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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi David,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><i><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">>></span> A loop which stores to a global variable has no side effects?<o:p></o:p></i></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"></a><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D">This seems to be an oversight.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D">How about we check that the pointer operand of the store is not a GLobalValue?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Does this seem reasonable?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Pankaj<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><a name="_____replyseparator"></a><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> David Majnemer [<a href="mailto:david.majnemer@gmail.com">mailto:david.majnemer@gmail.com</a>]
<br>
<b>Sent:</b> Friday, September 16, 2016 6:58 PM<br>
<b>To:</b> Kreitzer, David L <<a href="mailto:david.l.kreitzer@intel.com">david.l.kreitzer@intel.com</a>><br>
<b>Cc:</b> llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org">llvm-commits@lists.llvm.org</a>><br>
<b>Subject:</b> Re: [llvm] r281732 - Reapplying r278731 after fixing the problem that caused it to be reverted.<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Fri, Sep 16, 2016 at 7:38 AM, David L Kreitzer via llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<p class="MsoNormal">Author: dlkreitz<br>
Date: Fri Sep 16 09:38:13 2016<br>
New Revision: 281732<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=281732&view=rev" target="_blank">
http://llvm.org/viewvc/llvm-project?rev=281732&view=rev</a><br>
Log:<br>
Reapplying r278731 after fixing the problem that caused it to be reverted.<br>
<br>
Enhance SCEV to compute the trip count for some loops with unknown stride.<br>
<br>
Patch by Pankaj Chawla<br>
<br>
Differential Revision: <a href="https://reviews.llvm.org/D22377" target="_blank">
https://reviews.llvm.org/D22377</a><br>
<br>
Added:<br>
llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll<br>
Modified:<br>
llvm/trunk/include/llvm/Analysis/ScalarEvolution.h<br>
llvm/trunk/lib/Analysis/ScalarEvolution.cpp<br>
<br>
Modified: llvm/trunk/include/llvm/Analysis/ScalarEvolution.h<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolution.h?rev=281732&r1=281731&r2=281732&view=diff" target="_blank">
http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolution.h?rev=281732&r1=281731&r2=281732&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/include/llvm/Analysis/ScalarEvolution.h (original)<br>
+++ llvm/trunk/include/llvm/Analysis/ScalarEvolution.h Fri Sep 16 09:38:13 2016<br>
@@ -803,6 +803,13 @@ namespace llvm {<br>
/// Cache for \c loopHasNoAbnormalExits.<br>
DenseMap<const Loop *, bool> LoopHasNoAbnormalExits;<br>
<br>
+ /// Cache for \c loopHasNoSideEffects.<br>
+ DenseMap<const Loop *, bool> LoopHasNoSideEffects;<br>
+<br>
+ /// Returns true if \p L contains no instruction that can have side effects<br>
+ /// (i.e. via throwing an exception, volatile or atomic access).<br>
+ bool loopHasNoSideEffects(const Loop *L);<br>
+<br>
/// Returns true if \p L contains no instruction that can abnormally exit<br>
/// the loop (i.e. via throwing an exception, by terminating the thread<br>
/// cleanly or by infinite looping in a called function). Strictly<br>
<br>
Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=281732&r1=281731&r2=281732&view=diff" target="_blank">
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=281732&r1=281731&r2=281732&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)<br>
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Fri Sep 16 09:38:13 2016<br>
@@ -4953,6 +4953,28 @@ bool ScalarEvolution::isAddRecNeverPoiso<br>
return LatchControlDependentOnPoison && loopHasNoAbnormalExits(L);<br>
}<br>
<br>
+bool ScalarEvolution::loopHasNoSideEffects(const Loop *L) {<br>
+ auto Itr = LoopHasNoSideEffects.find(L);<br>
+ if (Itr == LoopHasNoSideEffects.end()) {<br>
+ auto NoSideEffectsInBB = [&](BasicBlock *BB) {<br>
+ return all_of(*BB, [](Instruction &I) {<br>
+ // Non-atomic, non-volatile stores are ok.<br>
+ if (auto *SI = dyn_cast<StoreInst>(&I))<br>
+ return SI->isSimple();<span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
</blockquote>
<div>
<p class="MsoPlainText">A loop which stores to a global variable has no side effects?<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<p class="MsoNormal">+<br>
+ return !I.mayHaveSideEffects();<br>
+ });<br>
+ };<br>
+<br>
+ auto InsertPair = LoopHasNoSideEffects.insert(<br>
+ {L, all_of(L->getBlocks(), NoSideEffectsInBB)});<br>
+ assert(InsertPair.second && "We just checked!");<br>
+ Itr = InsertPair.first;<br>
+ }<br>
+<br>
+ return Itr->second;<br>
+}<br>
+<br>
bool ScalarEvolution::loopHasNoAbnormalExits(const Loop *L) {<br>
auto Itr = LoopHasNoAbnormalExits.find(L);<br>
if (Itr == LoopHasNoAbnormalExits.end()) {<br>
@@ -5540,6 +5562,7 @@ void ScalarEvolution::forgetLoop(const L<br>
forgetLoop(I);<br>
<br>
LoopHasNoAbnormalExits.erase(L);<br>
+ LoopHasNoSideEffects.erase(L);<br>
}<br>
<br>
void ScalarEvolution::forgetValue(Value *V) {<br>
@@ -8614,6 +8637,8 @@ bool ScalarEvolution::isImpliedCondOpera<br>
<br>
bool ScalarEvolution::doesIVOverflowOnLT(const SCEV *RHS, const SCEV *Stride,<br>
bool IsSigned, bool NoWrap) {<br>
+ assert(isKnownPositive(Stride) && "Positive stride expected!");<br>
+<br>
if (NoWrap) return false;<br>
<br>
unsigned BitWidth = getTypeSizeInBits(RHS->getType());<br>
@@ -8682,11 +8707,15 @@ ScalarEvolution::howManyLessThans(const<br>
return getCouldNotCompute();<br>
<br>
const SCEVAddRecExpr *IV = dyn_cast<SCEVAddRecExpr>(LHS);<br>
- if (!IV && AllowPredicates)<br>
+ bool PredicatedIV = false;<br>
+<br>
+ if (!IV && AllowPredicates) {<br>
// Try to make this an AddRec using runtime tests, in the first X<br>
// iterations of this loop, where X is the SCEV expression found by the<br>
// algorithm below.<br>
IV = convertSCEVToAddRecWithPredicates(LHS, L, P);<br>
+ PredicatedIV = true;<br>
+ }<br>
<br>
// Avoid weird loops<br>
if (!IV || IV->getLoop() != L || !IV->isAffine())<br>
@@ -8697,15 +8726,62 @@ ScalarEvolution::howManyLessThans(const<br>
<br>
const SCEV *Stride = IV->getStepRecurrence(*this);<br>
<br>
- // Avoid negative or zero stride values<br>
- if (!isKnownPositive(Stride))<br>
- return getCouldNotCompute();<br>
+ bool PositiveStride = isKnownPositive(Stride);<br>
<br>
- // Avoid proven overflow cases: this will ensure that the backedge taken count<br>
- // will not generate any unsigned overflow. Relaxed no-overflow conditions<br>
- // exploit NoWrapFlags, allowing to optimize in presence of undefined<br>
- // behaviors like the case of C language.<br>
- if (!Stride->isOne() && doesIVOverflowOnLT(RHS, Stride, IsSigned, NoWrap))<br>
+ // Avoid negative or zero stride values.<br>
+ if (!PositiveStride) {<br>
+ // We can compute the correct backedge taken count for loops with unknown<br>
+ // strides if we can prove that the loop is not an infinite loop with side<br>
+ // effects. Here's the loop structure we are trying to handle -<br>
+ //<br>
+ // i = start<br>
+ // do {<br>
+ // A[i] = i;<br>
+ // i += s;<br>
+ // } while (i < end);<br>
+ //<br>
+ // The backedge taken count for such loops is evaluated as -<br>
+ // (max(end, start + stride) - start - 1) /u stride<br>
+ //<br>
+ // The additional preconditions that we need to check to prove correctness<br>
+ // of the above formula is as follows -<br>
+ //<br>
+ // a) IV is either nuw or nsw depending upon signedness (indicated by the<br>
+ // NoWrap flag).<br>
+ // b) loop is single exit with no side effects.<br>
+ //<br>
+ //<br>
+ // Precondition a) implies that if the stride is negative, this is a single<br>
+ // trip loop. The backedge taken count formula reduces to zero in this case.<br>
+ //<br>
+ // Precondition b) implies that the unknown stride cannot be zero otherwise<br>
+ // we have UB.<br>
+ //<br>
+ // The positive stride case is the same as isKnownPositive(Stride) returning<br>
+ // true (original behavior of the function).<br>
+ //<br>
+ // We want to make sure that the stride is truly unknown as there are edge<br>
+ // cases where ScalarEvolution propagates no wrap flags to the<br>
+ // post-increment/decrement IV even though the increment/decrement operation<br>
+ // itself is wrapping. The computed backedge taken count may be wrong in<br>
+ // such cases. This is prevented by checking that the stride is not known to<br>
+ // be either positive or non-positive. For example, no wrap flags are<br>
+ // propagated to the post-increment IV of this loop with a trip count of 2 -<br>
+ //<br>
+ // unsigned char i;<br>
+ // for(i=127; i<128; i+=129)<br>
+ // A[i] = i;<br>
+ //<br>
+ if (PredicatedIV || !NoWrap || isKnownNonPositive(Stride) ||<br>
+ !loopHasNoSideEffects(L))<br>
+ return getCouldNotCompute();<br>
+<br>
+ } else if (!Stride->isOne() &&<br>
+ doesIVOverflowOnLT(RHS, Stride, IsSigned, NoWrap))<br>
+ // Avoid proven overflow cases: this will ensure that the backedge taken<br>
+ // count will not generate any unsigned overflow. Relaxed no-overflow<br>
+ // conditions exploit NoWrapFlags, allowing to optimize in presence of<br>
+ // undefined behaviors like the case of C language.<br>
return getCouldNotCompute();<br>
<br>
ICmpInst::Predicate Cond = IsSigned ? ICmpInst::ICMP_SLT<br>
@@ -8720,12 +8796,21 @@ ScalarEvolution::howManyLessThans(const<br>
APInt MinStart = IsSigned ? getSignedRange(Start).getSignedMin()<br>
: getUnsignedRange(Start).getUnsignedMin();<br>
<br>
- APInt MinStride = IsSigned ? getSignedRange(Stride).getSignedMin()<br>
- : getUnsignedRange(Stride).getUnsignedMin();<br>
-<br>
unsigned BitWidth = getTypeSizeInBits(LHS->getType());<br>
- APInt Limit = IsSigned ? APInt::getSignedMaxValue(BitWidth) - (MinStride - 1)<br>
- : APInt::getMaxValue(BitWidth) - (MinStride - 1);<br>
+<br>
+ APInt StrideForMaxBECount;<br>
+<br>
+ if (PositiveStride)<br>
+ StrideForMaxBECount = IsSigned ? getSignedRange(Stride).getSignedMin()<br>
+ : getUnsignedRange(Stride).getUnsignedMin();<br>
+ else<br>
+ // Using a stride of 1 is safe when computing max backedge taken count for<br>
+ // a loop with unknown stride.<br>
+ StrideForMaxBECount = APInt(BitWidth, 1, IsSigned);<br>
+<br>
+ APInt Limit =<br>
+ IsSigned ? APInt::getSignedMaxValue(BitWidth) - (StrideForMaxBECount - 1)<br>
+ : APInt::getMaxValue(BitWidth) - (StrideForMaxBECount - 1);<br>
<br>
// Although End can be a MAX expression we estimate MaxEnd considering only<br>
// the case End = RHS. This is safe because in the other case (End - Start)<br>
@@ -8739,7 +8824,7 @@ ScalarEvolution::howManyLessThans(const<br>
MaxBECount = BECount;<br>
else<br>
MaxBECount = computeBECount(getConstant(MaxEnd - MinStart),<br>
- getConstant(MinStride), false);<br>
+ getConstant(StrideForMaxBECount), false);<br>
<br>
if (isa<SCEVCouldNotCompute>(MaxBECount))<br>
MaxBECount = BECount;<br>
<br>
Added: llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll?rev=281732&view=auto" target="_blank">
http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll?rev=281732&view=auto</a><br>
==============================================================================<br>
--- llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll (added)<br>
+++ llvm/trunk/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll Fri Sep 16 09:38:13 2016<br>
@@ -0,0 +1,62 @@<br>
+; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s<br>
+<br>
+; ScalarEvolution should be able to compute trip count of the loop by proving<br>
+; that this is not an infinite loop with side effects.<br>
+<br>
+; CHECK: Determining loop execution counts for: @foo1<br>
+; CHECK: backedge-taken count is ((-1 + %n) /u %s)<br>
+<br>
+; We should have a conservative estimate for the max backedge taken count for<br>
+; loops with unknown stride.<br>
+; CHECK: max backedge-taken count is -1<br>
+<br>
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"<br>
+<br>
+; Function Attrs: norecurse nounwind<br>
+define void @foo1(i32* nocapture %A, i32 %n, i32 %s) #0 {<br>
+entry:<br>
+ %cmp4 = icmp sgt i32 %n, 0<br>
+ br i1 %cmp4, label %for.body, label %for.end<br>
+<br>
+for.body: ; preds = %entry, %for.body<br>
+ %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]<br>
+ %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05<br>
+ %0 = load i32, i32* %arrayidx, align 4<br>
+ %inc = add nsw i32 %0, 1<br>
+ store i32 %inc, i32* %arrayidx, align 4<br>
+ %add = add nsw i32 %i.05, %s<br>
+ %cmp = icmp slt i32 %add, %n<br>
+ br i1 %cmp, label %for.body, label %for.end<br>
+<br>
+for.end: ; preds = %for.body, %entry<br>
+ ret void<br>
+}<br>
+<br>
+<br>
+; Check that we are able to compute trip count of a loop without an entry guard.<br>
+; CHECK: Determining loop execution counts for: @foo2<br>
+; CHECK: backedge-taken count is ((-1 + (%n smax %s)) /u %s)<br>
+<br>
+; We should have a conservative estimate for the max backedge taken count for<br>
+; loops with unknown stride.<br>
+; CHECK: max backedge-taken count is -1<br>
+<br>
+; Function Attrs: norecurse nounwind<br>
+define void @foo2(i32* nocapture %A, i32 %n, i32 %s) #0 {<br>
+entry:<br>
+ br label %for.body<br>
+<br>
+for.body: ; preds = %entry, %for.body<br>
+ %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]<br>
+ %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05<br>
+ %0 = load i32, i32* %arrayidx, align 4<br>
+ %inc = add nsw i32 %0, 1<br>
+ store i32 %inc, i32* %arrayidx, align 4<br>
+ %add = add nsw i32 %i.05, %s<br>
+ %cmp = icmp slt i32 %add, %n<br>
+ br i1 %cmp, label %for.body, label %for.end<br>
+<br>
+for.end: ; preds = %for.body, %entry<br>
+ ret void<br>
+}<br>
+<br>
<br>
<br>
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