http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943
Summary: Unnecessary ZERO_EXTEND
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: ada
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: ja...@gcc.gnu.org
Target: x86_64-linux
In http://blog.regehr.org/archives/320 Example 4
unsigned long long v;
unsigned short
foo (signed char x, unsigned short y)
{
v = (unsigned long long) y;
return (unsigned short) ((int) y / 3);
}
we emit a redundant zero-extension:
movzwl %si, %eax
movq %rax, v(%rip)
movzwl %si, %eax
imull $43691, %eax, %eax
shrl $17, %eax
ret
The reason why the second movzwl %si, %eax wasn't CSEd with the first one is
because the first one is (set (reg:DI reg1) (zero_extend:DI (reg:HI reg2))
while the second one is (set (reg:SI reg3) (zero_extend:SI (reg:HI reg2))
Wonder if we can't teach CSE to optimize it (say that reg3 is actually
(subreg:SI (reg:DI reg1) 0), or if e.g. one of the zee/see passes (implicit-zee
e.g.) couldn't handle such cases. Combiner can't do anything here, as there is
no data dependency, so try_combine won't see them together.
