maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

This patch extends the LoopUnroll pass to be able to unroll loops 

with unknown trip counts. This is left off by default, and a
command-line option enables it. It also begins to separate loop
unrolling into a utility routine; eventually it might be made usable
from other passes.

It currently works by inserting conditional branches between each
unrolled iteration, unless it proves that the trip count is a
multiple of a constant integer > 1, which it currently only does in
the rare case that the trip count expression is a Mul operator with
a ConstantInt operand. Eventually this information might be provided
by other sources, for example by a pass that peels/splits the loop
for this purpose.

llvm-svn: 36990
This commit is contained in:
Dan Gohman 2007-05-11 20:53:41 +00:00
parent 600db3eb96
commit 2980d9da45
1 changed files with 196 additions and 61 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

257
llvm/lib/Transforms/Scalar/LoopUnroll.cpp
View File

@ -31,6 +31,7 @@
#include "llvm/Support/Compiler.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
@ -39,12 +40,19 @@
#include <algorithm>
using namespace llvm;
STATISTIC(NumUnrolled, "Number of loops completely unrolled");
STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
namespace {
cl::opt<unsigned>
UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
cl::desc("The cut-off point for loop unrolling"));
UnrollThreshold
("unroll-threshold", cl::init(100), cl::Hidden,
cl::desc("The cut-off point for automatic loop unrolling"));
cl::opt<unsigned>
UnrollCount
("unroll-count", cl::init(0), cl::Hidden,
cl::desc("Use this unroll count for all loops, for testing purposes"));
class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
LoopInfo *LI; // The current loop information
@ -52,7 +60,13 @@ namespace {
static char ID; // Pass ID, replacement for typeid
LoopUnroll() : LoopPass((intptr_t)&ID) {}
/// A magic value for use with the Threshold parameter to indicate
/// that the loop unroll should be performed regardless of how much
/// code expansion would result.
static const unsigned NoThreshold = UINT_MAX;
bool runOnLoop(Loop *L, LPPassManager &LPM);
bool unrollLoop(Loop *L, unsigned Count, unsigned Threshold);
BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB);
/// This transformation requires natural loop information & requires that
@ -162,43 +176,137 @@ BasicBlock *LoopUnroll::FoldBlockIntoPredecessor(BasicBlock *BB) {
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
// Unroll the loop.
if (!unrollLoop(L, UnrollCount, UnrollThreshold))
return false;
// Update the loop information for this loop.
// If we completely unrolled the loop, remove it from the parent.
if (L->getNumBackEdges() == 0)
LPM.deleteLoopFromQueue(L);
return true;
}
/// Unroll the given loop by UnrollCount, or by a heuristically-determined
/// value if Count is zero. If Threshold is non-NULL, it points to
/// a Threshold value to limit code size expansion. If the loop size would
/// expand beyond the threshold value, unrolling is suppressed. The return
/// value is false if no transformations are performed.
///
bool LoopUnroll::unrollLoop(Loop *L, unsigned Count, unsigned Threshold) {
assert(L->isLCSSAForm());
BasicBlock *Header = L->getHeader();
BasicBlock *LatchBlock = L->getLoopLatch();
BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
if (BI == 0) return Changed; // Must end in a conditional branch
ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount());
if (!TripCountC) return Changed; // Must have constant trip count!
// Guard against huge trip counts. This also guards against assertions in
// APInt from the use of getZExtValue, below.
if (TripCountC->getValue().getActiveBits() > 32)
return Changed; // More than 2^32 iterations???
uint64_t TripCountFull = TripCountC->getZExtValue();
if (TripCountFull == 0)
return Changed; // Zero iteraitons?
unsigned LoopSize = ApproximateLoopSize(L);
DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << " Loop Size = "
<< LoopSize << " Trip Count = " << TripCountFull << " - ";
uint64_t Size = (uint64_t)LoopSize*TripCountFull;
if (Size > UnrollThreshold) {
DOUT << "TOO LARGE: " << Size << ">" << UnrollThreshold << "\n";
return Changed;
<< "] Loop %" << Header->getName() << "\n";
if (!BI || BI->isUnconditional()) {
// The loop-rorate pass can be helpful to avoid this in many cases.
DOUT << " Can't unroll; loop not terminated by a conditional branch.\n";
return false;
}
// Determine the trip count and/or trip multiple. A TripCount value of zero
// is used to mean an unknown trip count. The TripMultiple value is the
// greatest known integer multiple of the trip count.
unsigned TripCount = 0;
unsigned TripMultiple = 1;
if (Value *TripCountValue = L->getTripCount()) {
if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCountValue)) {
// Guard against huge trip counts. This also guards against assertions in
// APInt from the use of getZExtValue, below.
if (TripCountC->getValue().getActiveBits() <= 32) {
TripCount = (unsigned)TripCountC->getZExtValue();
}
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCountValue)) {
switch (BO->getOpcode()) {
case BinaryOperator::Mul:
if (ConstantInt *MultipleC = dyn_cast<ConstantInt>(BO->getOperand(1))) {
if (MultipleC->getValue().getActiveBits() <= 32) {
TripMultiple = (unsigned)MultipleC->getZExtValue();
}
}
break;
default: break;
}
}
}
if (TripCount != 0)
DOUT << " Trip Count = " << TripCount << "\n";
if (TripMultiple != 1)
DOUT << " Trip Multiple = " << TripMultiple << "\n";
// Automatically select an unroll count.
if (Count == 0) {
// Conservative heuristic: if we know the trip count, see if we can
// completely unroll (subject to the threshold, checked below); otherwise
// don't unroll.
if (TripCount != 0) {
Count = TripCount;
} else {
return false;
}
}
// Effectively "DCE" unrolled iterations that are beyond the tripcount
// and will never be executed.
if (TripCount != 0 && Count > TripCount)
Count = TripCount;
assert(Count > 0);
assert(TripMultiple > 0);
assert(TripCount == 0 || TripCount % TripMultiple == 0);
// Enforce the threshold.
if (Threshold != NoThreshold) {
unsigned LoopSize = ApproximateLoopSize(L);
DOUT << " Loop Size = " << LoopSize << "\n";
uint64_t Size = (uint64_t)LoopSize*Count;
if (TripCount != 1 && Size > Threshold) {
DOUT << " TOO LARGE TO UNROLL: "
<< Size << ">" << Threshold << "\n";
return false;
}
}
// Are we eliminating the loop control altogether?
bool CompletelyUnroll = Count == TripCount;
// If we know the trip count, we know the multiple...
unsigned BreakoutTrip = 0;
if (TripCount != 0) {
BreakoutTrip = TripCount % Count;
TripMultiple = 0;
} else {
// Figure out what multiple to use.
BreakoutTrip = TripMultiple =
(unsigned)GreatestCommonDivisor64(Count, TripMultiple);
}
if (CompletelyUnroll) {
DOUT << "COMPLETELY UNROLLING loop %" << Header->getName()
<< " with trip count " << TripCount << "!\n";
} else {
DOUT << "UNROLLING loop %" << Header->getName()
<< " by " << Count;
if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
DOUT << " with a breakout at trip " << BreakoutTrip;
} else if (TripMultiple != 1) {
DOUT << " with " << TripMultiple << " trips per branch";
}
DOUT << "!\n";
}
DOUT << "UNROLLING!\n";
std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
unsigned TripCount = (unsigned)TripCountFull;
BasicBlock *LoopExit = BI->getSuccessor(L->contains(BI->getSuccessor(0)));
bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
// For the first iteration of the loop, we should use the precloned values for
// PHI nodes. Insert associations now.
@ -214,16 +322,12 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
LastValueMap[I] = I;
}
// Remove the exit branch from the loop
LatchBlock->getInstList().erase(BI);
std::vector<BasicBlock*> Headers;
std::vector<BasicBlock*> Latches;
Headers.push_back(Header);
Latches.push_back(LatchBlock);
assert(TripCount != 0 && "Trip count of 0 is impossible!");
for (unsigned It = 1; It != TripCount; ++It) {
for (unsigned It = 1; It != Count; ++It) {
char SuffixBuffer[100];
sprintf(SuffixBuffer, ".%d", It);
@ -277,9 +381,18 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
// we can insert the proper branches later.
if (*BB == Header)
Headers.push_back(New);
if (*BB == LatchBlock)
if (*BB == LatchBlock) {
Latches.push_back(New);
// Also, clear out the new latch's back edge so that it doesn't look
// like a new loop, so that it's amenable to being merged with adjacent
// blocks later on.
TerminatorInst *Term = New->getTerminator();
assert(L->contains(Term->getSuccessor(!ContinueOnTrue)));
assert(Term->getSuccessor(ContinueOnTrue) == LoopExit);
Term->setSuccessor(!ContinueOnTrue, NULL);
}
NewBlocks.push_back(New);
}
@ -289,12 +402,11 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
E = NewBlocks[i]->end(); I != E; ++I)
RemapInstruction(I, LastValueMap);
}
// The latch block exits the loop. If there are any PHI nodes in the
// successor blocks, update them to use the appropriate values computed as the
// last iteration of the loop.
if (TripCount > 1) {
if (Count != 1) {
SmallPtrSet<PHINode*, 8> Users;
for (Value::use_iterator UI = LatchBlock->use_begin(),
UE = LatchBlock->use_end(); UI != UE; ++UI)
@ -316,28 +428,54 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
}
}
// Now loop over the PHI nodes in the original block, setting them to their
// incoming values.
BasicBlock *Preheader = L->getLoopPreheader();
for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
PHINode *PN = OrigPHINode[i];
PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
Header->getInstList().erase(PN);
}
// Insert the branches that link the different iterations together
for (unsigned i = 0; i < Latches.size()-1; ++i) {
new BranchInst(Headers[i+1], Latches[i]);
if (BasicBlock *Fold = FoldBlockIntoPredecessor(Headers[i+1])) {
std::replace(Latches.begin(), Latches.end(), Headers[i+1], Fold);
std::replace(Headers.begin(), Headers.end(), Headers[i+1], Fold);
// Now, if we're doing complete unrolling, loop over the PHI nodes in the
// original block, setting them to their incoming values.
if (CompletelyUnroll) {
BasicBlock *Preheader = L->getLoopPreheader();
for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
PHINode *PN = OrigPHINode[i];
PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
Header->getInstList().erase(PN);
}
}
// Now that all the basic blocks for the unrolled iterations are in place,
// set up the branches to connect them.
for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
// The original branch was replicated in each unrolled iteration.
BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
// The branch destination.
unsigned j = (i + 1) % e;
BasicBlock *Dest = Headers[j];
bool NeedConditional = true;
// For a complete unroll, make the last iteration end with a branch
// to the exit block.
if (CompletelyUnroll && j == 0) {
Dest = LoopExit;
NeedConditional = false;
}
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
NeedConditional = false;
}
if (NeedConditional) {
// Update the conditional branch's successor for the following
// iteration.
Term->setSuccessor(!ContinueOnTrue, Dest);
} else {
Term->setUnconditionalDest(Dest);
// Merge adjacent basic blocks, if possible.
if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest)) {
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
std::replace(Headers.begin(), Headers.end(), Dest, Fold);
}
}
}
// Finally, add an unconditional branch to the block to continue into the exit
// block.
new BranchInst(LoopExit, Latches[Latches.size()-1]);
FoldBlockIntoPredecessor(LoopExit);
// At this point, the code is well formed. We now do a quick sweep over the
// inserted code, doing constant propagation and dead code elimination as we
@ -356,10 +494,7 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
}
}
// Update the loop information for this loop.
// Remove the loop from the parent.
LPM.deleteLoopFromQueue(L);
NumCompletelyUnrolled += CompletelyUnroll;
++NumUnrolled;
return true;
}