propagate 'unpredictable' metadata on select instructions

[spatel]

Summary

This is similar to D18133 where we allowed profile weights on select instructions. This extends that change to also allow the 'unpredictable' attribute of branches to apply to selects.

A test to check that 'unpredictable' metadata is preserved when cloning instructions was checked in at:
http://reviews.llvm.org/rL263648

[davidxl]

lgtm

include/llvm/IR/IRBuilder.h

public:
   }
 
   CallInst *CreateCall(Function *Callee, ArrayRef<Value *> Args,
                        const Twine &Name = "", MDNode *FPMathTag = nullptr) {
     return CreateCall(Callee->getFunctionType(), Callee, Args, Name, FPMathTag);
   }
 
   Value *CreateSelect(Value *C, Value *True, Value *False,
-                      const Twine &Name = "", MDNode *ProfWeights = nullptr) {
+                      const Twine &Name = "", Instruction *MDFrom = nullptr) {
     if (Constant *CC = dyn_cast<Constant>(C))
       if (Constant *TC = dyn_cast<Constant>(True))
         if (Constant *FC = dyn_cast<Constant>(False))
           return Insert(Folder.CreateSelect(CC, TC, FC), Name);
 
     SelectInst *Sel = SelectInst::Create(C, True, False);
-    // TODO: "unpredictable" metadata can apply to a select too.
-    Sel->setMetadata(LLVMContext::MD_prof, ProfWeights);
+    if (MDFrom) {
+      MDNode *Prof = MDFrom->getMetadata(LLVMContext::MD_prof);
+      MDNode *Unpred = MDFrom->getMetadata(LLVMContext::MD_unpredictable);
+      Sel = addBranchMetadata(Sel, Prof, Unpred);
+    }
     return Insert(Sel, Name);
   }
 
   VAArgInst *CreateVAArg(Value *List, Type *Ty, const Twine &Name = "") {
     return Insert(new VAArgInst(List, Ty), Name);
   }
 
   Value *CreateExtractElement(Value *Vec, Value *Idx,

lib/Transforms/Utils/SimplifyCFG.cpp

DomBlock->getInstList().splice(InsertPt->getIterator(),
                                    IfBlock2->getInstList(), IfBlock2->begin(),
                                    IfBlock2->getTerminator()->getIterator());
 
   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     // Change the PHI node into a select instruction.
     Value *TrueVal  = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
     Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
 
-    MDNode *MDN = InsertPt->getMetadata(LLVMContext::MD_prof);
-    Value *Select = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", MDN);
-    PN->replaceAllUsesWith(Select);
-    Select->takeName(PN);
+    Value *Sel = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", InsertPt);
+    PN->replaceAllUsesWith(Sel);
+    Sel->takeName(PN);
     PN->eraseFromParent();
   }
 
   // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
   // has been flattened.  Change DomBlock to jump directly to our new block to
   // avoid other simplifycfg's kicking in on the diamond.
   TerminatorInst *OldTI = DomBlock->getTerminator();
   Builder.SetInsertPoint(OldTI);

test/Transforms/SimplifyCFG/PhiEliminate2.ll

 ; RUN: opt < %s -simplifycfg -S | FileCheck %s
 
 ; Use a select to make this a single BB.
 ; Also, make sure the profile metadata is propagated to the select (PR26636).
 
 define i32 @FoldTwoEntryPHINode(i1 %C, i32 %V1, i32 %V2, i16 %V3) {
 entry:
-        br i1 %C, label %then, label %else, !prof !0
+        br i1 %C, label %then, label %else, !prof !0, !unpredictable !1
 then:
         %V4 = or i32 %V2, %V1
         br label %Cont
 else:
         %V5 = sext i16 %V3 to i32
         br label %Cont
 Cont:
         %V6 = phi i32 [ %V5, %else ], [ %V4, %then ]
         call i32 @FoldTwoEntryPHINode( i1 false, i32 0, i32 0, i16 0 )
         ret i32 %V1
 
 ; CHECK-LABEL: @FoldTwoEntryPHINode(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:  %V5 = sext i16 %V3 to i32
 ; CHECK-NEXT:  %V4 = or i32 %V2, %V1
-; CHECK-NEXT:  %V6 = select i1 %C, i32 %V4, i32 %V5, !prof !0
+; CHECK-NEXT:  %V6 = select i1 %C, i32 %V4, i32 %V5, !prof !0, !unpredictable !1
 ; CHECK-NEXT:  %0 = call i32 @FoldTwoEntryPHINode(i1 false, i32 0, i32 0, i16 0)
 ; CHECK-NEXT:  ret i32 %V1
 }
 
 !0 = !{!"branch_weights", i32 3, i32 5}
+!1 = !{}
 
 ; CHECK: !0 = !{!"branch_weights", i32 3, i32 5}
+; CHECK: !1 = !{}