https://bugs.llvm.org/show_bug.cgi?id=42700
Bug ID: 42700
Summary: Replace loop-based variable with loop invariant
variable
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: david.bolvan...@gmail.com
CC: llvm-bugs@lists.llvm.org
int f(bool* arr, bool x) {
int ss = 0;
for (int i = 0; i < 16; ++i) {
if (arr[i] == x) {
// [0]
ss += (arr[i]) ? 1 : -1;
}
}
return ss;
}
[0] x is not modified in this branch so try to replace:
ss += (arr[i]) ? 1 : -1;
with:
ss += (x) ? 1 : -1;
int f2(bool* arr, bool x) {
int ss = 0;
for (int i = 0; i < 16; ++i) {
if (arr[i] == x) {
ss += (x) ? 1 : -1;
}
}
return ss;
}
It may help LICM to hoist some code, help Loop
distribution/unswitch/vectorization pass, etc..
For my motivating example, current Clang trunk -O3 just unrolls 'f' (for N=16)
/ vectorizes 'f' (N=64+). Clang vectorizes 'f2' even for N=16+.
Godbolt: https://godbolt.org/z/c7HwFi
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