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

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope updated this revision to Diff 92617.
bjope added a comment.

Solution was changed into using the DominatorTree analysis to
determine if basic blocks are "dead". This makes the patch
local to LoadCombine.cpp.


https://reviews.llvm.org/D31032

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


Index: test/Transforms/LoadCombine/deadcode.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoadCombine/deadcode.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -load-combine -disable-output
+
+; 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() {
+  ret void
+
+bb1:
+  %_tmp4 = load i16, i16* %_tmp10, align 1
+  %_tmp10 = getelementptr i16, i16* %_tmp10, i16 1
+  br label %bb1
+}
Index: lib/Transforms/Scalar/LoadCombine.cpp
===================================================================
--- lib/Transforms/Scalar/LoadCombine.cpp
+++ 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 +54,21 @@
 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<DominatorTreeWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
   }
 
@@ -234,6 +238,14 @@
     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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