[llvm] [FMV][GlobalOpt] Statically resolve calls to versioned functions. (PR #87939)

[via llvm-commits]

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@@ -2642,6 +2642,145 @@ DeleteDeadIFuncs(Module &M,
   return Changed;
 }
 
+// Follows the use-def chain of \p V backwards until it finds a Function,
+// in which case it collects in \p Versions.
+static void collectVersions(Value *V, SmallVectorImpl<Function *> &Versions) {
+  if (auto *F = dyn_cast<Function>(V)) {
+    Versions.push_back(F);
+  } else if (auto *Sel = dyn_cast<SelectInst>(V)) {
+    collectVersions(Sel->getTrueValue(), Versions);
+    collectVersions(Sel->getFalseValue(), Versions);
+  } else if (auto *Phi = dyn_cast<PHINode>(V)) {
+    for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I)
+      collectVersions(Phi->getIncomingValue(I), Versions);
+  }
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goldsteinn wrote:

Err no I mean if there is a return value but you can't collect the assosiated function, couldn't we optimize to a static call when a better match existed? I.e im proposing adding:
```
  ...
  } else if (auto *Phi = dyn_cast<PHINode>(V)) {
    for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I)
      if(!collectVersions(Phi->getIncomingValue(I), Versions)) return false;
    return true;
  }
  else
    return false;
```

then `if(!collectVersions(...)) { // skip transform entirely }`

https://github.com/llvm/llvm-project/pull/87939