[llvm] [VPlan] Record estimated branch probabilities on VPlan0 for cost modeling (PR #216172)

[Mircea Trofin via llvm-commits]

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@@ -2144,8 +2145,12 @@ void VPIRPhi::printRecipe(raw_ostream &O, const Twine &Indent,
 #endif
 
 void VPIRMetadata::applyMetadata(Instruction &I) const {
-  for (const auto &[Kind, Node] : Metadata)
+  for (const auto &[Kind, Node] : Metadata) {
+    // Estimated weights must not be emitted as profile data, see setProfile.
+    if (Kind == LLVMContext::MD_prof && EstimatedProfile)
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mtrofin wrote:

IIUC if the original IR has `!prof` throughout, none of these will have `EstimatedProfile`, correct? I.e. we shouldn't try to set it to "unknown" here (`llvm::setExplicitlyUnknownBranchWeightsIfProfiled`), as it's not like we had information to compute it from.

Could you add a comment about that - so that if later profcheck fails and someone investigates and sees this, they don't get tempted to "fix" it by slapping "unknown" here, and rather figures out what dropped the profile in the first place.

https://github.com/llvm/llvm-project/pull/216172