On Wed, May 29, 2013 at 10:33 AM, Eric Botcazou <ebotca...@adacore.com> wrote: > Hi, > > on most platforms the Ada compiler doesn't enable trap-on-FP-exceptions, so > it's appropriate to set -fno-trapping-math there, which has been done in > http://gcc.gnu.org/ml/gcc-patches/2013-05/msg01461.html > > However doing so can have an adverse effect if the architecture doesn't have a > sufficiently rich set of FP comparison instructions because -fno-trapping-math > allows the middle-end and the optimizers to turn the negation of signaling FP > comparison operators into quiet FP unordered operators, e.g. not(<) into !>= > and the quiet unordered operators aren't supported universally. The effect is > that you need 2 comparisons instead of just 1 to achieve the desired result. > > The first (surprising) example is the IA-64, which has ORDERED and UNORDERED > but not UNLT/UNLE/UNGT/UNGE because the unordered comparison instructions are > signaling (since they are the negation of the signaling ordered comparisons). > Therefore the fix is to make it possible to use these unordered comparison > instructions, which are signaling, for the quiet unordered operators when the > flag -fno-trapping-math is in effect. As a matter of fact, the support was > already there (e.g. in ia64_print_operand) but it was rightfully disabled. > > Tested on IA-64/Linux and IA-64/HP-UX, OK for the mainline?
Did you check that this doesn't cause traps on SPEC CPU 2000 / 2006 when compiled with -ffast-math (something we generally want to support, even if it is on the border of validity)? Thanks, Richard. > 2013-05-29 Eric Botcazou <ebotca...@adacore.com> > > * config/ia64/predicates.md (ia64_cbranch_operator): Accept unordered > comparison operators when -fno-trapping-math is in effect. > * config/ia64/ia64.c (ia64_expand_compare): Add support for unordered > comparison operators in TFmode and assert that unsupported operators > cannot reach here. > (ia64_print_operand): Likewise. > > > -- > Eric Botcazou
