https://bugs.llvm.org/show_bug.cgi?id=42257
Bug ID: 42257
Summary: Missed optimization when a constant is a power of two
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: jameshamm1...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
The following function is correctly treated as a no-op by clang(trunk, -Ofast
--march=skylake)
bool zero(bool b) {
int x = 5;
return (x * b) + (-x * b);
}
=>
zero(bool):
xor eax, eax
ret
Messing around with this function on compiler explorer
(https://godbolt.org/z/grc5ZF), I noticed when the constant is set to a power
of two (where the power is not 0 or 1) or the negative of a power of two, the
code is no longer optimized away.
bool zero(bool b) {
int x = 4;
return (x * b) + (-x * b);
}
=>
zero(bool):
shl edi, 2
cmp edi, edi ;; Always sets the ZF
setne al ;; ZF is set, so al is always 0
ret
This missed optimization still happens when the return type is an int, and the
missed optimization is not obvious from the assembly
(https://godbolt.org/z/ZEwONp).
// Completely optimized
int zero(bool b) {
int x = 18;
return (x * b) + (-x * b);
}
=>
zero(bool):
xor eax, eax
ret
vs
// Missing opportunity
int zero(bool b) {
int x = 4;
return (x * b) + (-x * b);
}
=>
zero(bool):
mov eax, edi
shl eax, 2
xor dil, 1
movzx ecx, dil
lea eax, [rax + 4*rcx]
add eax, -4
ret
The problem disappears if the function parameter is widened,
int zero(int b) {
int x = 4;
return (x * b) + (-x * b);
}
or if the intermediate values are close to overflowing
(https://godbolt.org/z/yHPjz2),
int zero(bool b) {
int x = 1 << 31;
return (x * b) + (-x * b);
}
or (as a comment on stackoverflow pointed out
https://stackoverflow.com/questions/56416747/missed-optimization-in-clang-only-on-powers-of-two#comment99429730_56416747)
if the value is not known.
int zero(bool b, int x) {
return (x * b) + (-x * b);
}
All compile down to
zero(int):
xor eax, eax
ret
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
