BranchPredict: a smattering of guesses

Author: kavon

lib/IRGen/GenIntegerLiteral.cpp

@@ -262,7 +262,13 @@ void irgen::emitIntegerLiteralCheckedTrunc(IRGenFunction &IGF, Explosion &in,
                                         IGF.IGM.getSize(Size(resultWidth + 1)));
       hasOverflow = IGF.Builder.CreateOr(isNegative, tooBig);
     }
-    IGF.Builder.CreateCondBr(hasOverflow, invalidBB, validBB);
+
+    // Predict no overflow.
+    auto *weight =
+        IGF.getSILModule().getOptions().EnableStaticBranchPrediction ?
+        IGF.IGM.createProfileWeightsExpecting(false) : nullptr;
+
+    IGF.Builder.CreateCondBr(hasOverflow, invalidBB, validBB, weight);
   }
 
   // In the invalid block, we just need to construct the result.  This block

lib/IRGen/GenThunk.cpp

@@ -357,7 +357,13 @@ void IRGenThunk::emit() {
     llvm::Value *nil = llvm::ConstantPointerNull::get(
         cast<llvm::PointerType>(errorValue->getType()));
     auto *hasError = IGF.Builder.CreateICmpNE(errorValue, nil);
-    IGF.Builder.CreateCondBr(hasError, errorBB, successBB);
+
+    // Predict no error is thrown.
+    auto *weight =
+        IGF.IGM.getSILModule().getOptions().EnableStaticBranchPrediction ?
+        IGF.IGM.createProfileWeightsExpecting(false) : nullptr;
+
+    IGF.Builder.CreateCondBr(hasError, errorBB, successBB, weight);
 
     IGF.Builder.emitBlock(errorBB);
     if (isAsync) {

lib/IRGen/IRGenFunction.cpp

@@ -718,7 +718,13 @@ void IRGenFunction::emitAwaitAsyncContinuation(
     auto firstAtAwait = Builder.CreateExtractValue(results, 1);
     auto contBB = createBasicBlock("await.async.resume");
     auto abortBB = createBasicBlock("await.async.abort");
-    Builder.CreateCondBr(firstAtAwait, abortBB, contBB);
+
+    // Predict no abort.
+    auto *weight =
+        getSILModule().getOptions().EnableStaticBranchPrediction ?
+        IGM.createProfileWeightsExpecting(false) : nullptr;
+
+    Builder.CreateCondBr(firstAtAwait, abortBB, contBB, weight);
     Builder.emitBlock(abortBB);
     {
       // We were the first to the sync point. "Abort" (return from the
@@ -777,7 +783,13 @@ void IRGenFunction::emitAwaitAsyncContinuation(
     auto nullError = llvm::Constant::getNullValue(errorRes->getType());
     auto hasError = Builder.CreateICmpNE(errorRes, nullError);
     optionalErrorResult->addIncoming(errorRes, Builder.GetInsertBlock());
-    Builder.CreateCondBr(hasError, optionalErrorBB, normalContBB);
+
+    // Predict no error.
+    auto *weight =
+        getSILModule().getOptions().EnableStaticBranchPrediction ?
+        IGM.createProfileWeightsExpecting(false) : nullptr;
+
+    Builder.CreateCondBr(hasError, optionalErrorBB, normalContBB, weight);
     Builder.emitBlock(normalContBB);
   }
 

lib/IRGen/IRGenModule.cpp

@@ -2069,6 +2069,13 @@ llvm::MDNode *IRGenModule::createProfileWeights(uint64_t TrueCount,
   return MDHelper.createBranchWeights(ScaledTrueCount, ScaledFalseCount);
 }
 
+llvm::MDNode *IRGenModule::createProfileWeightsExpecting(bool condition) const {
+  if (condition)
+    return createProfileWeights(999, 0);
+  else
+    return createProfileWeights(0, 999);
+}
+
 void IRGenModule::unimplemented(SourceLoc loc, StringRef message) {
   Context.Diags.diagnose(loc, diag::irgen_unimplemented, message);
 }

lib/IRGen/IRGenModule.h

@@ -1981,6 +1981,10 @@ private:                                                                       \
   llvm::MDNode *createProfileWeights(uint64_t TrueCount,
                                      uint64_t FalseCount) const;
 
+  /// Create llvm metadata with synthetic branch weights where the expectation
+  /// is that the branch condition will be \c condition
+  llvm::MDNode *createProfileWeightsExpecting(bool condition) const;
+
 private:
   void emitGlobalDecl(Decl *D);
 };

lib/IRGen/IRGenSIL.cpp

@@ -4636,7 +4636,13 @@ void IRGenSILFunction::visitYieldInst(swift::YieldInst *i) {
   // Branch to the appropriate destination.
   auto unwindBB = getLoweredBB(i->getUnwindBB()).bb;
   auto resumeBB = getLoweredBB(i->getResumeBB()).bb;
-  Builder.CreateCondBr(isUnwind, unwindBB, resumeBB);
+
+  // Predict no unwind happens.
+  auto *weight =
+      IGM.getSILModule().getOptions().EnableStaticBranchPrediction ?
+      IGM.createProfileWeightsExpecting(false) : nullptr;
+
+  Builder.CreateCondBr(isUnwind, unwindBB, resumeBB, weight);
 }
 
 void IRGenSILFunction::visitBeginApplyInst(BeginApplyInst *i) {