[PATCH] D31032: [LoadCombine] Avoid analysing dead basic blocks

[Bjorn Pettersson via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rL300034: [LoadCombine] Avoid analysing dead basic blocks (authored by bjope).

Changed prior to commit:
  https://reviews.llvm.org/D31032?vs=93648&id=94941#toc

Repository:
  rL LLVM

https://reviews.llvm.org/D31032

Files:
  llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
  llvm/trunk/test/Transforms/LoadCombine/deadcode.ll


Index: llvm/trunk/test/Transforms/LoadCombine/deadcode.ll
===================================================================
--- llvm/trunk/test/Transforms/LoadCombine/deadcode.ll
+++ llvm/trunk/test/Transforms/LoadCombine/deadcode.ll
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -load-combine -S < %s | FileCheck %s
+
+; It has been detected that dead loops like the one in this test case can be
+; created by -jump-threading (it was detected by a csmith generated program).
+;
+; According to -verify this is valid input (even if it could be discussed if
+; the dead loop really satisfies SSA form).
+;
+; The problem found was that the -load-combine pass ends up in an infinite loop
+; when analysing the 'bb1' basic block.
+define void @test1() {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    ret void
+; CHECK:       bb1:
+; CHECK-NEXT:    [[_TMP4:%.*]] = load i16, i16* [[_TMP10:%.*]], align 1
+; CHECK-NEXT:    [[_TMP10]] = getelementptr i16, i16* [[_TMP10]], i16 1
+; CHECK-NEXT:    br label [[BB1:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    [[_TMP7:%.*]] = load i16, i16* [[_TMP12:%.*]], align 1
+; CHECK-NEXT:    [[_TMP12]] = getelementptr i16, i16* [[_TMP12]], i16 1
+; CHECK-NEXT:    br label [[BB2:%.*]]
+;
+  ret void
+
+bb1:
+  %_tmp4 = load i16, i16* %_tmp10, align 1
+  %_tmp10 = getelementptr i16, i16* %_tmp10, i16 1
+  br label %bb1
+
+; A second basic block. Running the test with -debug-pass=Executions shows
+; that we only run the Dominator Tree Construction one time for each function,
+; also when having multiple basic blocks in the function.
+bb2:
+  %_tmp7 = load i16, i16* %_tmp12, align 1
+  %_tmp12 = getelementptr i16, i16* %_tmp12, i16 1
+  br label %bb2
+
+}
Index: llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/TargetFolder.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
@@ -53,18 +54,20 @@
 class LoadCombine : public BasicBlockPass {
   LLVMContext *C;
   AliasAnalysis *AA;
+  DominatorTree *DT;
 
 public:
   LoadCombine() : BasicBlockPass(ID), C(nullptr), AA(nullptr) {
     initializeLoadCombinePass(*PassRegistry::getPassRegistry());
   }
-  
+
   using llvm::Pass::doInitialization;
   bool doInitialization(Function &) override;
   bool runOnBasicBlock(BasicBlock &BB) override;
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<AAResultsWrapperPass>();
+    AU.addRequired<DominatorTreeWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
   }
 
@@ -234,6 +237,14 @@
     return false;
 
   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+
+  // Skip analysing dead blocks (not forward reachable from function entry).
+  if (!DT->isReachableFromEntry(&BB)) {
+    DEBUG(dbgs() << "LC: skipping unreachable " << BB.getName() <<
+          " in " << BB.getParent()->getName() << "\n");
+    return false;
+  }
 
   IRBuilder<TargetFolder> TheBuilder(
       BB.getContext(), TargetFolder(BB.getModule()->getDataLayout()));


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