[llvm] r278853 - Revert "Enhance SCEV to compute the trip count for some loops with unknown stride."
Reid Kleckner via llvm-commits
llvm-commits at lists.llvm.org
Tue Aug 16 14:02:06 PDT 2016
Author: rnk
Date: Tue Aug 16 16:02:04 2016
New Revision: 278853
URL: http://llvm.org/viewvc/llvm-project?rev=278853&view=rev
Log:
Revert "Enhance SCEV to compute the trip count for some loops with unknown stride."
This reverts commit r278731. It caused http://crbug.com/638314
Removed:
llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
Modified:
llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
llvm/trunk/lib/Analysis/ScalarEvolution.cpp
Modified: llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolution.h?rev=278853&r1=278852&r2=278853&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ScalarEvolution.h (original)
+++ llvm/trunk/include/llvm/Analysis/ScalarEvolution.h Tue Aug 16 16:02:04 2016
@@ -803,13 +803,6 @@ namespace llvm {
/// Cache for \c loopHasNoAbnormalExits.
DenseMap<const Loop *, bool> LoopHasNoAbnormalExits;
- /// Cache for \c loopHasNoSideEffects.
- DenseMap<const Loop *, bool> LoopHasNoSideEffects;
-
- /// Returns true if \p L contains no instruction that can have side effects
- /// (i.e. via throwing an exception, volatile or atomic access).
- bool loopHasNoSideEffects(const Loop *L);
-
/// Returns true if \p L contains no instruction that can abnormally exit
/// the loop (i.e. via throwing an exception, by terminating the thread
/// cleanly or by infinite looping in a called function). Strictly
Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=278853&r1=278852&r2=278853&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Tue Aug 16 16:02:04 2016
@@ -4949,28 +4949,6 @@ bool ScalarEvolution::isAddRecNeverPoiso
return LatchControlDependentOnPoison && loopHasNoAbnormalExits(L);
}
-bool ScalarEvolution::loopHasNoSideEffects(const Loop *L) {
- auto Itr = LoopHasNoSideEffects.find(L);
- if (Itr == LoopHasNoSideEffects.end()) {
- auto NoSideEffectsInBB = [&](BasicBlock *BB) {
- return all_of(*BB, [](Instruction &I) {
- // Non-atomic, non-volatile stores are ok.
- if (auto *SI = dyn_cast<StoreInst>(&I))
- return SI->isSimple();
-
- return !I.mayHaveSideEffects();
- });
- };
-
- auto InsertPair = LoopHasNoSideEffects.insert(
- {L, all_of(L->getBlocks(), NoSideEffectsInBB)});
- assert(InsertPair.second && "We just checked!");
- Itr = InsertPair.first;
- }
-
- return Itr->second;
-}
-
bool ScalarEvolution::loopHasNoAbnormalExits(const Loop *L) {
auto Itr = LoopHasNoAbnormalExits.find(L);
if (Itr == LoopHasNoAbnormalExits.end()) {
@@ -5558,7 +5536,6 @@ void ScalarEvolution::forgetLoop(const L
forgetLoop(I);
LoopHasNoAbnormalExits.erase(L);
- LoopHasNoSideEffects.erase(L);
}
void ScalarEvolution::forgetValue(Value *V) {
@@ -8701,15 +8678,11 @@ ScalarEvolution::howManyLessThans(const
return getCouldNotCompute();
const SCEVAddRecExpr *IV = dyn_cast<SCEVAddRecExpr>(LHS);
- bool PredicatedIV = false;
-
- if (!IV && AllowPredicates) {
+ if (!IV && AllowPredicates)
// Try to make this an AddRec using runtime tests, in the first X
// iterations of this loop, where X is the SCEV expression found by the
// algorithm below.
IV = convertSCEVToAddRecWithPredicates(LHS, L, P);
- PredicatedIV = true;
- }
// Avoid weird loops
if (!IV || IV->getLoop() != L || !IV->isAffine())
@@ -8720,55 +8693,9 @@ ScalarEvolution::howManyLessThans(const
const SCEV *Stride = IV->getStepRecurrence(*this);
- bool PositiveStride = isKnownPositive(Stride);
-
- // Avoid negative or zero stride values.
- if (!PositiveStride)
- // We can compute the correct backedge taken count for loops with unknown
- // strides if we can prove that the loop is not an infinite loop with side
- // effects. Here's the loop structure we are trying to handle -
- //
- // i = start
- // do {
- // A[i] = i;
- // i += s;
- // } while (i < end);
- //
- // The backedge taken count for such loops is evaluated as -
- // (max(end, start + stride) - start - 1) /u stride
- //
- // The additional preconditions that we need to check to prove correctness
- // of the above formula is as follows -
- //
- // a) IV is either nuw or nsw depending upon signedness (indicated by the
- // NoWrap flag).
- // b) loop is single exit with no side effects.
- //
- //
- // Precondition a) implies that if the stride is negative, this is a single
- // trip loop. The backedge taken count formula reduces to zero in this case.
- //
- // Precondition b) implies that the unknown stride cannot be zero otherwise
- // we have UB.
- //
- // The positive stride case is the same as isKnownPositive(Stride) returning
- // true (original behavior of the function).
- //
- // We want to make sure that the stride is truly unknown as there are edge
- // cases where ScalarEvolution propagates no wrap flags to the
- // post-increment/decrement IV even though the increment/decrement operation
- // itself is wrapping. The computed backedge taken count may be wrong in
- // such cases. This is prevented by checking that the stride is not known to
- // be either positive or non-positive. For example, no wrap flags are
- // propagated to the post-increment IV of this loop with a trip count of 2 -
- //
- // unsigned char i;
- // for(i=127; i<128; i+=129)
- // A[i] = i;
- //
- if (PredicatedIV || !NoWrap || isKnownNonPositive(Stride) ||
- !loopHasNoSideEffects(L))
- return getCouldNotCompute();
+ // Avoid negative or zero stride values
+ if (!isKnownPositive(Stride))
+ return getCouldNotCompute();
// Avoid proven overflow cases: this will ensure that the backedge taken count
// will not generate any unsigned overflow. Relaxed no-overflow conditions
Removed: llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll?rev=278852&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll (original)
+++ llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll (removed)
@@ -1,54 +0,0 @@
-; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s
-
-; ScalarEvolution should be able to compute trip count of the loop by proving
-; that this is not an infinite loop with side effects.
-
-; CHECK: Determining loop execution counts for: @foo1
-; CHECK: backedge-taken count is ((-1 + %n) /u %s)
-
-target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
-
-; Function Attrs: norecurse nounwind
-define void @foo1(i32* nocapture %A, i32 %n, i32 %s) #0 {
-entry:
- %cmp4 = icmp sgt i32 %n, 0
- br i1 %cmp4, label %for.body, label %for.end
-
-for.body: ; preds = %entry, %for.body
- %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]
- %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05
- %0 = load i32, i32* %arrayidx, align 4
- %inc = add nsw i32 %0, 1
- store i32 %inc, i32* %arrayidx, align 4
- %add = add nsw i32 %i.05, %s
- %cmp = icmp slt i32 %add, %n
- br i1 %cmp, label %for.body, label %for.end
-
-for.end: ; preds = %for.body, %entry
- ret void
-}
-
-
-; Check that we are able to compute trip count of a loop without an entry guard.
-; CHECK: Determining loop execution counts for: @foo2
-; CHECK: backedge-taken count is ((-1 + (%n smax %s)) /u %s)
-
-; Function Attrs: norecurse nounwind
-define void @foo2(i32* nocapture %A, i32 %n, i32 %s) #0 {
-entry:
- br label %for.body
-
-for.body: ; preds = %entry, %for.body
- %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]
- %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05
- %0 = load i32, i32* %arrayidx, align 4
- %inc = add nsw i32 %0, 1
- store i32 %inc, i32* %arrayidx, align 4
- %add = add nsw i32 %i.05, %s
- %cmp = icmp slt i32 %add, %n
- br i1 %cmp, label %for.body, label %for.end
-
-for.end: ; preds = %for.body, %entry
- ret void
-}
-
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