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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/103406] gcc -O0 behaves differently on "DBL_MAX related operations" than gcc -O1 and above Date: Thu, 25 Nov 2021 19:40:16 +0000 [thread overview] Message-ID: <bug-103406-4-CYHVdr6e9v@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-103406-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103406 Roger Sayle <roger at nextmovesoftware dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|ASSIGNED |NEW Assignee|roger at nextmovesoftware dot com |unassigned at gcc dot gnu.org Summary|[12 Regression] gcc -O0 |gcc -O0 behaves differently |behaves differently on |on "DBL_MAX related |"DBL_MAX related |operations" than gcc -O1 |operations" than gcc -O1 |and above |and above | Target| |x86_64 --- Comment #13 from Roger Sayle <roger at nextmovesoftware dot com> --- The Inf - Inf => 0.0 regression should now be fixed on mainline. Hmm. As hinted by Richard Beiner's investigation, the underlying problem is even more pervasive. It turns out that on x86/IA64 chips, floating point addition is not commutative, i.e. x+y is not the same as y+x, as demonstrated by the test program below: #include <stdio.h> const double pn = __builtin_nan(""); const double mn = -__builtin_nan(""); __attribute__ ((noinline, noclone)) double plus(double x, double y) { return x + y; } int main() { printf("%lf\n",plus(pn,mn)); printf("%lf\n",plus(mn,pn)); return 0; } Output: nan -nan Unfortunately, GCC assumes almost everywhere the FP addition is commutative and (as per comments #8 and #9) associative with negation/minus. This appears to be target property, c.f. libgcc's _FP_CHOOSENAN, but could in theory be resolved by a -fstrict-math mode (that implies -ftrapping-math) that disables commutativity (swapping of operands) throughout the compiler, including reload/fold-const etc., on affected Intel-like targets. Perhaps this PR is a duplicate now that the regression has been fixed?
next prev parent reply other threads:[~2021-11-25 19:40 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-11-24 11:09 [Bug c/103406] New: " shaohua.li at inf dot ethz.ch 2021-11-24 11:21 ` [Bug c/103406] " pinskia at gcc dot gnu.org 2021-11-24 11:23 ` ubizjak at gmail dot com 2021-11-24 11:29 ` pinskia at gcc dot gnu.org 2021-11-24 11:54 ` rguenth at gcc dot gnu.org 2021-11-24 12:01 ` rguenth at gcc dot gnu.org 2021-11-24 12:02 ` rguenth at gcc dot gnu.org 2021-11-24 12:03 ` [Bug middle-end/103406] [12 Regression] " rguenth at gcc dot gnu.org 2021-11-24 12:29 ` roger at nextmovesoftware dot com 2021-11-24 13:02 ` rguenth at gcc dot gnu.org 2021-11-24 13:04 ` rguenth at gcc dot gnu.org 2021-11-24 13:05 ` rguenth at gcc dot gnu.org 2021-11-24 22:22 ` joseph at codesourcery dot com 2021-11-25 19:04 ` cvs-commit at gcc dot gnu.org 2021-11-25 19:40 ` roger at nextmovesoftware dot com [this message] 2021-11-25 20:29 ` [Bug middle-end/103406] " joseph at codesourcery dot com 2021-11-26 17:58 ` roger at nextmovesoftware dot com
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