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| author | Roger Sayle <roger@nextmovesoftware.com> | 2022-03-12 09:20:52 +0000 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2022-03-12 09:20:52 +0000 |
| commit | 72c243017dee611bf3c32ea7bfad6ac538021aaf (patch) | |
| tree | 29fdccad56796b5a6ffd64e9312a9a999448b6fd | |
| parent | 3cb27b85a7b977958d53e1a29596ba211d21dde2 (diff) | |
| download | gcc-72c243017dee611bf3c32ea7bfad6ac538021aaf.zip gcc-72c243017dee611bf3c32ea7bfad6ac538021aaf.tar.gz gcc-72c243017dee611bf3c32ea7bfad6ac538021aaf.tar.bz2 | |
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.
| -rw-r--r-- | gcc/match.pd | 5 | ||||
| -rw-r--r-- | gcc/testsuite/gcc.dg/pr98420.c | 8 |
2 files changed, 12 insertions, 1 deletions
diff --git a/gcc/match.pd b/gcc/match.pd index 97399e5..8b44b5c 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 0000000..c289b84 --- /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" } } */ |