From 6665b0ea688941c1c044a5c034ee45d45862168f Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Mon, 26 Oct 2009 21:55:43 +0000 Subject: [PATCH] Teach BasicAA how to analyze Select instructions, and make it more aggressive on PHI instructions. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85158 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Analysis/BasicAliasAnalysis.cpp | 78 ++++++++++++++++++++++++- test/Analysis/BasicAA/phi-and-select.ll | 73 +++++++++++++++++++++++ 2 files changed, 149 insertions(+), 2 deletions(-) create mode 100644 test/Analysis/BasicAA/phi-and-select.ll diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp index 6dc463facc2..17a90f5a04e 100644 --- a/lib/Analysis/BasicAliasAnalysis.cpp +++ b/lib/Analysis/BasicAliasAnalysis.cpp @@ -217,7 +217,7 @@ namespace { private: // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call. - SmallPtrSet VisitedPHIs; + SmallPtrSet VisitedPHIs; // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction // against another. @@ -229,6 +229,10 @@ namespace { AliasResult aliasPHI(const PHINode *PN, unsigned PNSize, const Value *V2, unsigned V2Size); + /// aliasSelect - Disambiguate a Select instruction against another value. + AliasResult aliasSelect(const SelectInst *SI, unsigned SISize, + const Value *V2, unsigned V2Size); + AliasResult aliasCheck(const Value *V1, unsigned V1Size, const Value *V2, unsigned V2Size); @@ -519,6 +523,41 @@ BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size, return MayAlias; } +// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select instruction +// against another. +AliasAnalysis::AliasResult +BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, + const Value *V2, unsigned V2Size) { + // If the values are Selects with the same condition, we can do a more precise + // check: just check for aliases between the values on corresponding arms. + if (const SelectInst *SI2 = dyn_cast(V2)) + if (SI->getCondition() == SI2->getCondition()) { + AliasResult Alias = + aliasCheck(SI->getTrueValue(), SISize, + SI2->getTrueValue(), V2Size); + if (Alias == MayAlias) + return MayAlias; + AliasResult ThisAlias = + aliasCheck(SI->getFalseValue(), SISize, + SI2->getFalseValue(), V2Size); + if (ThisAlias != Alias) + return MayAlias; + return Alias; + } + + // If both arms of the Select node NoAlias or MustAlias V2, then returns + // NoAlias / MustAlias. Otherwise, returns MayAlias. + AliasResult Alias = + aliasCheck(SI->getTrueValue(), SISize, V2, V2Size); + if (Alias == MayAlias) + return MayAlias; + AliasResult ThisAlias = + aliasCheck(SI->getFalseValue(), SISize, V2, V2Size); + if (ThisAlias != Alias) + return MayAlias; + return Alias; +} + // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction // against another. AliasAnalysis::AliasResult @@ -528,6 +567,28 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, if (!VisitedPHIs.insert(PN)) return MayAlias; + // If the values are PHIs in the same block, we can do a more precise + // as well as efficient check: just check for aliases between the values + // on corresponding edges. + if (const PHINode *PN2 = dyn_cast(V2)) + if (PN2->getParent() == PN->getParent()) { + AliasResult Alias = + aliasCheck(PN->getIncomingValue(0), PNSize, + PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), + V2Size); + if (Alias == MayAlias) + return MayAlias; + for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { + AliasResult ThisAlias = + aliasCheck(PN->getIncomingValue(i), PNSize, + PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), + V2Size); + if (ThisAlias != Alias) + return MayAlias; + } + return Alias; + } + SmallPtrSet UniqueSrc; SmallVector V1Srcs; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { @@ -542,7 +603,7 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, V1Srcs.push_back(PV1); } - AliasResult Alias = aliasCheck(V1Srcs[0], PNSize, V2, V2Size); + AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); // Early exit if the check of the first PHI source against V2 is MayAlias. // Other results are not possible. if (Alias == MayAlias) @@ -552,6 +613,12 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, // NoAlias / MustAlias. Otherwise, returns MayAlias. for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { Value *V = V1Srcs[i]; + + // If V2 is a PHI, the recursive case will have been caught in the + // above aliasCheck call, so these subsequent calls to aliasCheck + // don't need to assume that V2 is being visited recursively. + VisitedPHIs.erase(V2); + AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); if (ThisAlias != Alias || ThisAlias == MayAlias) return MayAlias; @@ -628,6 +695,13 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, if (const PHINode *PN = dyn_cast(V1)) return aliasPHI(PN, V1Size, V2, V2Size); + if (isa(V2) && !isa(V1)) { + std::swap(V1, V2); + std::swap(V1Size, V2Size); + } + if (const SelectInst *S1 = dyn_cast(V1)) + return aliasSelect(S1, V1Size, V2, V2Size); + return MayAlias; } diff --git a/test/Analysis/BasicAA/phi-and-select.ll b/test/Analysis/BasicAA/phi-and-select.ll new file mode 100644 index 00000000000..c69e824035a --- /dev/null +++ b/test/Analysis/BasicAA/phi-and-select.ll @@ -0,0 +1,73 @@ +; RUN: opt < %s -aa-eval -print-all-alias-modref-info -disable-output \ +; RUN: |& grep {NoAlias: double\\* \[%\]a, double\\* \[%\]b\$} | count 4 + +; BasicAA should detect NoAliases in PHIs and Selects. + +; Two PHIs in the same block. +define void @foo(i1 %m, double* noalias %x, double* noalias %y) { +entry: + br i1 %m, label %true, label %false + +true: + br label %exit + +false: + br label %exit + +exit: + %a = phi double* [ %x, %true ], [ %y, %false ] + %b = phi double* [ %x, %false ], [ %y, %true ] + volatile store double 0.0, double* %a + volatile store double 1.0, double* %b + ret void +} + +; Two selects with the same condition. +define void @bar(i1 %m, double* noalias %x, double* noalias %y) { +entry: + %a = select i1 %m, double* %x, double* %y + %b = select i1 %m, double* %y, double* %x + volatile store double 0.000000e+00, double* %a + volatile store double 1.000000e+00, double* %b + ret void +} + +; Two PHIs with disjoint sets of inputs. +define void @qux(i1 %m, double* noalias %x, double* noalias %y, + i1 %n, double* noalias %v, double* noalias %w) { +entry: + br i1 %m, label %true, label %false + +true: + br label %exit + +false: + br label %exit + +exit: + %a = phi double* [ %x, %true ], [ %y, %false ] + br i1 %n, label %ntrue, label %nfalse + +ntrue: + br label %nexit + +nfalse: + br label %nexit + +nexit: + %b = phi double* [ %v, %ntrue ], [ %w, %nfalse ] + volatile store double 0.0, double* %a + volatile store double 1.0, double* %b + ret void +} + +; Two selects with disjoint sets of arms. +define void @fin(i1 %m, double* noalias %x, double* noalias %y, + i1 %n, double* noalias %v, double* noalias %w) { +entry: + %a = select i1 %m, double* %x, double* %y + %b = select i1 %n, double* %v, double* %w + volatile store double 0.000000e+00, double* %a + volatile store double 1.000000e+00, double* %b + ret void +}