[llvm] [PGO] Add uniformity profile metadata and gate CHR (PR #221517)

[Matt Arsenault via llvm-commits]

================
@@ -1329,6 +1336,36 @@ static bool hasAtLeastTwoBiasedBranches(CHRScope *Scope) {
 void CHR::filterScopes(SmallVectorImpl<CHRScope *> &Input,
                        SmallVectorImpl<CHRScope *> &Output) {
   for (CHRScope *Scope : Input) {
+    // Branch weights count individual lanes. A divergent branch can therefore
+    // look strongly biased even when every wave executes both paths. Avoid
+    // applying CHR to such a scope because duplicating its control flow may
+    // increase live ranges and register pressure without eliminating control
+    // flow for the wave. Select-only scopes are unaffected because branch
+    // uniformity profile describes only conditional branches.
+    if (HasBlockUniformityProfile) {
+      Instruction *DivergentBranch = nullptr;
+      auto FindDivergentBranch = [&](const DenseSet<Region *> &Regions) {
+        for (Region *R : Regions) {
+          Instruction *Branch = R->getEntry()->getTerminator();
+          if (!Branch->getMetadata(LLVMContext::MD_branch_uniformity_profile)) {
+            DivergentBranch = Branch;
+            return;
+          }
+        }
+      };
+      FindDivergentBranch(Scope->TrueBiasedRegions);
+      if (!DivergentBranch)
+        FindDivergentBranch(Scope->FalseBiasedRegions);
+      if (DivergentBranch) {
+        ORE.emit([&]() {
+          return OptimizationRemarkMissed(DEBUG_TYPE, "DivergentBranch",
+                                          DivergentBranch)
+                 << "Drop scope containing a divergent branch";
----------------
arsenm wrote:

```suggestion
                 << "drop scope containing a divergent branch";
```

https://github.com/llvm/llvm-project/pull/221517