From: Dan Gohman <gohman@apple.com>
Date: Sun, 11 Apr 2010 23:10:12 +0000 (+0000)
Subject: Teach IndVarSimplify how to eliminate comparisons involving induction
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=3fddbb337dd5d8de5317df62e8027a6dcd778e73;p=oota-llvm.git

Teach IndVarSimplify how to eliminate comparisons involving induction
variables. For example, with code like this:

  for (i=0;i<n;++i)
    if (i<n)
      x[i] = 0;

IndVarSimplify will now recognize that i is always less than n inside
the loop, and eliminate the if.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101000 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/IndVarSimplify.cpp b/lib/Transforms/Scalar/IndVarSimplify.cpp
index f87d69360c1..46fb5ecb95b 100644
--- a/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -97,6 +97,7 @@ namespace {
 
   private:
 
+    void EliminateIVComparisons();
     void RewriteNonIntegerIVs(Loop *L);
 
     ICmpInst *LinearFunctionTestReplace(Loop *L, const SCEV *BackedgeTakenCount,
@@ -336,6 +337,40 @@ void IndVarSimplify::RewriteNonIntegerIVs(Loop *L) {
     SE->forgetLoop(L);
 }
 
+void IndVarSimplify::EliminateIVComparisons() {
+  // Look for ICmp users.
+  for (IVUsers::iterator I = IU->begin(), E = IU->end(); I != E;) {
+    IVStrideUse &UI = *I++;
+    ICmpInst *ICmp = dyn_cast<ICmpInst>(UI.getUser());
+    if (!ICmp) continue;
+
+    bool Swapped = UI.getOperandValToReplace() == ICmp->getOperand(1);
+    ICmpInst::Predicate Pred = ICmp->getPredicate();
+    if (Swapped) Pred = ICmpInst::getSwappedPredicate(Pred);
+
+    // Get the SCEVs for the ICmp operands.
+    const SCEV *S = IU->getReplacementExpr(UI);
+    const SCEV *X = SE->getSCEV(ICmp->getOperand(!Swapped));
+
+    // Simplify unnecessary loops away.
+    const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
+    S = SE->getSCEVAtScope(S, ICmpLoop);
+    X = SE->getSCEVAtScope(X, ICmpLoop);
+
+    // If the condition is always true or always false, replace it with
+    // a constant value.
+    if (SE->isKnownPredicate(Pred, S, X))
+      ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
+    else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
+      ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
+    else
+      continue;
+
+    DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
+    ICmp->eraseFromParent();
+  }
+}
+
 bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
   IU = &getAnalysis<IVUsers>();
   LI = &getAnalysis<LoopInfo>();
@@ -362,6 +397,9 @@ bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
   if (!isa<SCEVCouldNotCompute>(BackedgeTakenCount))
     RewriteLoopExitValues(L, Rewriter);
 
+  // Simplify ICmp IV users.
+  EliminateIVComparisons();
+
   // Compute the type of the largest recurrence expression, and decide whether
   // a canonical induction variable should be inserted.
   const Type *LargestType = 0;
diff --git a/test/Transforms/IndVarSimplify/eliminate-comparison.ll b/test/Transforms/IndVarSimplify/eliminate-comparison.ll
new file mode 100644
index 00000000000..d15697497ff
--- /dev/null
+++ b/test/Transforms/IndVarSimplify/eliminate-comparison.ll
@@ -0,0 +1,40 @@
+; RUN: opt -indvars -S < %s | FileCheck %s
+
+; Indvars should be able to simplify simple comparisons involving
+; induction variables.
+
+; CHECK: %cond = and i1 %tobool.not, true
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+
+@X = external global [0 x double]
+
+define void @foo(i64 %n, i32* nocapture %p) nounwind {
+entry:
+  %cmp9 = icmp sgt i64 %n, 0
+  br i1 %cmp9, label %pre, label %return
+
+pre:
+  %t3 = load i32* %p
+  %tobool.not = icmp ne i32 %t3, 0
+  br label %loop
+
+loop:
+  %i = phi i64 [ 0, %pre ], [ %inc, %for.inc ]
+  %cmp6 = icmp slt i64 %i, %n
+  %cond = and i1 %tobool.not, %cmp6
+  br i1 %cond, label %if.then, label %for.inc
+
+if.then:
+  %arrayidx = getelementptr [0 x double]* @X, i64 0, i64 %i
+  store double 3.200000e+00, double* %arrayidx
+  br label %for.inc
+
+for.inc:
+  %inc = add nsw i64 %i, 1
+  %exitcond = icmp sge i64 %inc, %n
+  br i1 %exitcond, label %return, label %loop
+
+return:
+  ret void
+}