from
http://embed.cs.utah.edu/embarrassing/dec_09/harvest/gcc-head_llvm-gcc-head/
union __anonunion___u_19
{
double __d;
int __i[2];
};
extern __attribute__ ((__nothrow__))
int __signbit (double __x) __attribute__ ((__const__));
extern __attribute__ ((__nothrow__))
int __signbit (double __x) __attribute__ ((__const__));
extern int __signbit (double __x)
{
union __anonunion___u_19 __u;
{
__u.__d = __x;
return (__u.__i[1] < 0);
}
}
/* Checksum = AEFB9790 */
generates with -O2 -m32 -fomit-frame-pointer
subl $12, %esp
fldl 16(%esp)
fstpl (%esp)
movl 4(%esp), %eax
addl $12, %esp
shrl $31, %eax
ret
the move through the x87 stack and the local frame is totally unnecessary;
the shr could be just done on the input stack value
in comparison llvm generates the much neater:
0: 0f b7 44 24 0c movzwl 0xc(%esp),%eax
5: c1 e8 0f shr $0xf,%eax
8: c3 ret
--
Summary: unnecessary move through x87 stack/local frame for union
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: andi-gcc at firstfloor dot org
GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux -m32
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42588
