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[LoopUnswitch] Code refactoring to separate trivial loop unswitch and…
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… non-trivial loop unswitch in processCurrentLoop()

Summary: The current code in LoopUnswtich::processCurrentLoop() mixes trivial loop unswitch and non-trivial loop unswitch together. It goes over all basic blocks in the loop and checks if a condition is trivial or non-trivial unswitch condition. However, trivial unswitch condition can only occur in the loop header basic block (where it controls whether or not the loop does something at all). This refactoring separate trivial loop unswitch and non-trivial loop unswitch. Before going over all basic blocks in the loop, it checks if the loop header contains a trivial unswitch condition. If so, unswitch it. Otherwise, go over all blocks like before but don't check trivial condition any more since they are not possible to be in the other blocks. This code has no functionality change.

Reviewers: meheff, reames, broune

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11276

llvm-svn: 242873
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@lic9
lic9 committed Jul 22, 2015
1 parent e345173 commit c0f3a15
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Showing 2 changed files with 140 additions and 96 deletions.
208 changes: 112 additions & 96 deletions llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
View file
Expand Up @@ -212,6 +212,8 @@ namespace {
/// Update the appropriate Phi nodes as we do so.
void SplitExitEdges(Loop *L, const SmallVectorImpl<BasicBlock *> &ExitBlocks);

bool TryTrivialLoopUnswitch(bool &Changed);

bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,
TerminatorInst *TI = nullptr);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
Expand All @@ -229,9 +231,6 @@ namespace {
TerminatorInst *TI);

void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = nullptr,
BasicBlock **LoopExit = nullptr);

};
}

Expand Down Expand Up @@ -460,6 +459,13 @@ bool LoopUnswitch::processCurrentLoop() {
AC))
return false;

// Trivial unswitch condition can only occur at loop header basic block because
// that condition needs to control whether or not the loop does anything at all.
// Try trivial unswitch first before loop over other basic blocks in the loop.
if (TryTrivialLoopUnswitch(Changed)) {
return true;
}

// Loop over all of the basic blocks in the loop. If we find an interior
// block that is branching on a loop-invariant condition, we can unswitch this
// loop.
Expand Down Expand Up @@ -578,105 +584,12 @@ static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
return nullptr;
}

/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
/// trivial: that is, that the condition controls whether or not the loop does
/// anything at all. If this is a trivial condition, unswitching produces no
/// code duplications (equivalently, it produces a simpler loop and a new empty
/// loop, which gets deleted).
///
/// If this is a trivial condition, return true, otherwise return false. When
/// returning true, this sets Cond and Val to the condition that controls the
/// trivial condition: when Cond dynamically equals Val, the loop is known to
/// exit. Finally, this sets LoopExit to the BB that the loop exits to when
/// Cond == Val.
///
bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
BasicBlock **LoopExit) {
BasicBlock *Header = currentLoop->getHeader();
TerminatorInst *HeaderTerm = Header->getTerminator();
LLVMContext &Context = Header->getContext();

BasicBlock *LoopExitBB = nullptr;
if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
// If the header block doesn't end with a conditional branch on Cond, we
// can't handle it.
if (!BI->isConditional() || BI->getCondition() != Cond)
return false;

// Check to see if a successor of the branch is guaranteed to
// exit through a unique exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(0)))) {
if (Val) *Val = ConstantInt::getTrue(Context);
} else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(1)))) {
if (Val) *Val = ConstantInt::getFalse(Context);
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
// If this isn't a switch on Cond, we can't handle it.
if (SI->getCondition() != Cond) return false;

// Check to see if a successor of the switch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
// Note that we can't trivially unswitch on the default case or
// on already unswitched cases.
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
BasicBlock *LoopExitCandidate;
if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
i.getCaseSuccessor()))) {
// Okay, we found a trivial case, remember the value that is trivial.
ConstantInt *CaseVal = i.getCaseValue();

// Check that it was not unswitched before, since already unswitched
// trivial vals are looks trivial too.
if (BranchesInfo.isUnswitched(SI, CaseVal))
continue;
LoopExitBB = LoopExitCandidate;
if (Val) *Val = CaseVal;
break;
}
}
} else
return false;

// If we didn't find a single unique LoopExit block, or if the loop exit block
// contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.

if (LoopExit) *LoopExit = LoopExitBB;

// We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any
// side-effecting instructions (e.g. stores, calls, volatile loads) in the
// part of the loop that the code *would* execute. We already checked the
// tail, check the header now.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
if (I->mayHaveSideEffects())
return false;
return true;
}

/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
/// unswitch the loop, reprocess the pieces, then return true.
bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,
TerminatorInst *TI) {
Function *F = loopHeader->getParent();
Constant *CondVal = nullptr;
BasicBlock *ExitBlock = nullptr;

if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
// If the condition is trivial, always unswitch. There is no code growth
// for this case.
UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock, TI);
return true;
}

// Check to see if it would be profitable to unswitch current loop.
if (!BranchesInfo.CostAllowsUnswitching()) {
Expand Down Expand Up @@ -830,6 +743,109 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
++NumTrivial;
}

/// TryTrivialLoopUnswitch - Check if loop header block's terminator is a trivial
/// unswitch condition: that is, the condition controls whether or not the loop
/// does anything at all (which means it can only occur in loop header). If it is
/// a trivial condition, unswitching produces no code duplications (equivalently,
/// it produces a simpler loop and a new empty loop, which gets deleted). Therefore
/// always unswitch trivial condition.
///
bool LoopUnswitch::TryTrivialLoopUnswitch(bool &Changed) {
BasicBlock *Header = currentLoop->getHeader();
TerminatorInst *HeaderTerm = Header->getTerminator();
LLVMContext &Context = Header->getContext();

// Check if this loop will execute any side-effecting instructions (e.g.
// stores, calls, volatile loads) in the part of the loop that the code
// *would* execute. Check the header first.
for (BasicBlock::iterator I :*Header)
if (I->mayHaveSideEffects())
return false;

// CondVal is the condition that controls the trivial condition.
// LoopExitBB is the BasicBlock that loop exits when meets trivial condition.
Constant *CondVal = nullptr;
BasicBlock *LoopExitBB = nullptr;

if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
// If this isn't branching on an invariant condition, we can't unswitch it.
if (!BI->isConditional())
return false;

Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
currentLoop, Changed);

// Unswitch only if the trivial condition itself is an LIV (not
// partial LIV which could occur in and/or)
if (!LoopCond || LoopCond != BI->getCondition())
return false;

// Check to see if a successor of the branch is guaranteed to
// exit through a unique exit block without having any
// side-effects. If so, determine the value of Cond that causes
// it to do this.
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(0)))) {
CondVal = ConstantInt::getTrue(Context);
} else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(1)))) {
CondVal = ConstantInt::getFalse(Context);
}

// If we didn't find a single unique LoopExit block, or if the loop exit block
// contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.

UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, HeaderTerm);
++NumBranches;
return true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
// If this isn't switching on an invariant condition, we can't unswitch it.
Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
currentLoop, Changed);

// Unswitch only if the trivial condition itself is an LIV (not
// partial LIV which could occur in and/or)
if (!LoopCond || LoopCond != SI->getCondition())
return false;

// Check to see if a successor of the switch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
// Note that we can't trivially unswitch on the default case or
// on already unswitched cases.
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
BasicBlock *LoopExitCandidate;
if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
i.getCaseSuccessor()))) {
// Okay, we found a trivial case, remember the value that is trivial.
ConstantInt *CaseVal = i.getCaseValue();

// Check that it was not unswitched before, since already unswitched
// trivial vals are looks trivial too.
if (BranchesInfo.isUnswitched(SI, CaseVal))
continue;
LoopExitBB = LoopExitCandidate;
CondVal = CaseVal;
break;
}
}

// If we didn't find a single unique LoopExit block, or if the loop exit block
// contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.

UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, nullptr);
++NumSwitches;
return true;
}
return false;
}

/// SplitExitEdges - Split all of the edges from inside the loop to their exit
/// blocks. Update the appropriate Phi nodes as we do so.
void LoopUnswitch::SplitExitEdges(Loop *L,
Expand Down
28 changes: 28 additions & 0 deletions llvm/test/Transforms/LoopUnswitch/LIV-loop-condtion.ll
View file
@@ -0,0 +1,28 @@
; RUN: opt < %s -loop-unswitch -loop-unswitch-threshold=0 -S 2>&1 | FileCheck %s

; This is to test trivial loop unswitch only happens when trivial condition
; itself is an LIV loop condition (not partial LIV which could occur in and/or).

define i32 @test(i1 %cond1, i32 %var1) {
entry:
br label %loop_begin

loop_begin:
%var3 = phi i32 [%var1, %entry], [%var2, %do_something]
%cond2 = icmp eq i32 %var3, 10
%cond.and = and i1 %cond1, %cond2

; %cond.and only has %cond1 as LIV so no unswitch should happen.
; CHECK: br i1 %cond.and, label %do_something, label %loop_exit
br i1 %cond.and, label %do_something, label %loop_exit

do_something:
%var2 = add i32 %var3, 1
call void @some_func() noreturn nounwind
br label %loop_begin

loop_exit:
ret i32 0
}

declare void @some_func() noreturn

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