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Teach LSR to optimize away SMAX operations for tripcounts in common
cases. See the comment above OptimizeSMax for the full story, and the testcase for an example. This cancels out a pessimization commonly attributed to indvars, and will allow us to lift some of the artificial throttles in indvars, rather than add new ones. llvm-svn: 56230
This commit is contained in:
Dan Gohman
parent
b980f023ca
commit
faa15214e0
@ -184,6 +184,11 @@ private:
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/// inside the loop then try to eliminate the cast opeation.
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void OptimizeShadowIV(Loop *L);
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/// OptimizeSMax - Rewrite the loop's terminating condition
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/// if it uses an smax computation.
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ICmpInst *OptimizeSMax(Loop *L, ICmpInst *Cond,
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IVStrideUse* &CondUse);
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bool FindIVUserForCond(ICmpInst *Cond, IVStrideUse *&CondUse,
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const SCEVHandle *&CondStride);
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bool RequiresTypeConversion(const Type *Ty, const Type *NewTy);
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@ -1695,6 +1700,123 @@ ICmpInst *LoopStrengthReduce::ChangeCompareStride(Loop *L, ICmpInst *Cond,
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return Cond;
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}
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/// OptimizeSMax - Rewrite the loop's terminating condition if it uses
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/// an smax computation.
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///
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/// This is a narrow solution to a specific, but acute, problem. For loops
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/// like this:
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///
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/// i = 0;
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/// do {
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/// p[i] = 0.0;
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/// } while (++i < n);
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///
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/// where the comparison is signed, the trip count isn't just 'n', because
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/// 'n' could be negative. And unfortunately this can come up even for loops
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/// where the user didn't use a C do-while loop. For example, seemingly
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/// well-behaved top-test loops will commonly be lowered like this:
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//
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/// if (n > 0) {
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/// i = 0;
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/// do {
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/// p[i] = 0.0;
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/// } while (++i < n);
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/// }
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///
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/// and then it's possible for subsequent optimization to obscure the if
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/// test in such a way that indvars can't find it.
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///
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/// When indvars can't find the if test in loops like this, it creates a
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/// signed-max expression, which allows it to give the loop a canonical
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/// induction variable:
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///
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/// i = 0;
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/// smax = n < 1 ? 1 : n;
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/// do {
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/// p[i] = 0.0;
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/// } while (++i != smax);
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///
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/// Canonical induction variables are necessary because the loop passes
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/// are designed around them. The most obvious example of this is the
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/// LoopInfo analysis, which doesn't remember trip count values. It
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/// expects to be able to rediscover the trip count each time it is
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/// needed, and it does this using a simple analyis that only succeeds if
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/// the loop has a canonical induction variable.
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///
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/// However, when it comes time to generate code, the maximum operation
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/// can be quite costly, especially if it's inside of an outer loop.
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///
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/// This function solves this problem by detecting this type of loop and
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/// rewriting their conditions from ICMP_NE back to ICMP_SLT, and deleting
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/// the instructions for the maximum computation.
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///
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ICmpInst *LoopStrengthReduce::OptimizeSMax(Loop *L, ICmpInst *Cond,
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IVStrideUse* &CondUse) {
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// Check that the loop matches the pattern we're looking for.
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if (Cond->getPredicate() != CmpInst::ICMP_EQ &&
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Cond->getPredicate() != CmpInst::ICMP_NE)
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return Cond;
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SelectInst *Sel = dyn_cast<SelectInst>(Cond->getOperand(1));
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if (!Sel || !Sel->hasOneUse()) return Cond;
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SCEVHandle IterationCount = SE->getIterationCount(L);
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if (isa<SCEVCouldNotCompute>(IterationCount))
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return Cond;
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SCEVHandle One = SE->getIntegerSCEV(1, IterationCount->getType());
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// Adjust for an annoying getIterationCount quirk.
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IterationCount = SE->getAddExpr(IterationCount, One);
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// Check for a max calculation that matches the pattern.
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SCEVSMaxExpr *SMax = dyn_cast<SCEVSMaxExpr>(IterationCount);
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if (!SMax || SMax != SE->getSCEV(Sel)) return Cond;
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SCEVHandle SMaxLHS = SMax->getOperand(0);
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SCEVHandle SMaxRHS = SMax->getOperand(1);
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if (!SMaxLHS || SMaxLHS != One) return Cond;
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// Check the relevant induction variable for conformance to
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// the pattern.
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SCEVHandle IV = SE->getSCEV(Cond->getOperand(0));
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SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(IV);
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if (!AR || !AR->isAffine() ||
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AR->getStart() != One ||
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AR->getStepRecurrence(*SE) != One)
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return Cond;
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// Check the right operand of the select, and remember it, as it will
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// be used in the new comparison instruction.
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Value *NewRHS = 0;
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if (SE->getSCEV(Sel->getOperand(1)) == SMaxRHS)
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NewRHS = Sel->getOperand(1);
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else if (SE->getSCEV(Sel->getOperand(2)) == SMaxRHS)
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NewRHS = Sel->getOperand(2);
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if (!NewRHS) return Cond;
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// Ok, everything looks ok to change the condition into an SLT or SGE and
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// delete the max calculation.
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ICmpInst *NewCond =
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new ICmpInst(Cond->getPredicate() == CmpInst::ICMP_NE ?
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CmpInst::ICMP_SLT :
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CmpInst::ICMP_SGE,
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Cond->getOperand(0), NewRHS, "scmp", Cond);
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// Delete the max calculation instructions.
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Cond->replaceAllUsesWith(NewCond);
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Cond->eraseFromParent();
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SE->deleteValueFromRecords(Cond);
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Instruction *Cmp = cast<Instruction>(Sel->getOperand(0));
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Sel->eraseFromParent();
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SE->deleteValueFromRecords(Sel);
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if (Cmp->use_empty()) {
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Cmp->eraseFromParent();
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SE->deleteValueFromRecords(Cmp);
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}
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CondUse->User = NewCond;
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return NewCond;
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}
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/// OptimizeShadowIV - If IV is used in a int-to-float cast
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/// inside the loop then try to eliminate the cast opeation.
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void LoopStrengthReduce::OptimizeShadowIV(Loop *L) {
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@ -1836,6 +1958,11 @@ void LoopStrengthReduce::OptimizeIndvars(Loop *L) {
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if (!FindIVUserForCond(Cond, CondUse, CondStride))
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return; // setcc doesn't use the IV.
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// If the trip count is computed in terms of an smax (due to ScalarEvolution
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// being unable to find a sufficient guard, for example), change the loop
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// comparison to use SLT instead of NE.
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Cond = OptimizeSMax(L, Cond, CondUse);
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// If possible, change stride and operands of the compare instruction to
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// eliminate one stride.
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Cond = ChangeCompareStride(L, Cond, CondUse, CondStride);
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236
test/CodeGen/X86/optimize-smax.ll
Normal file
236
test/CodeGen/X86/optimize-smax.ll
Normal file
@ -0,0 +1,236 @@
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; RUN: llvm-as < %s | llc -march=x86 | not grep cmov
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; LSR should be able to eliminate the smax computations by
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; making the loops use slt comparisons instead of ne comparisons.
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target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
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target triple = "i386-apple-darwin9"
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define void @foo(i8* %r, i32 %s, i32 %w, i32 %x, i8* %j, i32 %d) nounwind {
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entry:
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%0 = mul i32 %x, %w ; <i32> [#uses=2]
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%1 = mul i32 %x, %w ; <i32> [#uses=1]
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%2 = sdiv i32 %1, 4 ; <i32> [#uses=1]
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%.sum2 = add i32 %2, %0 ; <i32> [#uses=2]
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%cond = icmp eq i32 %d, 1 ; <i1> [#uses=1]
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br i1 %cond, label %bb29, label %bb10.preheader
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bb10.preheader: ; preds = %entry
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%3 = icmp sgt i32 %x, 0 ; <i1> [#uses=1]
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br i1 %3, label %bb.nph9, label %bb18.loopexit
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bb.nph7: ; preds = %bb7.preheader
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%4 = mul i32 %y.08, %w ; <i32> [#uses=1]
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%5 = mul i32 %y.08, %s ; <i32> [#uses=1]
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%6 = add i32 %5, 1 ; <i32> [#uses=1]
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%tmp8 = icmp sgt i32 1, %w ; <i1> [#uses=1]
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%smax9 = select i1 %tmp8, i32 1, i32 %w ; <i32> [#uses=1]
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br label %bb6
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bb6: ; preds = %bb7, %bb.nph7
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%x.06 = phi i32 [ 0, %bb.nph7 ], [ %indvar.next7, %bb7 ] ; <i32> [#uses=3]
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%7 = add i32 %x.06, %4 ; <i32> [#uses=1]
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%8 = shl i32 %x.06, 1 ; <i32> [#uses=1]
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%9 = add i32 %6, %8 ; <i32> [#uses=1]
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%10 = getelementptr i8* %r, i32 %9 ; <i8*> [#uses=1]
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%11 = load i8* %10, align 1 ; <i8> [#uses=1]
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%12 = getelementptr i8* %j, i32 %7 ; <i8*> [#uses=1]
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store i8 %11, i8* %12, align 1
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br label %bb7
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bb7: ; preds = %bb6
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%indvar.next7 = add i32 %x.06, 1 ; <i32> [#uses=2]
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%exitcond10 = icmp ne i32 %indvar.next7, %smax9 ; <i1> [#uses=1]
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br i1 %exitcond10, label %bb6, label %bb7.bb9_crit_edge
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bb7.bb9_crit_edge: ; preds = %bb7
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br label %bb9
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bb9: ; preds = %bb7.preheader, %bb7.bb9_crit_edge
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br label %bb10
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bb10: ; preds = %bb9
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%indvar.next11 = add i32 %y.08, 1 ; <i32> [#uses=2]
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%exitcond12 = icmp ne i32 %indvar.next11, %x ; <i1> [#uses=1]
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br i1 %exitcond12, label %bb7.preheader, label %bb10.bb18.loopexit_crit_edge
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bb10.bb18.loopexit_crit_edge: ; preds = %bb10
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br label %bb10.bb18.loopexit_crit_edge.split
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bb10.bb18.loopexit_crit_edge.split: ; preds = %bb.nph9, %bb10.bb18.loopexit_crit_edge
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br label %bb18.loopexit
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bb.nph9: ; preds = %bb10.preheader
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%13 = icmp sgt i32 %w, 0 ; <i1> [#uses=1]
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br i1 %13, label %bb.nph9.split, label %bb10.bb18.loopexit_crit_edge.split
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bb.nph9.split: ; preds = %bb.nph9
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br label %bb7.preheader
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bb7.preheader: ; preds = %bb.nph9.split, %bb10
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%y.08 = phi i32 [ 0, %bb.nph9.split ], [ %indvar.next11, %bb10 ] ; <i32> [#uses=3]
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br i1 true, label %bb.nph7, label %bb9
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bb.nph5: ; preds = %bb18.loopexit
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%14 = sdiv i32 %w, 2 ; <i32> [#uses=1]
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%15 = icmp slt i32 %w, 2 ; <i1> [#uses=1]
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%16 = sdiv i32 %x, 2 ; <i32> [#uses=2]
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br i1 %15, label %bb18.bb20_crit_edge.split, label %bb.nph5.split
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bb.nph5.split: ; preds = %bb.nph5
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%tmp2 = icmp sgt i32 1, %16 ; <i1> [#uses=1]
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%smax3 = select i1 %tmp2, i32 1, i32 %16 ; <i32> [#uses=1]
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br label %bb13
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bb13: ; preds = %bb18, %bb.nph5.split
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%y.14 = phi i32 [ 0, %bb.nph5.split ], [ %indvar.next1, %bb18 ] ; <i32> [#uses=4]
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%17 = mul i32 %14, %y.14 ; <i32> [#uses=2]
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%18 = shl i32 %y.14, 1 ; <i32> [#uses=1]
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%19 = srem i32 %y.14, 2 ; <i32> [#uses=1]
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%20 = add i32 %19, %18 ; <i32> [#uses=1]
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%21 = mul i32 %20, %s ; <i32> [#uses=2]
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br i1 true, label %bb.nph3, label %bb17
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bb.nph3: ; preds = %bb13
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%22 = add i32 %17, %0 ; <i32> [#uses=1]
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%23 = add i32 %17, %.sum2 ; <i32> [#uses=1]
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%24 = sdiv i32 %w, 2 ; <i32> [#uses=2]
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%tmp = icmp sgt i32 1, %24 ; <i1> [#uses=1]
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%smax = select i1 %tmp, i32 1, i32 %24 ; <i32> [#uses=1]
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br label %bb14
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bb14: ; preds = %bb15, %bb.nph3
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%x.12 = phi i32 [ 0, %bb.nph3 ], [ %indvar.next, %bb15 ] ; <i32> [#uses=5]
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%25 = shl i32 %x.12, 2 ; <i32> [#uses=1]
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%26 = add i32 %25, %21 ; <i32> [#uses=1]
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%27 = getelementptr i8* %r, i32 %26 ; <i8*> [#uses=1]
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%28 = load i8* %27, align 1 ; <i8> [#uses=1]
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%.sum = add i32 %22, %x.12 ; <i32> [#uses=1]
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%29 = getelementptr i8* %j, i32 %.sum ; <i8*> [#uses=1]
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store i8 %28, i8* %29, align 1
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%30 = shl i32 %x.12, 2 ; <i32> [#uses=1]
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%31 = or i32 %30, 2 ; <i32> [#uses=1]
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%32 = add i32 %31, %21 ; <i32> [#uses=1]
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%33 = getelementptr i8* %r, i32 %32 ; <i8*> [#uses=1]
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%34 = load i8* %33, align 1 ; <i8> [#uses=1]
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%.sum6 = add i32 %23, %x.12 ; <i32> [#uses=1]
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%35 = getelementptr i8* %j, i32 %.sum6 ; <i8*> [#uses=1]
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store i8 %34, i8* %35, align 1
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br label %bb15
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bb15: ; preds = %bb14
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%indvar.next = add i32 %x.12, 1 ; <i32> [#uses=2]
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%exitcond = icmp ne i32 %indvar.next, %smax ; <i1> [#uses=1]
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br i1 %exitcond, label %bb14, label %bb15.bb17_crit_edge
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bb15.bb17_crit_edge: ; preds = %bb15
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br label %bb17
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bb17: ; preds = %bb15.bb17_crit_edge, %bb13
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br label %bb18
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bb18.loopexit: ; preds = %bb10.bb18.loopexit_crit_edge.split, %bb10.preheader
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%36 = icmp slt i32 %x, 2 ; <i1> [#uses=1]
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br i1 %36, label %bb20, label %bb.nph5
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bb18: ; preds = %bb17
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%indvar.next1 = add i32 %y.14, 1 ; <i32> [#uses=2]
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%exitcond4 = icmp ne i32 %indvar.next1, %smax3 ; <i1> [#uses=1]
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br i1 %exitcond4, label %bb13, label %bb18.bb20_crit_edge
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bb18.bb20_crit_edge: ; preds = %bb18
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br label %bb18.bb20_crit_edge.split
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bb18.bb20_crit_edge.split: ; preds = %bb18.bb20_crit_edge, %bb.nph5
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br label %bb20
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bb20: ; preds = %bb18.bb20_crit_edge.split, %bb18.loopexit
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switch i32 %d, label %return [
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i32 3, label %bb22
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i32 1, label %bb29
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]
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bb22: ; preds = %bb20
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%37 = mul i32 %x, %w ; <i32> [#uses=1]
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%38 = sdiv i32 %37, 4 ; <i32> [#uses=1]
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%.sum3 = add i32 %38, %.sum2 ; <i32> [#uses=2]
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%39 = add i32 %x, 15 ; <i32> [#uses=1]
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%40 = and i32 %39, -16 ; <i32> [#uses=1]
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%41 = add i32 %w, 15 ; <i32> [#uses=1]
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%42 = and i32 %41, -16 ; <i32> [#uses=1]
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%43 = mul i32 %40, %s ; <i32> [#uses=1]
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%44 = icmp sgt i32 %x, 0 ; <i1> [#uses=1]
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br i1 %44, label %bb.nph, label %bb26
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bb.nph: ; preds = %bb22
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br label %bb23
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bb23: ; preds = %bb24, %bb.nph
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%y.21 = phi i32 [ 0, %bb.nph ], [ %indvar.next5, %bb24 ] ; <i32> [#uses=3]
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%45 = mul i32 %y.21, %42 ; <i32> [#uses=1]
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%.sum1 = add i32 %45, %43 ; <i32> [#uses=1]
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%46 = getelementptr i8* %r, i32 %.sum1 ; <i8*> [#uses=1]
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%47 = mul i32 %y.21, %w ; <i32> [#uses=1]
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%.sum5 = add i32 %47, %.sum3 ; <i32> [#uses=1]
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%48 = getelementptr i8* %j, i32 %.sum5 ; <i8*> [#uses=1]
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tail call void @llvm.memcpy.i32(i8* %48, i8* %46, i32 %w, i32 1)
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br label %bb24
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bb24: ; preds = %bb23
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%indvar.next5 = add i32 %y.21, 1 ; <i32> [#uses=2]
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%exitcond6 = icmp ne i32 %indvar.next5, %x ; <i1> [#uses=1]
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br i1 %exitcond6, label %bb23, label %bb24.bb26_crit_edge
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bb24.bb26_crit_edge: ; preds = %bb24
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br label %bb26
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bb26: ; preds = %bb24.bb26_crit_edge, %bb22
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%49 = mul i32 %x, %w ; <i32> [#uses=1]
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%.sum4 = add i32 %.sum3, %49 ; <i32> [#uses=1]
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%50 = getelementptr i8* %j, i32 %.sum4 ; <i8*> [#uses=1]
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%51 = mul i32 %x, %w ; <i32> [#uses=1]
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%52 = sdiv i32 %51, 2 ; <i32> [#uses=1]
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tail call void @llvm.memset.i32(i8* %50, i8 -128, i32 %52, i32 1)
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ret void
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bb29: ; preds = %bb20, %entry
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%53 = add i32 %w, 15 ; <i32> [#uses=1]
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%54 = and i32 %53, -16 ; <i32> [#uses=1]
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%55 = icmp sgt i32 %x, 0 ; <i1> [#uses=1]
|
||||
br i1 %55, label %bb.nph11, label %bb33
|
||||
|
||||
bb.nph11: ; preds = %bb29
|
||||
br label %bb30
|
||||
|
||||
bb30: ; preds = %bb31, %bb.nph11
|
||||
%y.310 = phi i32 [ 0, %bb.nph11 ], [ %indvar.next13, %bb31 ] ; <i32> [#uses=3]
|
||||
%56 = mul i32 %y.310, %54 ; <i32> [#uses=1]
|
||||
%57 = getelementptr i8* %r, i32 %56 ; <i8*> [#uses=1]
|
||||
%58 = mul i32 %y.310, %w ; <i32> [#uses=1]
|
||||
%59 = getelementptr i8* %j, i32 %58 ; <i8*> [#uses=1]
|
||||
tail call void @llvm.memcpy.i32(i8* %59, i8* %57, i32 %w, i32 1)
|
||||
br label %bb31
|
||||
|
||||
bb31: ; preds = %bb30
|
||||
%indvar.next13 = add i32 %y.310, 1 ; <i32> [#uses=2]
|
||||
%exitcond14 = icmp ne i32 %indvar.next13, %x ; <i1> [#uses=1]
|
||||
br i1 %exitcond14, label %bb30, label %bb31.bb33_crit_edge
|
||||
|
||||
bb31.bb33_crit_edge: ; preds = %bb31
|
||||
br label %bb33
|
||||
|
||||
bb33: ; preds = %bb31.bb33_crit_edge, %bb29
|
||||
%60 = mul i32 %x, %w ; <i32> [#uses=1]
|
||||
%61 = getelementptr i8* %j, i32 %60 ; <i8*> [#uses=1]
|
||||
%62 = mul i32 %x, %w ; <i32> [#uses=1]
|
||||
%63 = sdiv i32 %62, 2 ; <i32> [#uses=1]
|
||||
tail call void @llvm.memset.i32(i8* %61, i8 -128, i32 %63, i32 1)
|
||||
ret void
|
||||
|
||||
return: ; preds = %bb20
|
||||
ret void
|
||||
}
|
||||
|
||||
declare void @llvm.memcpy.i32(i8*, i8*, i32, i32) nounwind
|
||||
|
||||
declare void @llvm.memset.i32(i8*, i8, i32, i32) nounwind
|
||||
Loading…
Reference in New Issue
Block a user