[InstCombine] re-commit r218721 icmp-select-icmp optimization

Takes care of the assert that caused build fails.
Rather than asserting the code checks now that the definition
and use are in the same block, and does not attempt
to optimize when that is not the case.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219175 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Gerolf-Apple

lib/Transforms/InstCombine/InstCombine.h

@@ -14,6 +14,7 @@
 #include "llvm/Analysis/AssumptionTracker.h"
 #include "llvm/Analysis/TargetFolder.h"
 #include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstVisitor.h"
 #include "llvm/IR/IntrinsicInst.h"
@@ -97,8 +98,8 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
       public InstVisitor<InstCombiner, Instruction *> {
   AssumptionTracker *AT;
   const DataLayout *DL;
-  TargetLibraryInfo *TLI;
   DominatorTree *DT; // not required
+  TargetLibraryInfo *TLI;
   bool MadeIRChange;
   LibCallSimplifier *Simplifier;
   bool MinimizeSize;
@@ -113,7 +114,8 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
   BuilderTy *Builder;
 
   static char ID; // Pass identification, replacement for typeid
-  InstCombiner() : FunctionPass(ID), DL(nullptr), Builder(nullptr) {
+  InstCombiner()
+      : FunctionPass(ID), DL(nullptr), DT(nullptr), Builder(nullptr) {
     MinimizeSize = false;
     initializeInstCombinerPass(*PassRegistry::getPassRegistry());
   }
@@ -242,6 +244,11 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
 
   // visitInstruction - Specify what to return for unhandled instructions...
   Instruction *visitInstruction(Instruction &I) { return nullptr; }
+  bool dominatesAllUses(const Instruction *DI, const Instruction *UI,
+                        const BasicBlock *DB) const;
+  bool replacedSelectWithOperand(SelectInst *SI, const ICmpInst *Icmp,
+                                 const ConstantInt *CI1,
+                                 const ConstantInt *CI2);
 
 private:
   bool ShouldChangeType(Type *From, Type *To) const;

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -21,6 +21,7 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Target/TargetLibraryInfo.h"
+
 using namespace llvm;
 using namespace PatternMatch;
 
@@ -2429,6 +2430,128 @@ static bool swapMayExposeCSEOpportunities(const Value * Op0,
   return GlobalSwapBenefits > 0;
 }
 
+/// \brief Check that one use is in the same block as the definition and all
+/// other uses are in blocks dominated by a given block
+///
+/// \param DI Definition
+/// \param UI Use
+/// \param DB Block that must dominate all uses of \p DI outside
+///           the parent block
+/// \return true when \p UI is the only use of \p DI in the parent block
+/// and all other uses of \p DI are in blocks dominated by \p DB.
+///
+bool InstCombiner::dominatesAllUses(const Instruction *DI,
+                                    const Instruction *UI,
+                                    const BasicBlock *DB) const {
+  assert(DI && UI && "Instruction not defined\n");
+  if (DI->getParent() != UI->getParent())
+    return false;
+  // DominatorTree available?
+  if (!DT)
+    return false;
+  for (const User *U : DI->users()) {
+    auto *Usr = cast<Instruction>(U);
+    if (Usr != UI && !DT->dominates(DB, Usr->getParent()))
+      return false;
+  }
+  return true;
+}
+
+///
+/// true when the instruction sequence within a block is select-cmp-br.
+///
+static bool isChainSelectCmpBranch(const SelectInst *SI) {
+  const BasicBlock *BB = SI->getParent();
+  if (!BB)
+    return false;
+  auto *BI = dyn_cast_or_null<BranchInst>(BB->getTerminator());
+  if (!BI || BI->getNumSuccessors() != 2)
+    return false;
+  auto *IC = dyn_cast<ICmpInst>(BI->getCondition());
+  if (!IC || (IC->getOperand(0) != SI && IC->getOperand(1) != SI))
+    return false;
+  return true;
+}
+
+///
+/// \brief True when a select result is replaced by one of its operands
+/// in select-icmp sequence. This will eventually result in the elimination
+/// of the select.
+///
+/// \param SI   Select instruction
+/// \param Icmp Compare instruction
+/// \param CI1  'true' when first select operand is equal to RHSC of Icmp
+/// \param CI2  'true' when second select operand is equal to RHSC of Icmp
+///
+/// Notes:
+/// - The replacement is global and requires dominator information
+/// - The caller is responsible for the actual replacement
+///
+/// Example:
+///
+/// entry:
+///  %4 = select i1 %3, %C* %0, %C* null
+///  %5 = icmp eq %C* %4, null
+///  br i1 %5, label %9, label %7
+///  ...
+///  ; <label>:7                                       ; preds = %entry
+///  %8 = getelementptr inbounds %C* %4, i64 0, i32 0
+///  ...
+///
+/// can be transformed to
+///
+///  %5 = icmp eq %C* %0, null
+///  %6 = select i1 %3, i1 %5, i1 true
+///  br i1 %6, label %9, label %7
+///  ...
+///  ; <label>:7                                       ; preds = %entry
+///  %8 = getelementptr inbounds %C* %0, i64 0, i32 0  // replace by %0!
+///
+/// Similar when the first operand of the select is a constant or/and
+/// the compare is for not equal rather than equal.
+///
+/// FIXME: Currently the function considers equal compares only. It should be
+/// possbile to extend it to not equal compares also.
+///
+bool InstCombiner::replacedSelectWithOperand(SelectInst *SI,
+                                             const ICmpInst *Icmp,
+                                             const ConstantInt *CI1,
+                                             const ConstantInt *CI2) {
+  if (isChainSelectCmpBranch(SI) && Icmp->isEquality()) {
+    // Code sequence is select - icmp.[eq|ne] - br
+    unsigned ReplaceWithOpd = 0;
+    if (CI1 && !CI1->isZero())
+      // The first constant operand of the select and the RHS of
+      // the compare match, so try to substitute
+      // the select results with its second operand
+      // Example:
+      // %4 = select i1 %3, %C* null, %C* %0
+      // %5 = icmp eq %C* %4, null
+      // ==> could replace select with second operand
+      ReplaceWithOpd = 2;
+    else if (CI2 && !CI2->isZero())
+      // Similar when the second operand of the select is a constant
+      // Example:
+      // %4 = select i1 %3, %C* %0, %C* null
+      // %5 = icmp eq %C* %4, null
+      // ==> could replace select with first operand
+      ReplaceWithOpd = 1;
+    if (ReplaceWithOpd) {
+      // Replace select with operand on else path for EQ compares.
+      // Replace select with operand on then path for NE compares.
+      BasicBlock *Succ =
+          Icmp->getPredicate() == ICmpInst::ICMP_EQ
+              ? SI->getParent()->getTerminator()->getSuccessor(1)
+              : SI->getParent()->getTerminator()->getSuccessor(0);
+      if (InstCombiner::dominatesAllUses(SI, Icmp, Succ)) {
+        SI->replaceAllUsesWith(SI->getOperand(ReplaceWithOpd));
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
 Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   bool Changed = false;
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
@@ -2885,8 +3008,21 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         // fold to a constant (in which case the icmp is replaced with a select
         // which will usually simplify) or this is the only user of the
         // select (in which case we are trading a select+icmp for a simpler
-        // select+icmp).
-        if ((Op1 && Op2) || (LHSI->hasOneUse() && (Op1 || Op2))) {
+        // select+icmp) or all uses of the select can be replaced based on
+        // dominance information ("Global cases").
+        bool Transform = false;
+        if (Op1 && Op2)
+          Transform = true;
+        else if (Op1 || Op2) {
+          if (LHSI->hasOneUse())
+            Transform = true;
+          else
+            // Global cases
+            Transform = replacedSelectWithOperand(
+                cast<SelectInst>(LHSI), &I, dyn_cast_or_null<ConstantInt>(Op1),
+                dyn_cast_or_null<ConstantInt>(Op2));
+        }
+        if (Transform) {
           if (!Op1)
             Op1 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(1),
                                       RHSC, I.getName());

lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -90,13 +90,16 @@ INITIALIZE_PASS_BEGIN(InstCombiner, "instcombine",
                 "Combine redundant instructions", false, false)
 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_END(InstCombiner, "instcombine",
                 "Combine redundant instructions", false, false)
 
 void InstCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesCFG();
   AU.addRequired<AssumptionTracker>();
   AU.addRequired<TargetLibraryInfo>();
+  AU.addRequired<DominatorTreeWrapperPass>();
+  AU.addPreserved<DominatorTreeWrapperPass>();
 }
 
 
@@ -2933,12 +2936,9 @@ bool InstCombiner::runOnFunction(Function &F) {
   AT = &getAnalysis<AssumptionTracker>();
   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
   DL = DLP ? &DLP->getDataLayout() : nullptr;
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   TLI = &getAnalysis<TargetLibraryInfo>();
 
-  DominatorTreeWrapperPass *DTWP =
-      getAnalysisIfAvailable<DominatorTreeWrapperPass>();
-  DT = DTWP ? &DTWP->getDomTree() : nullptr;
-
   // Minimizing size?
   MinimizeSize = F.getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                                 Attribute::MinSize);

test/Transforms/InstCombine/pr12338.ll

@@ -2,11 +2,11 @@
 
 define void @entry() nounwind {
 entry:
+; CHECK: br label %for.cond
   br label %for.cond
 
 for.cond:
   %local = phi <1 x i32> [ <i32 0>, %entry ], [ %phi2, %cond.end47 ]
-; CHECK: sub <1 x i32> <i32 92>, %local
   %phi3 = sub <1 x i32> zeroinitializer, %local
   br label %cond.end