PR middle-end/98420: Don't fold x - x to 0.0 with -frounding-math
This patch addresses PR middle-end/98420, which is inappropriate constant folding of x - x to 0.0 (in match.pd) when -frounding-math is specified. Specifically, x - x may be -0.0 with FE_DOWNWARD as the rounding mode. To summarize, the desired IEEE behaviour, x - x for floating point x, (1) can't be folded to 0.0 by default, due to the possibility of NaN or Inf (2) can be folded to 0.0 with -ffinite-math-only (3) can't be folded to 0.0 with -ffinite-math-only -frounding-math (4) can be folded with -ffinite-math-only -frounding-math -fno-signed-zeros 2022-03-12 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog PR middle-end/98420 * match.pd (minus @0 @0): Additional checks for -fno-rounding-math (the defaut) or -fno-signed-zeros. gcc/testsuite/ChangeLog PR middle-end/98420 * gcc.dg/pr98420.c: New test case.
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@ -229,13 +229,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
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/* Simplify x - x.
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/* Simplify x - x.
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This is unsafe for certain floats even in non-IEEE formats.
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This is unsafe for certain floats even in non-IEEE formats.
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In IEEE, it is unsafe because it does wrong for NaNs.
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In IEEE, it is unsafe because it does wrong for NaNs.
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PR middle-end/98420: x - x may be -0.0 with FE_DOWNWARD.
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Also note that operand_equal_p is always false if an operand
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Also note that operand_equal_p is always false if an operand
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is volatile. */
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is volatile. */
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(simplify
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(simplify
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(minus @0 @0)
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(minus @0 @0)
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(if (!FLOAT_TYPE_P (type)
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(if (!FLOAT_TYPE_P (type)
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|| (!tree_expr_maybe_nan_p (@0)
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|| (!tree_expr_maybe_nan_p (@0)
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&& !tree_expr_maybe_infinite_p (@0)))
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&& !tree_expr_maybe_infinite_p (@0)
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&& (!HONOR_SIGN_DEPENDENT_ROUNDING (type)
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|| !HONOR_SIGNED_ZEROS (type))))
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{ build_zero_cst (type); }))
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{ build_zero_cst (type); }))
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(simplify
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(simplify
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(pointer_diff @@0 @0)
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(pointer_diff @@0 @0)
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@ -0,0 +1,8 @@
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/* { dg-do compile } */
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/* { dg-options "-O2 -ffinite-math-only -frounding-math -fdump-tree-optimized" } */
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double foo (double a)
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{
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return a - a;
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}
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/* { dg-final { scan-tree-dump " = a_\[0-9\]\\(D\\) - a_\[0-9\]\\(D\\);" "optimized" } } */
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