[llvm] r211038 - LowerSwitch: track bounding range for the condition tree.
Jim Grosbach
grosbach at apple.com
Mon Jun 16 09:55:21 PDT 2014
Author: grosbach
Date: Mon Jun 16 11:55:20 2014
New Revision: 211038
URL: http://llvm.org/viewvc/llvm-project?rev=211038&view=rev
Log:
LowerSwitch: track bounding range for the condition tree.
When LowerSwitch transforms a switch instruction into a tree of ifs it
is actually performing a binary search into the various case ranges, to
see if the current value falls into one cases range of values.
So, if we have a program with something like this:
switch (a) {
case 0:
do0();
break;
case 1:
do1();
break;
case 2:
do2();
break;
default:
break;
}
the code produced is something like this:
if (a < 1) {
if (a == 0) {
do0();
}
} else {
if (a < 2) {
if (a == 1) {
do1();
}
} else {
if (a == 2) {
do2();
}
}
}
This code is inefficient because the check (a == 1) to execute do1() is
not needed.
The reason is that because we already checked that (a >= 1) initially by
checking that also (a < 2) we basically already inferred that (a == 1)
without the need of an extra basic block spawned to check if actually (a
== 1).
The patch addresses this problem by keeping track of already
checked bounds in the LowerSwitch algorithm, so that when the time
arrives to produce a Leaf Block that checks the equality with the case
value / range the algorithm can decide if that block is really needed
depending on the already checked bounds .
For example, the above with "a = 1" would work like this:
the bounds start as LB: NONE , UB: NONE
as (a < 1) is emitted the bounds for the else path become LB: 1 UB:
NONE. This happens because by failing the test (a < 1) we know that the
value "a" cannot be smaller than 1 if we enter the else branch.
After the emitting the check (a < 2) the bounds in the if branch become
LB: 1 UB: 1. This is because by checking that "a" is smaller than 2 then
the upper bound becomes 2 - 1 = 1.
When it is time to emit the leaf block for "case 1:" we notice that 1
can be squeezed exactly in between the LB and UB, which means that if we
arrived to that block there is no need to emit a block that checks if (a
== 1).
Patch by: Marcello Maggioni <hayarms at gmail.com>
Added:
llvm/trunk/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll
llvm/trunk/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
Modified:
llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp
llvm/trunk/test/Transforms/LowerSwitch/feature.ll
Modified: llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp?rev=211038&r1=211037&r2=211038&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp Mon Jun 16 11:55:20 2014
@@ -14,11 +14,13 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/CFG.h"
#include "llvm/Pass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
@@ -58,16 +60,18 @@ namespace {
Low(low), High(high), BB(bb) { }
};
- typedef std::vector<CaseRange> CaseVector;
+ typedef std::vector<CaseRange> CaseVector;
typedef std::vector<CaseRange>::iterator CaseItr;
private:
void processSwitchInst(SwitchInst *SI);
- BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
- BasicBlock* OrigBlock, BasicBlock* Default);
- BasicBlock* newLeafBlock(CaseRange& Leaf, Value* Val,
- BasicBlock* OrigBlock, BasicBlock* Default);
- unsigned Clusterify(CaseVector& Cases, SwitchInst *SI);
+ BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
+ ConstantInt *LowerBound, ConstantInt *UpperBound,
+ Value *Val, BasicBlock *OrigBlock,
+ BasicBlock *Default);
+ BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val, BasicBlock *OrigBlock,
+ BasicBlock *Default);
+ unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
};
/// The comparison function for sorting the switch case values in the vector.
@@ -129,15 +133,26 @@ static raw_ostream& operator<<(raw_ostre
// switchConvert - Convert the switch statement into a binary lookup of
// the case values. The function recursively builds this tree.
-//
-BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
- Value* Val, BasicBlock* OrigBlock,
- BasicBlock* Default)
-{
+// LowerBound and UpperBound are used to keep track of the bounds for Val
+// that have already been checked by a block emitted by one of the previous
+// calls to switchConvert in the call stack.
+BasicBlock *LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
+ ConstantInt *LowerBound,
+ ConstantInt *UpperBound, Value *Val,
+ BasicBlock *OrigBlock,
+ BasicBlock *Default) {
unsigned Size = End - Begin;
- if (Size == 1)
+ if (Size == 1) {
+ // Check if the Case Range is perfectly squeezed in between
+ // already checked Upper and Lower bounds. If it is then we can avoid
+ // emitting the code that checks if the value actually falls in the range
+ // because the bounds already tell us so.
+ if (Begin->Low == LowerBound && Begin->High == UpperBound) {
+ return Begin->BB;
+ }
return newLeafBlock(*Begin, Val, OrigBlock, Default);
+ }
unsigned Mid = Size / 2;
std::vector<CaseRange> LHS(Begin, Begin + Mid);
@@ -145,15 +160,50 @@ BasicBlock* LowerSwitch::switchConvert(C
std::vector<CaseRange> RHS(Begin + Mid, End);
DEBUG(dbgs() << "RHS: " << RHS << "\n");
- CaseRange& Pivot = *(Begin + Mid);
- DEBUG(dbgs() << "Pivot ==> "
- << cast<ConstantInt>(Pivot.Low)->getValue() << " -"
- << cast<ConstantInt>(Pivot.High)->getValue() << "\n");
-
- BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
- OrigBlock, Default);
- BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
- OrigBlock, Default);
+ CaseRange &Pivot = *(Begin + Mid);
+ DEBUG(dbgs() << "Pivot ==> "
+ << cast<ConstantInt>(Pivot.Low)->getValue()
+ << " -" << cast<ConstantInt>(Pivot.High)->getValue() << "\n");
+
+ // NewLowerBound here should never be the integer minimal value.
+ // This is because it is computed from a case range that is never
+ // the smallest, so there is always a case range that has at least
+ // a smaller value.
+ ConstantInt *NewLowerBound = cast<ConstantInt>(Pivot.Low);
+ ConstantInt *NewUpperBound;
+
+ // If we don't have a Default block then it means that we can never
+ // have a value outside of a case range, so set the UpperBound to the highest
+ // value in the LHS part of the case ranges.
+ if (Default != nullptr) {
+ // Because NewLowerBound is never the smallest representable integer
+ // it is safe here to subtract one.
+ NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
+ NewLowerBound->getValue() - 1);
+ } else {
+ CaseItr LastLHS = LHS.begin() + LHS.size() - 1;
+ NewUpperBound = cast<ConstantInt>(LastLHS->High);
+ }
+
+ DEBUG(dbgs() << "LHS Bounds ==> ";
+ if (LowerBound) {
+ dbgs() << cast<ConstantInt>(LowerBound)->getSExtValue();
+ } else {
+ dbgs() << "NONE";
+ }
+ dbgs() << " - " << NewUpperBound->getSExtValue() << "\n";
+ dbgs() << "RHS Bounds ==> ";
+ dbgs() << NewLowerBound->getSExtValue() << " - ";
+ if (UpperBound) {
+ dbgs() << cast<ConstantInt>(UpperBound)->getSExtValue() << "\n";
+ } else {
+ dbgs() << "NONE\n";
+ });
+
+ BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
+ NewUpperBound, Val, OrigBlock, Default);
+ BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
+ UpperBound, Val, OrigBlock, Default);
// Create a new node that checks if the value is < pivot. Go to the
// left branch if it is and right branch if not.
@@ -291,13 +341,19 @@ void LowerSwitch::processSwitchInst(Swit
return;
}
+ const bool DefaultIsUnreachable =
+ Default->size() == 1 && isa<UnreachableInst>(Default->getTerminator());
// Create a new, empty default block so that the new hierarchy of
// if-then statements go to this and the PHI nodes are happy.
- BasicBlock* NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
- F->getBasicBlockList().insert(Default, NewDefault);
-
- BranchInst::Create(Default, NewDefault);
+ // if the default block is set as an unreachable we avoid creating one
+ // because will never be a valid target.
+ BasicBlock *NewDefault = nullptr;
+ if (!DefaultIsUnreachable) {
+ NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
+ F->getBasicBlockList().insert(Default, NewDefault);
+ BranchInst::Create(Default, NewDefault);
+ }
// If there is an entry in any PHI nodes for the default edge, make sure
// to update them as well.
for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
@@ -316,12 +372,31 @@ void LowerSwitch::processSwitchInst(Swit
DEBUG(dbgs() << "Cases: " << Cases << "\n");
(void)numCmps;
- BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
- OrigBlock, NewDefault);
+ ConstantInt *UpperBound = nullptr;
+ ConstantInt *LowerBound = nullptr;
+
+ // Optimize the condition where Default is an unreachable block. In this case
+ // we can make the bounds tightly fitted around the case value ranges,
+ // because we know that the value passed to the switch should always be
+ // exactly one of the case values.
+ if (DefaultIsUnreachable) {
+ CaseItr LastCase = Cases.begin() + Cases.size() - 1;
+ UpperBound = cast<ConstantInt>(LastCase->High);
+ LowerBound = cast<ConstantInt>(Cases.begin()->Low);
+ }
+ BasicBlock *SwitchBlock =
+ switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
+ OrigBlock, NewDefault);
// Branch to our shiny new if-then stuff...
BranchInst::Create(SwitchBlock, OrigBlock);
// We are now done with the switch instruction, delete it.
CurBlock->getInstList().erase(SI);
+
+ pred_iterator PI = pred_begin(Default), E = pred_end(Default);
+ // If the Default block has no more predecessors just remove it
+ if (PI == E) {
+ DeleteDeadBlock(Default);
+ }
}
Added: llvm/trunk/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll?rev=211038&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll (added)
+++ llvm/trunk/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll Mon Jun 16 11:55:20 2014
@@ -0,0 +1,27 @@
+; RUN: opt < %s -lowerswitch -S | FileCheck %s
+; CHECK-NOT: icmp eq i32 %0, 1
+
+define i32 @foo(i32 %a) #0 {
+entry:
+ %retval = alloca i32, align 4
+ %a.addr = alloca i32, align 4
+ store i32 %a, i32* %a.addr, align 4
+ %0 = load i32* %a.addr, align 4
+ switch i32 %0, label %sw.default [
+ i32 0, label %sw.bb
+ i32 1, label %sw.bb1
+ i32 2, label %sw.bb2
+ ]
+
+sw.bb:
+ ret i32 12
+
+sw.bb1:
+ ret i32 4
+
+sw.bb2:
+ ret i32 2
+
+sw.default:
+ ret i32 9
+}
Added: llvm/trunk/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll?rev=211038&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll (added)
+++ llvm/trunk/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll Mon Jun 16 11:55:20 2014
@@ -0,0 +1,41 @@
+; RUN: opt < %s -lowerswitch -S | FileCheck %s
+; CHECK-NOT: {{.*}}icmp eq{{.*}}
+;
+;int foo(int a) {
+;
+; switch (a) {
+; case 0:
+; return 10;
+; case 1:
+; return 3;
+; default:
+; __builtin_unreachable();
+; }
+;
+;}
+
+define i32 @foo(i32 %a) nounwind ssp uwtable {
+ %1 = alloca i32, align 4
+ %2 = alloca i32, align 4
+ store i32 %a, i32* %2, align 4
+ %3 = load i32* %2, align 4
+ switch i32 %3, label %6 [
+ i32 0, label %4
+ i32 1, label %5
+ ]
+
+; <label>:4
+ store i32 10, i32* %1
+ br label %7
+
+; <label>:5
+ store i32 3, i32* %1
+ br label %7
+
+; <label>:6
+ unreachable
+
+; <label>:7
+ %8 = load i32* %1
+ ret i32 %8
+}
Modified: llvm/trunk/test/Transforms/LowerSwitch/feature.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LowerSwitch/feature.ll?rev=211038&r1=211037&r2=211038&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LowerSwitch/feature.ll (original)
+++ llvm/trunk/test/Transforms/LowerSwitch/feature.ll Mon Jun 16 11:55:20 2014
@@ -3,93 +3,57 @@
; We have switch on input.
; On output we should got binary comparison tree. Check that all is fine.
-;CHECK: entry:
-;CHECK-NEXT: br label %NodeBlock37
+;CHECK: entry:
+;CHECK-NEXT: br label %NodeBlock19
-;CHECK: NodeBlock37: ; preds = %entry
-;CHECK-NEXT: %Pivot38 = icmp slt i32 %tmp158, 10
-;CHECK-NEXT: br i1 %Pivot38, label %NodeBlock13, label %NodeBlock35
-
-;CHECK: NodeBlock35: ; preds = %NodeBlock37
-;CHECK-NEXT: %Pivot36 = icmp slt i32 %tmp158, 13
-;CHECK-NEXT: br i1 %Pivot36, label %NodeBlock23, label %NodeBlock33
-
-;CHECK: NodeBlock33: ; preds = %NodeBlock35
-;CHECK-NEXT: %Pivot34 = icmp slt i32 %tmp158, 14
-;CHECK-NEXT: br i1 %Pivot34, label %LeafBlock25, label %NodeBlock31
-
-;CHECK: NodeBlock31: ; preds = %NodeBlock33
-;CHECK-NEXT: %Pivot32 = icmp slt i32 %tmp158, 15
-;CHECK-NEXT: br i1 %Pivot32, label %LeafBlock27, label %LeafBlock29
-
-;CHECK: LeafBlock29: ; preds = %NodeBlock31
-;CHECK-NEXT: %SwitchLeaf30 = icmp eq i32 %tmp158, 15
-;CHECK-NEXT: br i1 %SwitchLeaf30, label %bb334, label %NewDefault
-
-;CHECK: LeafBlock27: ; preds = %NodeBlock31
-;CHECK-NEXT: %SwitchLeaf28 = icmp eq i32 %tmp158, 14
-;CHECK-NEXT: br i1 %SwitchLeaf28, label %bb332, label %NewDefault
-
-;CHECK: LeafBlock25: ; preds = %NodeBlock33
-;CHECK-NEXT: %SwitchLeaf26 = icmp eq i32 %tmp158, 13
-;CHECK-NEXT: br i1 %SwitchLeaf26, label %bb330, label %NewDefault
-
-;CHECK: NodeBlock23: ; preds = %NodeBlock35
-;CHECK-NEXT: %Pivot24 = icmp slt i32 %tmp158, 11
-;CHECK-NEXT: br i1 %Pivot24, label %LeafBlock15, label %NodeBlock21
-
-;CHECK: NodeBlock21: ; preds = %NodeBlock23
-;CHECK-NEXT: %Pivot22 = icmp slt i32 %tmp158, 12
-;CHECK-NEXT: br i1 %Pivot22, label %LeafBlock17, label %LeafBlock19
-
-;CHECK: LeafBlock19: ; preds = %NodeBlock21
-;CHECK-NEXT: %SwitchLeaf20 = icmp eq i32 %tmp158, 12
-;CHECK-NEXT: br i1 %SwitchLeaf20, label %bb328, label %NewDefault
-
-;CHECK: LeafBlock17: ; preds = %NodeBlock21
-;CHECK-NEXT: %SwitchLeaf18 = icmp eq i32 %tmp158, 11
-;CHECK-NEXT: br i1 %SwitchLeaf18, label %bb326, label %NewDefault
-
-;CHECK: LeafBlock15: ; preds = %NodeBlock23
-;CHECK-NEXT: %SwitchLeaf16 = icmp eq i32 %tmp158, 10
-;CHECK-NEXT: br i1 %SwitchLeaf16, label %bb324, label %NewDefault
-
-;CHECK: NodeBlock13: ; preds = %NodeBlock37
-;CHECK-NEXT: %Pivot14 = icmp slt i32 %tmp158, 7
-;CHECK-NEXT: br i1 %Pivot14, label %NodeBlock, label %NodeBlock11
-
-;CHECK: NodeBlock11: ; preds = %NodeBlock13
-;CHECK-NEXT: %Pivot12 = icmp slt i32 %tmp158, 8
-;CHECK-NEXT: br i1 %Pivot12, label %LeafBlock3, label %NodeBlock9
-
-;CHECK: NodeBlock9: ; preds = %NodeBlock11
-;CHECK-NEXT: %Pivot10 = icmp slt i32 %tmp158, 9
-;CHECK-NEXT: br i1 %Pivot10, label %LeafBlock5, label %LeafBlock7
-
-;CHECK: LeafBlock7: ; preds = %NodeBlock9
-;CHECK-NEXT: %SwitchLeaf8 = icmp eq i32 %tmp158, 9
-;CHECK-NEXT: br i1 %SwitchLeaf8, label %bb322, label %NewDefault
-
-;CHECK: LeafBlock5: ; preds = %NodeBlock9
-;CHECK-NEXT: %SwitchLeaf6 = icmp eq i32 %tmp158, 8
-;CHECK-NEXT: br i1 %SwitchLeaf6, label %bb338, label %NewDefault
-
-;CHECK: LeafBlock3: ; preds = %NodeBlock11
-;CHECK-NEXT: %SwitchLeaf4 = icmp eq i32 %tmp158, 7
-;CHECK-NEXT: br i1 %SwitchLeaf4, label %bb, label %NewDefault
-
-;CHECK: NodeBlock: ; preds = %NodeBlock13
-;CHECK-NEXT: %Pivot = icmp slt i32 %tmp158, 0
-;CHECK-NEXT: br i1 %Pivot, label %LeafBlock, label %LeafBlock1
-
-;CHECK: LeafBlock1: ; preds = %NodeBlock
-;CHECK-NEXT: %SwitchLeaf2 = icmp ule i32 %tmp158, 6
-;CHECK-NEXT: br i1 %SwitchLeaf2, label %bb338, label %NewDefault
-
-;CHECK: LeafBlock: ; preds = %NodeBlock
-;CHECK-NEXT: %tmp158.off = add i32 %tmp158, 6
-;CHECK-NEXT: %SwitchLeaf = icmp ule i32 %tmp158.off, 4
-;CHECK-NEXT: br i1 %SwitchLeaf, label %bb338, label %NewDefault
+;CHECK: NodeBlock19: ; preds = %entry
+;CHECK-NEXT: %Pivot20 = icmp slt i32 %tmp158, 10
+;CHECK-NEXT: br i1 %Pivot20, label %NodeBlock5, label %NodeBlock17
+
+;CHECK: NodeBlock17: ; preds = %NodeBlock19
+;CHECK-NEXT: %Pivot18 = icmp slt i32 %tmp158, 13
+;CHECK-NEXT: br i1 %Pivot18, label %NodeBlock9, label %NodeBlock15
+
+;CHECK: NodeBlock15: ; preds = %NodeBlock17
+;CHECK-NEXT: %Pivot16 = icmp slt i32 %tmp158, 14
+;CHECK-NEXT: br i1 %Pivot16, label %bb330, label %NodeBlock13
+
+;CHECK: NodeBlock13: ; preds = %NodeBlock15
+;CHECK-NEXT: %Pivot14 = icmp slt i32 %tmp158, 15
+;CHECK-NEXT: br i1 %Pivot14, label %bb332, label %LeafBlock11
+
+;CHECK: LeafBlock11: ; preds = %NodeBlock13
+;CHECK-NEXT: %SwitchLeaf12 = icmp eq i32 %tmp158, 15
+;CHECK-NEXT: br i1 %SwitchLeaf12, label %bb334, label %NewDefault
+
+;CHECK: NodeBlock9: ; preds = %NodeBlock17
+;CHECK-NEXT: %Pivot10 = icmp slt i32 %tmp158, 11
+;CHECK-NEXT: br i1 %Pivot10, label %bb324, label %NodeBlock7
+
+;CHECK: NodeBlock7: ; preds = %NodeBlock9
+;CHECK-NEXT: %Pivot8 = icmp slt i32 %tmp158, 12
+;CHECK-NEXT: br i1 %Pivot8, label %bb326, label %bb328
+
+;CHECK: NodeBlock5: ; preds = %NodeBlock19
+;CHECK-NEXT: %Pivot6 = icmp slt i32 %tmp158, 7
+;CHECK-NEXT: br i1 %Pivot6, label %NodeBlock, label %NodeBlock3
+
+;CHECK: NodeBlock3: ; preds = %NodeBlock5
+;CHECK-NEXT: %Pivot4 = icmp slt i32 %tmp158, 8
+;CHECK-NEXT: br i1 %Pivot4, label %bb, label %NodeBlock1
+
+;CHECK: NodeBlock1: ; preds = %NodeBlock3
+;CHECK-NEXT: %Pivot2 = icmp slt i32 %tmp158, 9
+;CHECK-NEXT: br i1 %Pivot2, label %bb338, label %bb322
+
+;CHECK: NodeBlock: ; preds = %NodeBlock5
+;CHECK-NEXT: %Pivot = icmp slt i32 %tmp158, 0
+;CHECK-NEXT: br i1 %Pivot, label %LeafBlock, label %bb338
+
+;CHECK: LeafBlock: ; preds = %NodeBlock
+;CHECK-NEXT: %tmp158.off = add i32 %tmp158, 6
+;CHECK-NEXT: %SwitchLeaf = icmp ule i32 %tmp158.off, 4
+;CHECK-NEXT: br i1 %SwitchLeaf, label %bb338, label %NewDefault
define i32 @main(i32 %tmp158) {
entry:
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