From e95cc25a2267128436bb83af6cb57c07323c8693 Mon Sep 17 00:00:00 2001
From: Duncan Sands <baldrick@free.fr>
Date: Tue, 21 Dec 2010 16:12:03 +0000
Subject: [PATCH] If an instruction simplifies, try again to simplify any uses
 of it.  This is not very important since the pass is only used for testing,
 but it does make it more realistic.  Suggested by Frits van Bommel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122336 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Utils/SimplifyInstructions.cpp | 36 ++++++++++++++++---
 1 file changed, 32 insertions(+), 4 deletions(-)

diff --git a/lib/Transforms/Utils/SimplifyInstructions.cpp b/lib/Transforms/Utils/SimplifyInstructions.cpp
index 548bc510248..6074a5f2c21 100644
--- a/lib/Transforms/Utils/SimplifyInstructions.cpp
+++ b/lib/Transforms/Utils/SimplifyInstructions.cpp
@@ -23,6 +23,7 @@
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
 using namespace llvm;
 
 STATISTIC(NumSimplified, "Number of redundant instructions removed");
@@ -40,19 +41,46 @@ namespace {
 
     /// runOnFunction - Remove instructions that simplify.
     bool runOnFunction(Function &F) {
-      bool Changed = false;
       const TargetData *TD = getAnalysisIfAvailable<TargetData>();
       const DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
+      bool Changed = false;
+
+      // Add all interesting instructions to the worklist.
+      std::set<Instruction*> Worklist;
       for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
         for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
           Instruction *I = BI++;
-          if (Value *V = SimplifyInstruction(I, TD, DT)) {
-            I->replaceAllUsesWith(V);
+          // Zap any dead instructions.
+          if (isInstructionTriviallyDead(I)) {
             I->eraseFromParent();
             Changed = true;
-            ++NumSimplified;
+            continue;
           }
+          // Add all others to the worklist.
+          Worklist.insert(I);
         }
+
+      // Simplify everything in the worklist until the cows come home.
+      while (!Worklist.empty()) {
+        Instruction *I = *Worklist.begin();
+        Worklist.erase(Worklist.begin());
+        Value *V = SimplifyInstruction(I, TD, DT);
+        if (!V) continue;
+
+        // This instruction simplifies!  Replace it with its simplification and
+        // add all uses to the worklist, since they may now simplify.
+        I->replaceAllUsesWith(V);
+        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
+             UI != UE; ++UI)
+          // In unreachable code an instruction can use itself, in which case
+          // don't add it to the worklist since we are about to erase it.
+          if (*UI != I) Worklist.insert(cast<Instruction>(*UI));
+        if (isInstructionTriviallyDead(I))
+          I->eraseFromParent();
+        ++NumSimplified;
+        Changed = true;
+      }
+
       return Changed;
     }
   };