https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101063
Bug ID: 101063 Summary: #pragma STDC FENV_ACCESS ON: wrong code generation: instructions leading to side effects may not be generated Product: gcc Version: 11.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: pavel.morozkin at gmail dot com Target Milestone: --- Bug-triggering code (t0.c): #include <stdio.h> #include <fenv.h> #include <float.h> #pragma STDC FENV_ACCESS ON int main( void ) { volatile double d1 = DBL_MAX; volatile double d2 = DBL_MAX; #ifdef __STDC__ printf("__STDC__ %d\n", __STDC__); #endif #ifdef __STDC_VERSION__ printf("__STDC_VERSION__ %ld\n", __STDC_VERSION__); #endif #ifdef __STDC_IEC_559__ printf("__STDC_IEC_559__ %d\n", __STDC_IEC_559__); #endif feclearexcept( FE_ALL_EXCEPT ); ( void )( d1 * d2 ); if ( fetestexcept( FE_OVERFLOW ) ) { return 0; } printf("error: no FE_OVERFLOW is raised\n"); return 1; } Invocation: gcc t0.c -lm && ./a.out Expected output: __STDC__ 1 __STDC_VERSION__ 201710 __STDC_IEC_559__ 1 Actual output: __STDC__ 1 __STDC_VERSION__ 201710 __STDC_IEC_559__ 1 error: no FE_OVERFLOW is raised Note 1: Under -Wall gcc generates warning: <source>:5: warning: ignoring '#pragma STDC FENV_ACCESS' [-Wunknown-pragmas] 5 | #pragma STDC FENV_ACCESS ON Note 2: Comparison with clang: https://godbolt.org/z/6GnqTYvaa. Brief difference: gcc: call feclearexcept movsd xmm0, QWORD PTR [rbp-8] movsd xmm0, QWORD PTR [rbp-16] mov edi, 8 call fetestexcept clang: call feclearexcept movsd xmm0, qword ptr [rbp - 16] # xmm0 = mem[0],zero movsd xmm1, qword ptr [rbp - 24] # xmm1 = mem[0],zero mulsd xmm0, xmm1 mov edi, 8 call fetestexcept Here we see that gcc does not generate "mulsd", which is a bug, because execution of "mulsd" leads to side effects.