https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77664
Bug ID: 77664
Summary: Missed optimization: signed int >= 0 && < unsigned
short
Product: gcc
Version: 6.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: achurch+gcc at achurch dot org
Target Milestone: ---
Given the following code:
extern void foo(void);
void bar(int a, unsigned short b)
{
if (a >= 0 && a < b)
foo();
}
the "a >= 0 && a < b" test could be implemented as a single unsigned
comparison, e.g. on x86-64:
bar:
cmp %esi,%edi
jb foo
ret
GCC, however, misses this optimization (even at -O3) and generates two separate
comparisons:
bar:
test %edi,%edi
js 0f
movzwl %si,%esi
cmp %esi,%edi
jl 1f
0: repz ret
nopl (%rax)
1: jmp foo
(The movzwl is also unnecessary per my reading of the x86-64 ABI, but that's a
different issue.)
FWIW, Clang 3.8.1 successfully optimizes this at -O and higher.
(Component is a guess; please move if appropriate.)
