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Teach IndVarSimplify how to eliminate comparisons involving induction

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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
This commit is contained in:
Dan Gohman 2010-04-11 23:10:12 +00:00
parent 53c66eacc4
commit 3fddbb337d
2 changed files with 78 additions and 0 deletions
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38
lib/Transforms/Scalar/IndVarSimplify.cpp
View File

@ -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;

40
test/Transforms/IndVarSimplify/eliminate-comparison.ll Normal file
View File

@ -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
}