Hello!
I would just like to ask if -mno-fp-ret-in-387 (and now also -mno-80387)
violates ABI in some way (just like -mregparm=X). Consider this code:
#include <math.h>
double test(double x) {
double b;
b = sin(x);
return b + 1.0;
}
compiled with gcc -O2 -march=pentium4 -mfpmath=sse -mno-80387:
test:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
movsd 8(%ebp), %xmm0
movsd %xmm0, (%esp)
call sin
movl %eax, -8(%ebp)
movl %edx, -4(%ebp)
movsd -8(%ebp), %xmm0
addsd .LC1, %xmm0
movsd %xmm0, -8(%ebp)
movl -8(%ebp), %eax
movl -4(%ebp), %edx
leave
ret
In this code, it is assumed that sin() function returns its result in
%eax/%edx pair. However, a "normal" libm sin() function returns its
value in top FP register, so there we have an ABI mismatch. When normal
libm is linked with this code, wrong results will be produced.
In -mregparm case, there is a big warning in the documentation:
*Warning:* if you use this switch, and NUM is nonzero, then you
must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
I think that something similar should be added to -mno-fp-ret-in-387 /
-m-no-80387.