https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68282
Bug ID: 68282
Summary: Optimization fails to remove unnecessary sign
extension instruction
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: stanshebs at earthlink dot net
Target Milestone: ---
For the code
int table[256];
int func(unsigned char c)
{
return table[(c >> 2) + 1];
}
x86-64 gcc -m64 -O2 generates
movzbl %dil, %eax
sarl $2, %eax
addl $1, %eax
cltq
movl table(,%rax,4), %eax
ret
where the cltq is not really needed.
Current Clang produces
shrl $2, %edi
incl %edi
movl table(,%rdi,4), %eax
retq
which is more like what we would want.
