[CGP] Look through bitcasts when duplicating returns for tail calls

The simple case of:

```
int *callee();
void *caller(void *a) {
  if (a == NULL)
    return callee();
  return a;
}
```

would generate a regular call instead of a tail call because we don't
look through the bitcast of the call to `callee` when duplicating the
return blocks.

Differential Revision: https://reviews.llvm.org/D60837

llvm-svn: 359041

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -2032,7 +2032,9 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB, bool &ModifiedDT
   SmallVector<CallInst*, 4> TailCalls;
   if (PN) {
     for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) {
-      CallInst *CI = dyn_cast<CallInst>(PN->getIncomingValue(I));
+      // Look through bitcasts.
+      Value *IncomingVal = PN->getIncomingValue(I)->stripPointerCasts();
+      CallInst *CI = dyn_cast<CallInst>(IncomingVal);
       // Make sure the phi value is indeed produced by the tail call.
       if (CI && CI->hasOneUse() && CI->getParent() == PN->getIncomingBlock(I) &&
           TLI->mayBeEmittedAsTailCall(CI) &&

llvm/test/CodeGen/X86/tailcall-cgp-dup.ll

@@ -118,21 +118,18 @@ define i8* @f_ret8(i8* %obj) nounwind {
 ; OPT:       if.then:
 ; OPT-NEXT:    [[PTR:%.*]] = tail call i32* @g_ret32()
 ; OPT-NEXT:    [[CASTED:%.*]] = bitcast i32* [[PTR]] to i8*
-; OPT-NEXT:    br label [[RETURN]]
+; OPT-NEXT:    ret i8* [[CASTED]]
 ; OPT:       return:
-; OPT-NEXT:    [[RETVAL:%.*]] = phi i8* [ [[CASTED]], [[IF_THEN]] ], [ [[OBJ]], [[ENTRY:%.*]] ]
-; OPT-NEXT:    ret i8* [[RETVAL]]
+; OPT-NEXT:    ret i8* [[OBJ]]
 ;
 ; CHECK-LABEL: f_ret8:
 ; CHECK:       ## %bb.0: ## %entry
-; CHECK-NEXT:    movq %rdi, %rax
 ; CHECK-NEXT:    testq %rdi, %rdi
-; CHECK-NEXT:    je LBB3_2
-; CHECK-NEXT:  ## %bb.1: ## %if.then
-; CHECK-NEXT:    pushq %rax
-; CHECK-NEXT:    callq _g_ret32
-; CHECK-NEXT:    addq $8, %rsp
-; CHECK-NEXT:  LBB3_2: ## %return
+; CHECK-NEXT:    je LBB3_1
+; CHECK-NEXT:  ## %bb.2: ## %if.then
+; CHECK-NEXT:    jmp _g_ret32 ## TAILCALL
+; CHECK-NEXT:  LBB3_1: ## %return
+; CHECK-NEXT:    movq %rdi, %rax
 ; CHECK-NEXT:    retq
 entry:
   %cmp = icmp eq i8* %obj, null