[llvm] r219223 - Two case switch to select optimization
Marcello Maggioni
hayarms at gmail.com
Tue Oct 7 11:16:44 PDT 2014
Author: mggm
Date: Tue Oct 7 13:16:44 2014
New Revision: 219223
URL: http://llvm.org/viewvc/llvm-project?rev=219223&view=rev
Log:
Two case switch to select optimization
This optimization tries to convert switch instructions that are used to select a value with only 2 unique cases + default block
to a select or a couple of selects (depending if the default block is reachable or not).
The typical case this optimization wants to be able to optimize is this one:
Example:
switch (a) {
case 10: %0 = icmp eq i32 %a, 10
return 10; %1 = select i1 %0, i32 10, i32 4
case 20: ----> %2 = icmp eq i32 %a, 20
return 2; %3 = select i1 %2, i32 2, i32 %1
default:
return 4;
}
It also sets the base for further optimizations that are planned and being reviewed.
Added:
llvm/trunk/test/Transforms/SimplifyCFG/switch-to-select-two-case.ll
Modified:
llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
llvm/trunk/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=219223&r1=219222&r2=219223&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Tue Oct 7 13:16:44 2014
@@ -76,6 +76,16 @@ STATISTIC(NumSinkCommons, "Number of com
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
namespace {
+ // The first field contains the value that the switch produces when a certain
+ // case group is selected, and the second field is a vector containing the cases
+ // composing the case group.
+ typedef SmallVector<std::pair<Constant *, SmallVector<ConstantInt *, 4>>, 2>
+ SwitchCaseResultVectorTy;
+ // The first field contains the phi node that generates a result of the switch
+ // and the second field contains the value generated for a certain case in the switch
+ // for that PHI.
+ typedef SmallVector<std::pair<PHINode *, Constant *>, 4> SwitchCaseResultsTy;
+
/// ValueEqualityComparisonCase - Represents a case of a switch.
struct ValueEqualityComparisonCase {
ConstantInt *Value;
@@ -3440,6 +3450,163 @@ GetCaseResults(SwitchInst *SI,
return Res.size() > 0;
}
+// MapCaseToResult - Helper function used to
+// add CaseVal to the list of cases that generate Result.
+static void MapCaseToResult(ConstantInt *CaseVal,
+ SwitchCaseResultVectorTy &UniqueResults,
+ Constant *Result) {
+ for (auto &I : UniqueResults) {
+ if (I.first == Result) {
+ I.second.push_back(CaseVal);
+ return;
+ }
+ }
+ UniqueResults.push_back(std::make_pair(Result,
+ SmallVector<ConstantInt*, 4>(1, CaseVal)));
+}
+
+// InitializeUniqueCases - Helper function that initializes a map containing
+// results for the PHI node of the common destination block for a switch
+// instruction. Returns false if multiple PHI nodes have been found or if
+// there is not a common destination block for the switch.
+static bool InitializeUniqueCases(
+ SwitchInst *SI, const DataLayout *DL, PHINode *&PHI,
+ BasicBlock *&CommonDest,
+ SwitchCaseResultVectorTy &UniqueResults,
+ Constant *&DefaultResult) {
+ for (auto &I : SI->cases()) {
+ ConstantInt *CaseVal = I.getCaseValue();
+
+ // Resulting value at phi nodes for this case value.
+ SwitchCaseResultsTy Results;
+ if (!GetCaseResults(SI, CaseVal, I.getCaseSuccessor(), &CommonDest, Results,
+ DL))
+ return false;
+
+ // Only one value per case is permitted
+ if (Results.size() > 1)
+ return false;
+ MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
+
+ // Check the PHI consistency.
+ if (!PHI)
+ PHI = Results[0].first;
+ else if (PHI != Results[0].first)
+ return false;
+ }
+ // Find the default result value.
+ SmallVector<std::pair<PHINode *, Constant *>, 1> DefaultResults;
+ BasicBlock *DefaultDest = SI->getDefaultDest();
+ GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResults,
+ DL);
+ // If the default value is not found abort unless the default destination
+ // is unreachable.
+ DefaultResult =
+ DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
+ if ((!DefaultResult &&
+ !isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())))
+ return false;
+
+ return true;
+}
+
+// ConvertTwoCaseSwitch - Helper function that checks if it is possible to
+// transform a switch with only two cases (or two cases + default)
+// that produces a result into a value select.
+// Example:
+// switch (a) {
+// case 10: %0 = icmp eq i32 %a, 10
+// return 10; %1 = select i1 %0, i32 10, i32 4
+// case 20: ----> %2 = icmp eq i32 %a, 20
+// return 2; %3 = select i1 %2, i32 2, i32 %1
+// default:
+// return 4;
+// }
+static Value *
+ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy &ResultVector,
+ Constant *DefaultResult, Value *Condition,
+ IRBuilder<> &Builder) {
+ assert(ResultVector.size() == 2 &&
+ "We should have exactly two unique results at this point");
+ // If we are selecting between only two cases transform into a simple
+ // select or a two-way select if default is possible.
+ if (ResultVector[0].second.size() == 1 &&
+ ResultVector[1].second.size() == 1) {
+ ConstantInt *const FirstCase = ResultVector[0].second[0];
+ ConstantInt *const SecondCase = ResultVector[1].second[0];
+
+ bool DefaultCanTrigger = DefaultResult;
+ Value *SelectValue = ResultVector[1].first;
+ if (DefaultCanTrigger) {
+ Value *const ValueCompare =
+ Builder.CreateICmpEQ(Condition, SecondCase, "switch.selectcmp");
+ SelectValue = Builder.CreateSelect(ValueCompare, ResultVector[1].first,
+ DefaultResult, "switch.select");
+ }
+ Value *const ValueCompare =
+ Builder.CreateICmpEQ(Condition, FirstCase, "switch.selectcmp");
+ return Builder.CreateSelect(ValueCompare, ResultVector[0].first, SelectValue,
+ "switch.select");
+ }
+
+ return nullptr;
+}
+
+// RemoveSwitchAfterSelectConversion - Helper function to cleanup a switch
+// instruction that has been converted into a select, fixing up PHI nodes and
+// basic blocks.
+static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
+ Value *SelectValue,
+ IRBuilder<> &Builder) {
+ BasicBlock *SelectBB = SI->getParent();
+ if (PHI->getBasicBlockIndex(SelectBB) >= 0)
+ PHI->removeIncomingValue(SelectBB);
+ PHI->addIncoming(SelectValue, SelectBB);
+
+ Builder.CreateBr(PHI->getParent());
+
+ // Remove the switch.
+ for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
+ BasicBlock *Succ = SI->getSuccessor(i);
+
+ if (Succ == PHI->getParent())
+ continue;
+ Succ->removePredecessor(SelectBB);
+ }
+ SI->eraseFromParent();
+}
+
+/// SwitchToSelect - If the switch is only used to initialize one or more
+/// phi nodes in a common successor block with only two different
+/// constant values, replace the switch with select.
+static bool SwitchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
+ const DataLayout *DL, AssumptionTracker *AT) {
+ Value *const Cond = SI->getCondition();
+ PHINode *PHI = nullptr;
+ BasicBlock *CommonDest = nullptr;
+ Constant *DefaultResult;
+ SwitchCaseResultVectorTy UniqueResults;
+ // Collect all the cases that will deliver the same value from the switch.
+ if (!InitializeUniqueCases(SI, DL, PHI, CommonDest, UniqueResults,
+ DefaultResult))
+ return false;
+ // Selects choose between maximum two values.
+ if (UniqueResults.size() != 2)
+ return false;
+ assert(PHI != nullptr && "PHI for value select not found");
+
+ Builder.SetInsertPoint(SI);
+ Value *SelectValue = ConvertTwoCaseSwitch(
+ UniqueResults,
+ DefaultResult, Cond, Builder);
+ if (SelectValue) {
+ RemoveSwitchAfterSelectConversion(SI, PHI, SelectValue, Builder);
+ return true;
+ }
+ // The switch couldn't be converted into a select.
+ return false;
+}
+
namespace {
/// SwitchLookupTable - This class represents a lookup table that can be used
/// to replace a switch.
@@ -3938,6 +4105,9 @@ bool SimplifyCFGOpt::SimplifySwitch(Swit
if (EliminateDeadSwitchCases(SI, DL, AT))
return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
+ if (SwitchToSelect(SI, Builder, DL, AT))
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
+
if (ForwardSwitchConditionToPHI(SI))
return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
Modified: llvm/trunk/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/UnreachableEliminate.ll?rev=219223&r1=219222&r2=219223&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/UnreachableEliminate.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/UnreachableEliminate.ll Tue Oct 7 13:16:44 2014
@@ -47,7 +47,7 @@ T:
}
; PR9450
-define i32 @test4(i32 %v) {
+define i32 @test4(i32 %v, i32 %w) {
; CHECK: entry:
; CHECK-NEXT: switch i32 %v, label %T [
; CHECK-NEXT: i32 3, label %V
@@ -67,7 +67,7 @@ SWITCH:
default:
unreachable
U:
- ret i32 1
+ ret i32 %w
T:
ret i32 2
}
Modified: llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll?rev=219223&r1=219222&r2=219223&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll Tue Oct 7 13:16:44 2014
@@ -915,8 +915,12 @@ return:
%x = phi i32 [ 3, %sw.default ], [ 7, %sw.bb1 ], [ 9, %entry ]
ret i32 %x
; CHECK-LABEL: @twocases(
-; CHECK: switch i32
+; CHECK-NOT: switch i32
; CHECK-NOT: @switch.table
+; CHECK: %switch.selectcmp
+; CHECK-NEXT: %switch.select
+; CHECK-NEXT: %switch.selectcmp1
+; CHECK-NEXT: %switch.select2
}
; Don't build tables for switches with TLS variables.
Added: llvm/trunk/test/Transforms/SimplifyCFG/switch-to-select-two-case.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/switch-to-select-two-case.ll?rev=219223&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/switch-to-select-two-case.ll (added)
+++ llvm/trunk/test/Transforms/SimplifyCFG/switch-to-select-two-case.ll Tue Oct 7 13:16:44 2014
@@ -0,0 +1,72 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; int foo1_with_default(int a) {
+; switch(a) {
+; case 10:
+; return 10;
+; case 20:
+; return 2;
+; }
+; return 4;
+; }
+
+define i32 @foo1_with_default(i32 %a) {
+; CHECK-LABEL: @foo1_with_default
+; CHECK: %switch.selectcmp = icmp eq i32 %a, 20
+; CHECK-NEXT: %switch.select = select i1 %switch.selectcmp, i32 2, i32 4
+; CHECK-NEXT: %switch.selectcmp1 = icmp eq i32 %a, 10
+; CHECK-NEXT: %switch.select2 = select i1 %switch.selectcmp1, i32 10, i32 %switch.select
+entry:
+ switch i32 %a, label %sw.epilog [
+ i32 10, label %sw.bb
+ i32 20, label %sw.bb1
+ ]
+
+sw.bb:
+ br label %return
+
+sw.bb1:
+ br label %return
+
+sw.epilog:
+ br label %return
+
+return:
+ %retval.0 = phi i32 [ 4, %sw.epilog ], [ 2, %sw.bb1 ], [ 10, %sw.bb ]
+ ret i32 %retval.0
+}
+
+; int foo1_without_default(int a) {
+; switch(a) {
+; case 10:
+; return 10;
+; case 20:
+; return 2;
+; }
+; __builtin_unreachable();
+; }
+
+define i32 @foo1_without_default(i32 %a) {
+; CHECK-LABEL: @foo1_without_default
+; CHECK: %switch.selectcmp = icmp eq i32 %a, 10
+; CHECK-NEXT: %switch.select = select i1 %switch.selectcmp, i32 10, i32 2
+; CHECK-NOT: %switch.selectcmp1
+entry:
+ switch i32 %a, label %sw.epilog [
+ i32 10, label %sw.bb
+ i32 20, label %sw.bb1
+ ]
+
+sw.bb:
+ br label %return
+
+sw.bb1:
+ br label %return
+
+sw.epilog:
+ unreachable
+
+return:
+ %retval.0 = phi i32 [ 2, %sw.bb1 ], [ 10, %sw.bb ]
+ ret i32 %retval.0
+}
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