remove cyclic undef PHIs (#1245)

Ref #1209 #1208
google · Oct 4, 2023 · e3f4744 · e3f4744
1 parent 60c4f7b
commit e3f4744
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diff --git a/lib/FixupStructuredCFGPass.cpp b/lib/FixupStructuredCFGPass.cpp
@@ -14,6 +14,7 @@
 
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/Constants.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -28,10 +29,52 @@ clspv::FixupStructuredCFGPass::run(Function &F, FunctionAnalysisManager &FAM) {
   // Run after isolateContinue since this can invalidate loop info.
   breakConditionalHeader(F, FAM);
 
+  removeUndefPHI(F);
+
   PreservedAnalyses PA;
   return PA;
 }
 
+void clspv::FixupStructuredCFGPass::removeUndefPHI(Function &F) {
+  SmallVector<PHINode *> ToBeDeleted;
+  DenseMap<PHINode *, SmallVector<PHINode *>> dict;
+  for (auto &BB : F) {
+    for (auto &I : BB) {
+      if (auto phi = dyn_cast<PHINode>(&I)) {
+        if (!phi->getType()->isPointerTy()) {
+          continue;
+        }
+        bool phiIsUndef = true;
+        for (unsigned i = 0; i < phi->getNumIncomingValues(); i++) {
+          auto Val = phi->getIncomingValue(i);
+          if (auto phi2 = dyn_cast<PHINode>(Val)) {
+            for (unsigned j = 0; j < phi2->getNumIncomingValues(); j++) {
+              auto Val2 = phi2->getIncomingValue(i);
+              if (auto phi3 = dyn_cast<PHINode>(Val2)) {
+                if (phi3 != phi) {
+                  phiIsUndef = false;
+                }
+              } else if (!isa<UndefValue>(Val2)) {
+                phiIsUndef = false;
+              }
+            }
+          } else if (!isa<UndefValue>(Val)) {
+            phiIsUndef = false;
+          }
+        }
+        if (!phiIsUndef) {
+          continue;
+        }
+        phi->replaceAllUsesWith(UndefValue::get(phi->getType()));
+        ToBeDeleted.push_back(phi);
+      }
+    }
+  }
+  for (auto phi : ToBeDeleted) {
+    phi->eraseFromParent();
+  }
+}
+
 void clspv::FixupStructuredCFGPass::breakConditionalHeader(
     Function &F, FunctionAnalysisManager &FAM) {
   auto &LI = FAM.getResult<LoopAnalysis>(F);

diff --git a/lib/FixupStructuredCFGPass.h b/lib/FixupStructuredCFGPass.h
@@ -24,6 +24,7 @@ struct FixupStructuredCFGPass : llvm::PassInfoMixin<FixupStructuredCFGPass> {
                               llvm::FunctionAnalysisManager &FAM);
 
 private:
+  void removeUndefPHI(llvm::Function &F);
   void breakConditionalHeader(llvm::Function &F, llvm::FunctionAnalysisManager &FAM);
   void isolateContinue(llvm::Function &F, llvm::FunctionAnalysisManager &FAM);
 

diff --git a/test/FixupStructuredCFG/UndefPHI.ll b/test/FixupStructuredCFG/UndefPHI.ll
@@ -0,0 +1,30 @@
+; RUN: clspv-opt %s -o %t.ll --passes=fixup-structured-cfg
+; RUN: FileCheck %s < %t.ll
+
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024"
+target triple = "spir64-unknown-unknown"
+
+define dso_local spir_kernel void @foo(i1 %cmp1, i1 %cmp2, i1 %cmp3) {
+entry:
+; CHECK: [[alloca:%[^ ]+]] = alloca i32, align 4
+  %alloca = alloca i32, align 4
+  br i1 %cmp1, label %b1, label %b2
+
+; CHECK: b1:
+; CHECK-NEXT: br
+b1:
+  %b1_phi = phi ptr [ undef, %entry ], [ %b2_phi, %b2 ]
+  br i1 %cmp2, label %b2, label %exit
+
+; CHECK: b2:
+; CHECK-NEXT: br
+b2:
+  %b2_phi = phi ptr [ undef, %entry ], [ %b1_phi, %b1 ]
+  br i1 %cmp3, label %b1, label %exit
+
+; CHECK: exit:
+; CHECK-NEXT: phi ptr [ undef, %b1 ], [ [[alloca]], %b2 ]
+exit:
+  %exit_phi = phi ptr [ %b1_phi, %b1 ], [ %alloca, %b2 ]
+  ret void
+}