https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80450
Bug ID: 80450
Summary: -std=c99 breaks -frounding-math on i686
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: nsz at gcc dot gnu.org
Target Milestone: ---
on i686 the following code is miscompiled with -std=c99:
void f() { volatile double x = 0x1p-1000*0x1p-1000; }
with -S -O2 -frounding-math:
f:
subl $20, %esp
fldl .LC0 ////// 0x1p-1000
fmul %st(0), %st
fstpl 8(%esp)
addl $20, %esp
ret
with -S -O2 -frounding-math -std=c99 a.c:
f:
subl $20, %esp
fldz ////// unconditional 0.0
fstpl 8(%esp)
addl $20, %esp
ret
note that there is double rounding (first to 80bit floats then to 64bit),
but with upward rounding the result is non-zero either way so the -std=c99
code is wrong (in c99 only static initializers are evaluated in default fenv).
