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Improve 'tail' call marking in TRE. A bootstrap of clang goes from 375k calls marked tail in the IR to 470k, however this improvement does not carry into an improvement of the call/jmp ratio on x86. The most common pattern is a tail call + br to a block with nothing but a 'ret'.

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The number of tail call to loop conversions remains the same (1618 by my count).

The new algorithm does a local scan over the use-def chains to identify local "alloca-derived" values, as well as points where the alloca could escape. Then, a visit over the CFG marks blocks as being before or after the allocas have escaped, and annotates the calls accordingly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208017 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2014-05-05 23:59:03 +00:00
parent 59c397de1a
commit 05da4dd998
3 changed files with 275 additions and 79 deletions
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5
include/llvm/IR/CallSite.h
View File

@ -166,6 +166,11 @@ public:
return isCall() && cast<CallInst>(getInstruction())->isMustTailCall();
}
/// \brief Tests if this call site is marked as a tail call.
bool isTailCall() const {
return isCall() && cast<CallInst>(getInstruction())->isTailCall();
}
#define CALLSITE_DELEGATE_GETTER(METHOD) \
InstrTy *II = getInstruction(); \
return isCall() \

326
lib/Transforms/Scalar/TailRecursionElimination.cpp
View File

@ -55,6 +55,7 @@
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
@ -95,6 +96,9 @@ namespace {
bool runOnFunction(Function &F) override;
private:
bool runTRE(Function &F);
bool markTails(Function &F, bool &AllCallsAreTailCalls);
CallInst *FindTRECandidate(Instruction *I,
bool CannotTailCallElimCallsMarkedTail);
bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
@ -146,35 +150,232 @@ static bool CanTRE(AllocaInst *AI) {
isa<ConstantInt>(AI->getArraySize());
}
namespace {
struct AllocaCaptureTracker : public CaptureTracker {
AllocaCaptureTracker() : Captured(false) {}
void tooManyUses() override { Captured = true; }
bool shouldExplore(const Use *U) override {
Value *V = U->getUser();
if (isa<CallInst>(V) || isa<InvokeInst>(V))
UsesAlloca.insert(V);
return true;
}
bool captured(const Use *U) override {
if (isa<ReturnInst>(U->getUser()))
return false;
Captured = true;
return true;
}
bool Captured;
SmallPtrSet<const Value *, 16> UsesAlloca;
};
} // end anonymous namespace
bool TailCallElim::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
bool AllCallsAreTailCalls = false;
bool Modified = markTails(F, AllCallsAreTailCalls);
if (AllCallsAreTailCalls)
Modified |= runTRE(F);
return Modified;
}
namespace {
struct AllocaDerivedValueTracker {
// Start at a root value and walk its use-def chain to mark calls that use the
// value or a derived value in AllocaUsers, and places where it may escape in
// EscapePoints.
void walk(Value *Root) {
SmallVector<Use *, 32> Worklist;
SmallPtrSet<Use *, 32> Visited;
auto AddUsesToWorklist = [&](Value *V) {
for (auto &U : V->uses()) {
if (!Visited.insert(&U))
continue;
Worklist.push_back(&U);
}
};
AddUsesToWorklist(Root);
while (!Worklist.empty()) {
Use *U = Worklist.pop_back_val();
Instruction *I = cast<Instruction>(U->getUser());
switch (I->getOpcode()) {
case Instruction::Call:
case Instruction::Invoke: {
CallSite CS(I);
bool IsNocapture = !CS.isCallee(U) &&
CS.doesNotCapture(CS.getArgumentNo(U));
callUsesLocalStack(CS, IsNocapture);
if (IsNocapture) {
// If the alloca-derived argument is passed in as nocapture, then it
// can't propagate to the call's return. That would be capturing.
continue;
}
break;
}
case Instruction::Load: {
// The result of a load is not alloca-derived (unless an alloca has
// otherwise escaped, but this is a local analysis).
continue;
}
case Instruction::Store: {
if (U->getOperandNo() == 0)
EscapePoints.insert(I);
continue; // Stores have no users to analyze.
}
case Instruction::BitCast:
case Instruction::GetElementPtr:
case Instruction::PHI:
case Instruction::Select:
case Instruction::AddrSpaceCast:
break;
default:
EscapePoints.insert(I);
break;
}
AddUsesToWorklist(I);
}
}
void callUsesLocalStack(CallSite CS, bool IsNocapture) {
// Add it to the list of alloca users. If it's already there, skip further
// processing.
if (!AllocaUsers.insert(CS.getInstruction()))
return;
// If it's nocapture then it can't capture the alloca.
if (IsNocapture)
return;
// If it can write to memory, it can leak the alloca value.
if (!CS.onlyReadsMemory())
EscapePoints.insert(CS.getInstruction());
}
SmallPtrSet<Instruction *, 32> AllocaUsers;
SmallPtrSet<Instruction *, 32> EscapePoints;
};
}
bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
if (F.callsFunctionThatReturnsTwice())
return false;
AllCallsAreTailCalls = true;
// The local stack holds all alloca instructions and all byval arguments.
AllocaDerivedValueTracker Tracker;
for (Argument &Arg : F.args()) {
if (Arg.hasByValAttr())
Tracker.walk(&Arg);
}
for (auto &BB : F) {
for (auto &I : BB)
if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
Tracker.walk(AI);
}
bool Modified = false;
// Track whether a block is reachable after an alloca has escaped. Blocks that
// contain the escaping instruction will be marked as being visited without an
// escaped alloca, since that is how the block began.
enum VisitType {
UNVISITED,
UNESCAPED,
ESCAPED
};
DenseMap<BasicBlock *, VisitType> Visited;
// We propagate the fact that an alloca has escaped from block to successor.
// Visit the blocks that are propagating the escapedness first. To do this, we
// maintain two worklists.
SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
// We may enter a block and visit it thinking that no alloca has escaped yet,
// then see an escape point and go back around a loop edge and come back to
// the same block twice. Because of this, we defer setting tail on calls when
// we first encounter them in a block. Every entry in this list does not
// statically use an alloca via use-def chain analysis, but may find an alloca
// through other means if the block turns out to be reachable after an escape
// point.
SmallVector<CallInst *, 32> DeferredTails;
BasicBlock *BB = &F.getEntryBlock();
VisitType Escaped = UNESCAPED;
do {
for (auto &I : *BB) {
if (Tracker.EscapePoints.count(&I))
Escaped = ESCAPED;
CallInst *CI = dyn_cast<CallInst>(&I);
if (!CI || CI->isTailCall())
continue;
if (CI->doesNotAccessMemory()) {
// A call to a readnone function whose arguments are all things computed
// outside this function can be marked tail. Even if you stored the
// alloca address into a global, a readnone function can't load the
// global anyhow.
//
// Note that this runs whether we know an alloca has escaped or not. If
// it has, then we can't trust Tracker.AllocaUsers to be accurate.
bool SafeToTail = true;
for (auto &Arg : CI->arg_operands()) {
if (isa<Constant>(Arg.getUser()))
continue;
if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
if (!A->hasByValAttr())
continue;
SafeToTail = false;
break;
}
if (SafeToTail) {
F.getContext().emitOptimizationRemark(
"tailcallelim", F, CI->getDebugLoc(),
"found readnone tail call candidate");
CI->setTailCall();
Modified = true;
continue;
}
}
if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
DeferredTails.push_back(CI);
} else {
AllCallsAreTailCalls = false;
}
}
for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
auto &State = Visited[SuccBB];
if (State < Escaped) {
State = Escaped;
if (State == ESCAPED)
WorklistEscaped.push_back(SuccBB);
else
WorklistUnescaped.push_back(SuccBB);
}
}
if (!WorklistEscaped.empty()) {
BB = WorklistEscaped.pop_back_val();
Escaped = ESCAPED;
} else {
BB = nullptr;
while (!WorklistUnescaped.empty()) {
auto *NextBB = WorklistUnescaped.pop_back_val();
if (Visited[NextBB] == UNESCAPED) {
BB = NextBB;
Escaped = UNESCAPED;
break;
}
}
}
} while (BB);
for (CallInst *CI : DeferredTails) {
if (Visited[CI->getParent()] != ESCAPED) {
// If the escape point was part way through the block, calls after the
// escape point wouldn't have been put into DeferredTails.
F.getContext().emitOptimizationRemark(
"tailcallelim", F, CI->getDebugLoc(), "found tail call candidate");
CI->setTailCall();
Modified = true;
} else {
AllCallsAreTailCalls = false;
}
}
return Modified;
}
bool TailCallElim::runTRE(Function &F) {
// If this function is a varargs function, we won't be able to PHI the args
// right, so don't even try to convert it...
if (F.getFunctionType()->isVarArg()) return false;
@ -191,46 +392,30 @@ bool TailCallElim::runOnFunction(Function &F) {
// doesn't).
bool CanTRETailMarkedCall = true;
// Find calls that can be marked tail.
AllocaCaptureTracker ACT;
// Find dynamic allocas.
for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
CanTRETailMarkedCall &= CanTRE(AI);
PointerMayBeCaptured(AI, &ACT);
// If any allocas are captured, exit.
if (ACT.Captured)
return false;
}
}
}
// If any byval or inalloca args are captured, exit. They are also allocated
// in our stack frame.
for (Argument &Arg : F.args()) {
if (Arg.hasByValOrInAllocaAttr())
PointerMayBeCaptured(&Arg, &ACT);
if (ACT.Captured)
return false;
}
// Second pass, change any tail recursive calls to loops.
// Change any tail recursive calls to loops.
//
// FIXME: The code generator produces really bad code when an 'escaping
// alloca' is changed from being a static alloca to being a dynamic alloca.
// Until this is resolved, disable this transformation if that would ever
// happen. This bug is PR962.
if (ACT.UsesAlloca.empty()) {
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs, !CanTRETailMarkedCall);
if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
TailCallsAreMarkedTail, ArgumentPHIs,
!CanTRETailMarkedCall);
MadeChange |= Change;
}
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs, !CanTRETailMarkedCall);
if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
TailCallsAreMarkedTail, ArgumentPHIs,
!CanTRETailMarkedCall);
MadeChange |= Change;
}
}
@ -239,34 +424,13 @@ bool TailCallElim::runOnFunction(Function &F) {
// with themselves. Check to see if we did and clean up our mess if so. This
// occurs when a function passes an argument straight through to its tail
// call.
if (!ArgumentPHIs.empty()) {
for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
PHINode *PN = ArgumentPHIs[i];
for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
PHINode *PN = ArgumentPHIs[i];
// If the PHI Node is a dynamic constant, replace it with the value it is.
if (Value *PNV = SimplifyInstruction(PN)) {
PN->replaceAllUsesWith(PNV);
PN->eraseFromParent();
}
}
}
// At this point, we know that the function does not have any captured
// allocas. If additionally the function does not call setjmp, mark all calls
// in the function that do not access stack memory with the tail keyword. This
// implies ensuring that there does not exist any path from a call that takes
// in an alloca but does not capture it and the call which we wish to mark
// with "tail".
if (!F.callsFunctionThatReturnsTwice()) {
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
if (CallInst *CI = dyn_cast<CallInst>(I)) {
if (!ACT.UsesAlloca.count(CI)) {
CI->setTailCall();
MadeChange = true;
}
}
}
// If the PHI Node is a dynamic constant, replace it with the value it is.
if (Value *PNV = SimplifyInstruction(PN)) {
PN->replaceAllUsesWith(PNV);
PN->eraseFromParent();
}
}
@ -520,6 +684,10 @@ bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
BasicBlock *BB = Ret->getParent();
Function *F = BB->getParent();
F->getContext().emitOptimizationRemark(
"tailcallelim", *F, CI->getDebugLoc(),
"transforming tail recursion to loop");
// OK! We can transform this tail call. If this is the first one found,
// create the new entry block, allowing us to branch back to the old entry.
if (!OldEntry) {

23
test/Transforms/TailCallElim/basic.ll
View File

@ -151,3 +151,26 @@ define void @test9(i32* byval %a) {
call void @use(i32* %a)
ret void
}
%struct.X = type { i8* }
declare void @ctor(%struct.X*)
define void @test10(%struct.X* noalias sret %agg.result, i1 zeroext %b) {
; CHECK-LABEL @test10
entry:
%x = alloca %struct.X, align 8
br i1 %b, label %if.then, label %if.end
if.then: ; preds = %entry
call void @ctor(%struct.X* %agg.result)
; CHECK: tail call void @ctor
br label %return
if.end:
call void @ctor(%struct.X* %x)
; CHECK: call void @ctor
br label %return
return:
ret void
}