[llvm] [VPlan] Preserve branch weights from VPlan0 through to codegen. (PR #213143)

[Mircea Trofin via llvm-commits]

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@@ -112,6 +114,23 @@ class VPPredicator {
 };
 } // namespace
 
+/// Returns the branch weights describing how often a block executing with
+/// probability \p Prob is entered, or nullptr if \p Prob carries no useful
+/// information, i.e. the block is never or always executed, or its probability
+/// is unknown.
+static MDNode *getWeightsForProbability(BranchProbability Prob,
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mtrofin wrote:

For readability, would it make sense to use a local metadata value instead of MD_prof? IIUC the idea is to use the existing mechanism in VPlan for carrying metadata, and in this case the metadata would be pairs of i32 codifying the probability of executing a VPlan Block or Instruction. There are some differences from the IR MD_prof, iiuc:

- any VPlan instruction can carry these
- MD_prof encodes 2 complementary probabilities (well, OK, N in the case of `select`s). Here, we really mean to encode one probability, `first_number / (first_number + second_number)`


https://github.com/llvm/llvm-project/pull/213143