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From: Roger Sayle <sayle@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r12-7621] PR middle-end/98420: Don't fold x - x to 0.0 with -frounding-math Date: Sat, 12 Mar 2022 09:23:25 +0000 (GMT) [thread overview] Message-ID: <20220312092325.1CCB53858C27@sourceware.org> (raw) https://gcc.gnu.org/g:72c243017dee611bf3c32ea7bfad6ac538021aaf commit r12-7621-g72c243017dee611bf3c32ea7bfad6ac538021aaf Author: Roger Sayle <roger@nextmovesoftware.com> Date: Sat Mar 12 09:20:52 2022 +0000 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. Diff: --- gcc/match.pd | 5 ++++- gcc/testsuite/gcc.dg/pr98420.c | 8 ++++++++ 2 files changed, 12 insertions(+), 1 deletion(-) diff --git a/gcc/match.pd b/gcc/match.pd index 97399e580a4..8b44b5cc92c 100644 --- a/gcc/match.pd +++ b/gcc/match.pd @@ -229,13 +229,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) /* 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. + PR middle-end/98420: x - x may be -0.0 with FE_DOWNWARD. Also note that operand_equal_p is always false if an operand is volatile. */ (simplify (minus @0 @0) (if (!FLOAT_TYPE_P (type) || (!tree_expr_maybe_nan_p (@0) - && !tree_expr_maybe_infinite_p (@0))) + && !tree_expr_maybe_infinite_p (@0) + && (!HONOR_SIGN_DEPENDENT_ROUNDING (type) + || !HONOR_SIGNED_ZEROS (type)))) { build_zero_cst (type); })) (simplify (pointer_diff @@0 @0) diff --git a/gcc/testsuite/gcc.dg/pr98420.c b/gcc/testsuite/gcc.dg/pr98420.c new file mode 100644 index 00000000000..c289b845d63 --- /dev/null +++ b/gcc/testsuite/gcc.dg/pr98420.c @@ -0,0 +1,8 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -ffinite-math-only -frounding-math -fdump-tree-optimized" } */ +double foo (double a) +{ + return a - a; +} + +/* { dg-final { scan-tree-dump " = a_\[0-9\]\\(D\\) - a_\[0-9\]\\(D\\);" "optimized" } } */
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