http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51007
--- Comment #3 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org>
2011-11-07 11:39:12 UTC ---
Going further: I tried to compare the trees generated by the simple function
below:
#include <stdint.h>
typedef union
{
__float128 value;
struct
{
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
unsigned negative:1;
unsigned exponent:15;
uint64_t mant_high:48;
uint64_t mant_low:64;
#else
uint64_t mant_low:64;
uint64_t mant_high:48;
unsigned exponent:15;
unsigned negative:1;
#endif
} ieee;
} ieee854_float128;
unsigned foo (__float128 x)
{
ieee854_float128 d;
d.value = x;
return d.ieee.exponent;
}
AFAICT, both the original and the optimized tree for darwin and mingw are
identical. The assembly code generated for a common arch (-march=core2 -O0)
differs:
***** on mingw *****
_foo:
pushl %ebp
movl %esp, %ebp
subl $40, %esp
movdqa 8(%ebp), %xmm0
movdqa %xmm0, -40(%ebp)
movzwl -24(%ebp), %eax
andw $32767, %ax
movzwl %ax, %eax
leave
ret
***** on darwin *****
_foo:
pushq %rbp
movq %rsp, %rbp
movdqa %xmm0, -32(%rbp)
movdqa -32(%rbp), %xmm0
movdqa %xmm0, -16(%rbp)
movzwl -2(%rbp), %eax
andw $32767, %ax
movzwl %ax, %eax
popq %rbp
ret
Now, my assembly skills are pretty close to nil, so I can't figure out if the
differences are meaningful, and what they mean. I only hope that I got it close
enough that someone can, from here, understand where this code generation come
from and how to fix it.
