[SimplifyCFG] Fix weight calculation for simplifySwitchOfPowersOfTwo (#165956)

Continued from #165804

This maintains the BFI of the default branch. Originally `10/63`​, post-pass, it ends up being `5/63 + 58/63 * 5/58`​(first term is from `PROF`​, second is the probability of going to the switch lookup times the probability, there, of taking the default branch)

Issue #147390

Author: mtrofin

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -7731,19 +7731,24 @@ static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
       // label. The other is those powers of 2 that don't appear in the case
       // statement. We don't know the distribution of the values coming in, so
       // the safest is to split 50-50 the original probability to `default`.
-      uint64_t OrigDenominator = sum_of(map_range(
-          Weights, [](const auto &V) { return static_cast<uint64_t>(V); }));
+      uint64_t OrigDenominator =
+          sum_of(map_range(Weights, StaticCastTo<uint64_t>));
       SmallVector<uint64_t> NewWeights(2);
       NewWeights[1] = Weights[0] / 2;
       NewWeights[0] = OrigDenominator - NewWeights[1];
       setFittedBranchWeights(*BI, NewWeights, /*IsExpected=*/false);
-
-      // For the original switch, we reduce the weight of the default by the
-      // amount by which the previous branch contributes to getting to default,
-      // and then make sure the remaining weights have the same relative ratio
-      // wrt eachother.
+      // The probability of executing the default block stays constant. It was
+      //  p_d = Weights[0] / OrigDenominator
+      //  we rewrite as W/D
+      // We want to find the probability of the default branch of the switch
+      // statement. Let's call it X. We have W/D = W/2D + X * (1-W/2D)
+      // i.e. the original probability is the probability we go to the default
+      // branch from the BI branch, or we take the default branch on the SI.
+      // Meaning X = W / (2D - W), or (W/2) / (D - W/2)
+      // This matches using W/2 for the default branch probability numerator and
+      // D-W/2 as the denominator.
+      Weights[0] = NewWeights[1];
       uint64_t CasesDenominator = OrigDenominator - Weights[0];
-      Weights[0] /= 2;
       for (auto &W : drop_begin(Weights))
         W = NewWeights[0] * static_cast<double>(W) / CasesDenominator;
 

llvm/test/Transforms/SimplifyCFG/X86/switch-of-powers-of-two.ll

@@ -141,5 +141,5 @@ return:
 ;.
 ; CHECK: [[PROF0]] = !{!"function_entry_count", i32 10}
 ; CHECK: [[PROF1]] = !{!"branch_weights", i32 58, i32 5}
-; CHECK: [[PROF2]] = !{!"branch_weights", i32 56, i32 5}
+; CHECK: [[PROF2]] = !{!"branch_weights", i32 53, i32 5}
 ;.