https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95351
Bug ID: 95351 Summary: Comparison with NAN optimizes incorrectly with -ffast-math disabled Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: chris.dahlberg at cytovale dot com Target Milestone: --- Created attachment 48610 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48610&action=edit nan_bug.ii: preprocessed output from --save-temps The function bool Foo(double possiblyNAN, double b, double c) { return (possiblyNAN <= 2.0) || ((possiblyNAN > 2.0) && (b > c)); } is optimized incorrectly with -O1. It gets reduced to just '(b > c)', ignoring the possibility that 'possiblyNAN' is NAN and both comparisons vs 2.0 are false. -ffinite-math-only is off (both by default and explicitly, no difference) The command line I'm using is: "g++ -Wall -Wextra -O -fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations -fno-finite-math-only nan_bug.cpp" No warnings are output Looking at the assembly, 'possiblyNAN' is never accessed at all Tried this on these gcc versions and all have the same behavior: - gcc version 8.3.1 20190311 (Red Hat 8.3.1-3) (GCC) - gcc version 8.4.0 (Ubuntu 8.4.0-1ubuntu1~18.04) - gcc version 10.1.0 (Compiler-Explorer-Build) - gcc version 11.0.0 20200525 (experimental) (Compiler-Explorer-Build)