https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66872
Bug ID: 66872
Summary: fold a & ((1 << b) - 1) to a & ~(-1 << b)
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: bonzini at gnu dot org
Target Milestone: ---
This can save one or two instructions on some architectures. For example, when
compiling
int f(int x, int t)
{
return x & ((1 << t) - 1);
}
vs.
int f(int x, int t)
{
return x & ~(-1 << t);
}
with -march=haswell you get before:
movl $1, %edx
shlx %esi, %edx, %esi
subl $1, %esi # not sure why no lea here?
movl %esi, %eax
andl %edi, %eax
ret
and after:
movl $-1, %edx
shlx %esi, %edx, %esi
andn %edi, %esi, %eax
ret
Even if you account for the strange register allocation in the first assembly
listing, using andn can save one instruction. Also, with -mtune=generic the
size is the same. Before:
movl %esi, %ecx
movl $1, %edx
sall %cl, %edx
subl $1, %edx
movl %edx, %eax
andl %edi, %eax
ret
After:
movl %esi, %ecx
movl $-1, %edx
sall %cl, %edx
notl %edx
movl %edx, %eax
andl %edi, %eax
ret
