Replace constant conditional branches by unconditional branches

src/IceCfg.cpp

@@ -283,7 +283,7 @@ void Cfg::computeInOutEdges() {
 
   TimerMarker T(TimerStack::TT_phiValidation, this);
   for (CfgNode *Node : Nodes)
-    Node->validatePhis();
+    Node->enforcePhiConsistency();
 }
 
 void Cfg::renumberInstructions() {

src/IceCfgNode.cpp

@@ -84,11 +84,15 @@ void CfgNode::computeSuccessors() {
   OutEdges = Insts.rbegin()->getTerminatorEdges();
 }
 
-// Validate each Phi instruction in the node with respect to control flow. For
-// every phi argument, its label must appear in the predecessor list. For each
-// predecessor, there must be a phi argument with that label. We don't check
-// that phi arguments with the same label have the same value.
-void CfgNode::validatePhis() {
+// Ensure each Phi instruction in the node is consistent with respect to control
+// flow.  For each predecessor, there must be a phi argument with that label.
+// If a phi argument's label doesn't appear in the predecessor list (which can
+// happen as a result of e.g. unreachable node elimination), its value is
+// modified to be zero, to maintain consistency in liveness analysis.  This
+// allows us to remove some dead control flow without a major rework of the phi
+// instructions.  We don't check that phi arguments with the same label have the
+// same value.
+void CfgNode::enforcePhiConsistency() {
   for (Inst &Instr : Phis) {
     auto *Phi = llvm::cast<InstPhi>(&Instr);
     // We do a simple O(N^2) algorithm to check for consistency. Even so, it
@@ -106,8 +110,11 @@ void CfgNode::validatePhis() {
           break;
         }
       }
-      if (!Found)
-        llvm::report_fatal_error("Phi error: label for bad incoming edge");
+      if (!Found) {
+        // Predecessor was unreachable, so if (impossibly) the control flow
+        // enters from that predecessor, the value should be zero.
+        Phi->clearOperandForTarget(Label);
+      }
     }
     for (CfgNode *InNode : getInEdges()) {
       bool Found = false;

src/IceCfgNode.h

@@ -90,7 +90,7 @@ public:
   CfgNode *splitIncomingEdge(CfgNode *Pred, SizeT InEdgeIndex);
   /// @}
 
-  void validatePhis();
+  void enforcePhiConsistency();
   void placePhiLoads();
   void placePhiStores();
   void deletePhis();

src/IceInst.cpp

@@ -304,7 +304,13 @@ InstBr::InstBr(Cfg *Func, Operand *Source, CfgNode *TargetTrue_,
                CfgNode *TargetFalse_)
     : InstHighLevel(Func, Inst::Br, 1, nullptr), TargetFalse(TargetFalse_),
       TargetTrue(TargetTrue_) {
-  if (TargetTrue == TargetFalse) {
+  if (auto *Constant = llvm::dyn_cast<ConstantInteger32>(Source)) {
+    int32_t C32 = Constant->getValue();
+    if (C32 != 0) {
+      TargetFalse = TargetTrue;
+    }
+    TargetTrue = nullptr; // turn into unconditional version
+  } else if (TargetTrue == TargetFalse) {
     TargetTrue = nullptr; // turn into unconditional version
   } else {
     addSource(Source);
@@ -392,8 +398,7 @@ void InstPhi::addArgument(Operand *Source, CfgNode *Label) {
 }
 
 // Find the source operand corresponding to the incoming edge for the given
-// node. TODO: This uses a linear-time search, which could be improved if it
-// becomes a problem.
+// node.
 Operand *InstPhi::getOperandForTarget(CfgNode *Target) const {
   for (SizeT I = 0; I < getSrcSize(); ++I) {
     if (Labels[I] == Target)
@@ -403,6 +408,19 @@ Operand *InstPhi::getOperandForTarget(CfgNode *Target) const {
   return nullptr;
 }
 
+// Replace the source operand corresponding to the incoming edge for the given
+// node by a zero of the appropriate type.
+void InstPhi::clearOperandForTarget(CfgNode *Target) {
+  for (SizeT I = 0; I < getSrcSize(); ++I) {
+    if (getLabel(I) == Target) {
+      Type Ty = Dest->getType();
+      Srcs[I] = Target->getCfg()->getContext()->getConstantZero(Ty);
+      return;
+    }
+  }
+  llvm_unreachable("Phi target not found");
+}
+
 // Updates liveness for a particular operand based on the given predecessor
 // edge. Doesn't mark the operand as live if the Phi instruction is dead or
 // deleted.

src/IceInst.h

@@ -622,6 +622,7 @@ public:
   }
   void addArgument(Operand *Source, CfgNode *Label);
   Operand *getOperandForTarget(CfgNode *Target) const;
+  void clearOperandForTarget(CfgNode *Target);
   CfgNode *getLabel(SizeT Index) const { return Labels[Index]; }
   void livenessPhiOperand(LivenessBV &Live, CfgNode *Target,
                           Liveness *Liveness);

tests_lit/reader_tests/forwardref.ll

@@ -72,18 +72,9 @@ b6:
 ; CHECK-NEXT: b1:
 ; CHECK-NEXT:   %v0 = add i32 %p0, %v3
 ; CHECK-NEXT:   br label %b6
-; CHECK-NEXT: b2:
-; CHECK-NEXT:   %v1 = add i32 %p0, %v4
-; CHECK-NEXT:   br label %b6
-; CHECK-NEXT: b3:
-; CHECK-NEXT:   %v2 = add i32 %p0, %v3
-; CHECK-NEXT:   br label %b6
 ; CHECK-NEXT: b4:
 ; CHECK-NEXT:   %v3 = add i32 %p0, %p0
-; CHECK-NEXT:   br i1 true, label %b1, label %b5
-; CHECK-NEXT: b5:
-; CHECK-NEXT:   %v4 = add i32 %v3, %p0
-; CHECK-NEXT:   br i1 true, label %b2, label %b3
+; CHECK-NEXT:   br label %b1
 ; CHECK-NEXT: b6:
 ; CHECK-NEXT:   ret void
 ; CHECK-NEXT: }