The following shows a missing easy optimization for GCC:
int
in_canforward(unsigned int in)
{
if ((in & ~0xffffff0f) == 0xf0 || (in & ~0xffffff0f) == 0xe0)
return 0;
return 1;
}
results in (@ -O2):
in_canforward:
andl $240, %edi
cmpl $240, %edi
sete %al
cmpl $224, %edi
sete %dl
orl %edx, %eax
xorl $1, %eax
movzbl %al, %eax
ret
given that 0xf0 and 0xe0 only differ by one bit, there is no reason to test for
that bit so the comparision could be: (in & 0xffffff1f) == 0xe0. More
generally
the optimization is:
given (x & m) == a0 || (x & m) == a1
where m, a0, and a1 are all constant
let b = (a0 ^ a1)
then if (b & (b - 1)) == 0 [b is a power of 2]
rewrite to: (x & (m|b)) == (a0 & ~b)
--
Summary: Missing simple optimization
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: matt at 3am-software dot com
GCC build triplet: x86_64--netbsd
GCC host triplet: x86_64--netbsd
GCC target triplet: x86_64--netbsd
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31271
