labrinea wrote:
@erichkeane while I agree that Clang might not be the best place for such an
optimization, I have some concerns about implementing it in LLVM:
* We cannot distinguish a FMV resolver from any other ifunc resolver.
* There is no information at the LLVM IR level about function versions or which
resolver they are associated with.
* We cannot use target-features to determine version priority since this
information is encoded via front-end features in the TargetParser. We can only
rely on the resolver's basic block layout under the assumption that predecessor
basic blocks correspond to versions of higher priority than successor basic
blocks. This is fragile and unreliable:
```
void discoverResolvedIFuncsInPriorityOrder(GlobalIFunc *IFunc) {
DenseMap<GlobalIFunc *, SmallVector<Function *>> ResolvedIFuncs;
std::function<void(Value *)> visitValue = [&](Value *V) {
if (auto *Func = dyn_cast<Function>(V)) {
ResolvedIFuncs[IFunc].push_back(Func);
} else if (auto *Sel = dyn_cast<SelectInst>(V)) {
visitValue(Sel->getTrueValue());
visitValue(Sel->getFalseValue());
} else if (auto *Phi = dyn_cast<PHINode>(V)) {
for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I)
visitValue(Phi->getIncomingValue(I));
}
};
for (BasicBlock &BB : *IFunc->getResolverFunction())
if (auto *Ret = dyn_cast_or_null<ReturnInst>(BB.getTerminator()))
visitValue(Ret->getReturnValue());
// discard default
if (!ResolvedIFuncs[IFunc].empty())
ResolvedIFuncs[IFunc].pop_back();
}
```
https://github.com/llvm/llvm-project/pull/80093
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