-torture/execute/ieee/pr84235.c 2018-02-06 15:05:06.836341334 +0100
@@ -0,0 +1,11 @@
+/* PR tree-optimization/84235 */
+
+int
+main ()
+{
+ double d = 1.0 / 0.0;
+ _Bool b = d == d && (d - d) != (d - d);
+ if (!b)
+ __builtin_abort ();
+ return 0;
+}
Jakub
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