https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85501
Bug ID: 85501
Summary: missed if-conversion / phiopt trick
Product: gcc
Version: 8.0.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rguenth at gcc dot gnu.org
Blocks: 53947, 85466
Target Milestone: ---
PR85466 shows
For the "if" case, llvm turns:
if (myVector[n] > 0.5){
result[n] = 0.8f;
}
else {
result[n] = 0.1f;
}
into
const float tab[2] = { .8f, .1f };
result[n] = tab[item > .5f];
which is a neat trick. Profitability is not without questions though
(if you'd do this to scalar code).
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85466
[Bug 85466] Performance is slow when doing 'branchless' conditional style math
operations
