[PowerPC] Fix fast-isel when compare is split from branch

When the compare feeding a branch was in a different BB from the branch, we'd
try to "regenerate" the compare in the block with the branch, possibly trying
to make use of values not available there. Copy a page from AArch64's play book
here to fix the problem (at least in terms of correctness).

Fixes PR23640.

llvm-svn: 238097

1 files changed, 32 insertions, 19 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCFastISel.cpp b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
index 002616babd4..0b8e23c4ebf 100644
--- a/llvm/lib/Target/PowerPC/PPCFastISel.cpp
+++ b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
@@ -144,6 +144,7 @@ class PPCFastISel final : public FastISel {
   private:
     bool isTypeLegal(Type *Ty, MVT &VT);
     bool isLoadTypeLegal(Type *Ty, MVT &VT);
+    bool isValueAvailable(const Value *V) const;
     bool isVSFRCRegister(unsigned Register) const {
       return MRI.getRegClass(Register)->getID() == PPC::VSFRCRegClassID;
     }
@@ -283,6 +284,17 @@ bool PPCFastISel::isLoadTypeLegal(Type *Ty, MVT &VT) {
   return false;
 }
 
+bool PPCFastISel::isValueAvailable(const Value *V) const {
+  if (!isa<Instruction>(V))
+    return true;
+
+  const auto *I = cast<Instruction>(V);
+  if (FuncInfo.MBBMap[I->getParent()] == FuncInfo.MBB)
+    return true;
+
+  return false;
+}
+
 // Given a value Obj, create an Address object Addr that represents its
 // address.  Return false if we can't handle it.
 bool PPCFastISel::PPCComputeAddress(const Value *Obj, Address &Addr) {
@@ -731,30 +743,31 @@ bool PPCFastISel::SelectBranch(const Instruction *I) {
 
   // For now, just try the simplest case where it's fed by a compare.
   if (const CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition())) {
-    Optional<PPC::Predicate> OptPPCPred = getComparePred(CI->getPredicate());
-    if (!OptPPCPred)
-      return false;
-
-    PPC::Predicate PPCPred = OptPPCPred.getValue();
+    if (isValueAvailable(CI)) {
+      Optional<PPC::Predicate> OptPPCPred = getComparePred(CI->getPredicate());
+      if (!OptPPCPred)
+        return false;
 
-    // Take advantage of fall-through opportunities.
-    if (FuncInfo.MBB->isLayoutSuccessor(TBB)) {
-      std::swap(TBB, FBB);
-      PPCPred = PPC::InvertPredicate(PPCPred);
-    }
+      PPC::Predicate PPCPred = OptPPCPred.getValue();
 
-    unsigned CondReg = createResultReg(&PPC::CRRCRegClass);
+      // Take advantage of fall-through opportunities.
+      if (FuncInfo.MBB->isLayoutSuccessor(TBB)) {
+        std::swap(TBB, FBB);
+        PPCPred = PPC::InvertPredicate(PPCPred);
+      }
 
-    if (!PPCEmitCmp(CI->getOperand(0), CI->getOperand(1), CI->isUnsigned(),
-                    CondReg))
-      return false;
+      unsigned CondReg = createResultReg(&PPC::CRRCRegClass);
 
-    BuildMI(*BrBB, FuncInfo.InsertPt, DbgLoc, TII.get(PPC::BCC))
-      .addImm(PPCPred).addReg(CondReg).addMBB(TBB);
-    fastEmitBranch(FBB, DbgLoc);
-    FuncInfo.MBB->addSuccessor(TBB);
-    return true;
+      if (!PPCEmitCmp(CI->getOperand(0), CI->getOperand(1), CI->isUnsigned(),
+                      CondReg))
+        return false;
 
+      BuildMI(*BrBB, FuncInfo.InsertPt, DbgLoc, TII.get(PPC::BCC))
+        .addImm(PPCPred).addReg(CondReg).addMBB(TBB);
+      fastEmitBranch(FBB, DbgLoc);
+      FuncInfo.MBB->addSuccessor(TBB);
+      return true;
+    }
   } else if (const ConstantInt *CI =
              dyn_cast<ConstantInt>(BI->getCondition())) {
     uint64_t Imm = CI->getZExtValue();