[llvm] [SimplifyCFG] Set branch weights when merging conditional store to address (PR #154841)

[Joel E. Denny via llvm-commits]

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@@ -190,5 +191,33 @@ LLVM_ABI bool hasExplicitlyUnknownBranchWeights(const Instruction &I);
 /// Scaling the profile data attached to 'I' using the ratio of S/T.
 LLVM_ABI void scaleProfData(Instruction &I, uint64_t S, uint64_t T);
 
+/// Get the branch weights of a branch conditioned on b1 || b2, where b1 and b2
+/// are 2 booleans that are the conditions of 2 branches for which we have the
+/// branch weights B1 and B2, respectively. In both B1 and B2, the first
+/// position (index 0) is for the 'true' branch, and the second position (index
+/// 1) is for the 'false' branch.
+inline SmallVector<uint64_t, 2>
+getDisjunctionWeights(const SmallVector<uint32_t, 2> &B1,
+                      const SmallVector<uint32_t, 2> &B2) {
+  // For the first conditional branch, the probability the "true" case is taken
+  // is p(b1) = B1[0] / (B1[0] + B1[1]). The "false" case's probability is
+  // p(not b1) = B1[1] / (B1[0] + B1[1]).
+  // Similarly for the second conditional branch and B2.
+  //
+  // The probability of the new branch NOT being taken is:
+  // not P = p((not b1) and (not b2)) =
+  //       = B1[1] / (B1[0]+B1[1]) * B2[1] / (B2[0]+B2[1]) =
+  //       = B1[1] * B2[1] / (B1[0] + B1[1]) * (B2[0] + B2[1])
+  // Then the probability of it being taken is: P = 1 - (not P).
+  // The denominator will be the same as above, and the numerator of P will be
+  // (B1[0] + B1[1]) * (B2[0] + B2[1]) - B1[1]*B2[1]
+  // Which then reduces to what's shown below (out of the 4 terms coming out of
+  // the product of sums, the subtracted one cancels out)
----------------
jdenny-ornl wrote:

```suggestion
  // The denominator will be the same as above, and the numerator of P will be:
  // (B1[0] + B1[1]) * (B2[0] + B2[1]) - B1[1]*B2[1]
  // Which then reduces to what's shown below (out of the 4 terms coming out of
  // the product of sums, the subtracted one cancels out).
```

https://github.com/llvm/llvm-project/pull/154841