[SimplifyCFG] Don't speculatively execute BB if it's predictably not taken

If the branch isn't `unpredictable`, and it is predicted to *not* branch
to the block we are considering speculatively executing,
then it seems counter-productive to execute the code that is predicted not to be executed.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D106650

Author: LebedevRI

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2372,6 +2372,20 @@ bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
   }
   assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
 
+  // If the branch is non-unpredictable, and is predicted to *not* branch to
+  // the `then` block, then avoid speculating it.
+  if (!BI->getMetadata(LLVMContext::MD_unpredictable)) {
+    uint64_t TWeight, FWeight;
+    if (BI->extractProfMetadata(TWeight, FWeight) && (TWeight + FWeight) != 0) {
+      uint64_t EndWeight = Invert ? TWeight : FWeight;
+      BranchProbability BIEndProb =
+          BranchProbability::getBranchProbability(EndWeight, TWeight + FWeight);
+      BranchProbability Likely = TTI.getPredictableBranchThreshold();
+      if (BIEndProb >= Likely)
+        return false;
+    }
+  }
+
   // Keep a count of how many times instructions are used within ThenBB when
   // they are candidates for sinking into ThenBB. Specifically:
   // - They are defined in BB, and

llvm/test/Transforms/SimplifyCFG/speculatively-execute-block-profmd.ll

@@ -83,11 +83,12 @@ define i32 @predictably_nontaken(i1 %c, i32 %a, i32 %b) {
 ; CHECK:       dispatch:
 ; CHECK-NEXT:    call void @sideeffect1()
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[END]], label [[COND_TRUE:%.*]], !prof [[PROF0]]
+; CHECK:       cond.true:
 ; CHECK-NEXT:    [[VAL:%.*]] = add i32 [[A]], [[B]]
-; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[CMP]], i32 0, i32 [[VAL]], !prof [[PROF0]]
 ; CHECK-NEXT:    br label [[END]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[RES:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ [[SPEC_SELECT]], [[DISPATCH]] ]
+; CHECK-NEXT:    [[RES:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ 0, [[DISPATCH]] ], [ [[VAL]], [[COND_TRUE]] ]
 ; CHECK-NEXT:    call void @sideeffect2()
 ; CHECK-NEXT:    ret i32 [[RES]]
 ;