Teach memdep to phi translate bitcasts. This allows us to compile

the example in GCC PR16799 to:

LBB1_2:                                                     ## %bb1
	movl	%eax, %eax
	subq	%rax, %rdi
	movq	%rdi, (%rcx)
	movl	(%rdi), %eax
	testl	%eax, %eax
	je	LBB1_2

instead of:

LBB1_2:                                                     ## %bb1
	movl	(%rdi), %ecx
	subq	%rcx, %rdi
	movq	%rdi, (%rax)
	cmpl	$0, (%rdi)
	je	LBB1_2



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89978 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 27 insertions, 5 deletions

diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index c4647b1f50..780d73ec23 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -690,10 +690,15 @@ static bool isPHITranslatable(Instruction *Inst) {
   if (isa<PHINode>(Inst))
     return true;
   
-  // TODO: BITCAST, GEP.
-
-  // ...
+  // We can handle bitcast of a PHI, but the PHI needs to be in the same block
+  // as the bitcast.
+  if (BitCastInst *BC = dyn_cast<BitCastInst>(Inst))
+    if (PHINode *PN = dyn_cast<PHINode>(BC->getOperand(0)))
+      if (PN->getParent() == BC->getParent())
+        return true;
   
+  // TODO: GEP, ...
+
   //   cerr << "MEMDEP: Could not PHI translate: " << *Pointer;
   //   if (isa<BitCastInst>(PtrInst) || isa<GetElementPtrInst>(PtrInst))
   //     cerr << "OP:\t\t\t\t" << *PtrInst->getOperand(0);
@@ -708,6 +713,25 @@ static Value *PHITranslateForPred(Instruction *Inst, BasicBlock *Pred) {
   if (PHINode *PN = dyn_cast<PHINode>(Inst))
     return PN->getIncomingValueForBlock(Pred);
   
+  if (BitCastInst *BC = dyn_cast<BitCastInst>(Inst)) {
+    PHINode *BCPN = cast<PHINode>(BC->getOperand(0));
+    Value *PHIIn = BCPN->getIncomingValueForBlock(Pred);
+    
+    // Constants are trivial to phi translate.
+    if (Constant *C = dyn_cast<Constant>(PHIIn))
+      return ConstantExpr::getBitCast(C, BC->getType());
+    
+    // Otherwise we have to see if a bitcasted version of the incoming pointer
+    // is available.  If so, we can use it, otherwise we have to fail.
+    for (Value::use_iterator UI = PHIIn->use_begin(), E = PHIIn->use_end();
+         UI != E; ++UI) {
+      if (BitCastInst *BCI = dyn_cast<BitCastInst>(*UI))
+        if (BCI->getType() == BC->getType())
+          return BCI;
+    }
+    return 0;
+  }
+
   return 0;
 }
 
diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 2db580824c..6bac630f50 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1284,8 +1284,6 @@ http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35287 [LPRE crit edge splitting]
 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34677 (licm does this, LPRE crit edge)
   llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | opt -mem2reg -simplifycfg -gvn | llvm-dis
 
-http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16799 [BITCAST PHI TRANS]
-
 //===---------------------------------------------------------------------===//
 
 Type based alias analysis: