On Fri, Mar 11, 2022 at 12:31 AM Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > 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 > > Technically, this is a regression from GCC 4.1 (according to godbolt.org) > so hopefully this patch is suitable during stage4. > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > and make -k check with no new failures. Ok for mainline?
+ && !tree_expr_maybe_infinite_p (@0) + && (!flag_rounding_math || !HONOR_SIGNED_ZEROS (type)))) { build_zero_cst (type); })) HONOR_SIGN_DEPENDENT_ROUNDING (type) instead of flag_rounding_math? OK with that change. Richard. > > 2022-03-10 Roger Sayle <ro...@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. > > > Thanks in advance, > Roger > -- >