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Use a loop to simplify the runtime unrolling prologue.

Browse Source 
Runtime unrolling will create a prologue to execute the extra
iterations which is can't divided by the unroll factor. It
generates an if-then-else sequence to jump into a factor -1
times unrolled loop body, like

    extraiters = tripcount % loopfactor
    if (extraiters == 0) jump Loop:
    if (extraiters == loopfactor) jump L1
    if (extraiters == loopfactor-1) jump L2
    ...
    L1:  LoopBody;
    L2:  LoopBody;
    ...
    if tripcount < loopfactor jump End
    Loop:
    ...
    End:

It means if the unroll factor is 4, the loop body will be 7
times unrolled, 3 are in loop prologue, and 4 are in the loop.
This commit is to use a loop to execute the extra iterations
in prologue, like

        extraiters = tripcount % loopfactor
        if (extraiters == 0) jump Loop:
        else jump Prol
 Prol:  LoopBody;
        extraiters -= 1                 // Omitted if unroll factor is 2.
        if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
        if (tripcount < loopfactor) jump End
 Loop:
 ...
 End:

Then when unroll factor is 4, the loop body will be copied by
only 5 times, 1 in the prologue loop, 4 in the original loop.
And if the unroll factor is 2, new loop won't be created, just
as the original solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218604 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Kevin Qin
2014-09-29 11:15:00 +00:00
parent 017c6111a8
commit dbaeb6e7cb
5 changed files with 150 additions and 135 deletions
Download Patch File Download Diff File Expand all files Collapse all files

244
lib/Transforms/Utils/LoopUnrollRuntime.cpp
View File

@ -28,6 +28,7 @@
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
@ -57,7 +58,7 @@ STATISTIC(NumRuntimeUnrolled,
static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count,
BasicBlock *LastPrologBB, BasicBlock *PrologEnd,
BasicBlock *OrigPH, BasicBlock *NewPH,
ValueToValueMapTy &LVMap, Pass *P) {
ValueToValueMapTy &VMap, Pass *P) {
BasicBlock *Latch = L->getLoopLatch();
assert(Latch && "Loop must have a latch");
@ -86,7 +87,7 @@ static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count,
Value *V = PN->getIncomingValueForBlock(Latch);
if (Instruction *I = dyn_cast<Instruction>(V)) {
if (L->contains(I)) {
V = LVMap[I];
V = VMap[I];
}
}
// Adding a value to the new PHI node from the last prolog block
@ -127,76 +128,122 @@ static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count,
}
/// Create a clone of the blocks in a loop and connect them together.
/// This function doesn't create a clone of the loop structure.
/// If UnrollProlog is true, loop structure will not be cloned, otherwise a new
/// loop will be created including all cloned blocks, and the iterator of it
/// switches to count NewIter down to 0.
///
/// There are two value maps that are defined and used. VMap is
/// for the values in the current loop instance. LVMap contains
/// the values from the last loop instance. We need the LVMap values
/// to update the initial values for the current loop instance.
///
static void CloneLoopBlocks(Loop *L,
bool FirstCopy,
BasicBlock *InsertTop,
BasicBlock *InsertBot,
static void CloneLoopBlocks(Loop *L, Value *NewIter, const bool UnrollProlog,
BasicBlock *InsertTop, BasicBlock *InsertBot,
std::vector<BasicBlock *> &NewBlocks,
LoopBlocksDFS &LoopBlocks,
ValueToValueMapTy &VMap,
ValueToValueMapTy &LVMap,
LoopBlocksDFS &LoopBlocks, ValueToValueMapTy &VMap,
LoopInfo *LI) {
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
BasicBlock *Latch = L->getLoopLatch();
Function *F = Header->getParent();
LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
Loop *NewLoop = 0;
Loop *ParentLoop = L->getParentLoop();
if (!UnrollProlog) {
NewLoop = new Loop();
if (ParentLoop)
ParentLoop->addChildLoop(NewLoop);
else
LI->addTopLevelLoop(NewLoop);
}
// For each block in the original loop, create a new copy,
// and update the value map with the newly created values.
for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".unr", F);
BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".prol", F);
NewBlocks.push_back(NewBB);
if (Loop *ParentLoop = L->getParentLoop())
if (NewLoop)
NewLoop->addBasicBlockToLoop(NewBB, LI->getBase());
else if (ParentLoop)
ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
VMap[*BB] = NewBB;
if (Header == *BB) {
// For the first block, add a CFG connection to this newly
// created block
// created block.
InsertTop->getTerminator()->setSuccessor(0, NewBB);
// Change the incoming values to the ones defined in the
// previously cloned loop.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *NewPHI = cast<PHINode>(VMap[I]);
if (FirstCopy) {
// We replace the first phi node with the value from the preheader
VMap[I] = NewPHI->getIncomingValueForBlock(Preheader);
NewBB->getInstList().erase(NewPHI);
} else {
// Update VMap with values from the previous block
unsigned idx = NewPHI->getBasicBlockIndex(Latch);
Value *InVal = NewPHI->getIncomingValue(idx);
if (Instruction *I = dyn_cast<Instruction>(InVal))
if (L->contains(I))
InVal = LVMap[InVal];
NewPHI->setIncomingValue(idx, InVal);
NewPHI->setIncomingBlock(idx, InsertTop);
}
if (Latch == *BB) {
// For the last block, if UnrollProlog is true, create a direct jump to
// InsertBot. If not, create a loop back to cloned head.
VMap.erase((*BB)->getTerminator());
BasicBlock *FirstLoopBB = cast<BasicBlock>(VMap[Header]);
BranchInst *LatchBR = cast<BranchInst>(NewBB->getTerminator());
if (UnrollProlog) {
LatchBR->eraseFromParent();
BranchInst::Create(InsertBot, NewBB);
} else {
PHINode *NewIdx = PHINode::Create(NewIter->getType(), 2, "prol.iter",
FirstLoopBB->getFirstNonPHI());
IRBuilder<> Builder(LatchBR);
Value *IdxSub =
Builder.CreateSub(NewIdx, ConstantInt::get(NewIdx->getType(), 1),
NewIdx->getName() + ".sub");
Value *IdxCmp =
Builder.CreateIsNotNull(IdxSub, NewIdx->getName() + ".cmp");
BranchInst::Create(FirstLoopBB, InsertBot, IdxCmp, NewBB);
NewIdx->addIncoming(NewIter, InsertTop);
NewIdx->addIncoming(IdxSub, NewBB);
LatchBR->eraseFromParent();
}
}
}
// Change the incoming values to the ones defined in the preheader or
// cloned loop.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *NewPHI = cast<PHINode>(VMap[I]);
if (UnrollProlog) {
VMap[I] = NewPHI->getIncomingValueForBlock(Preheader);
cast<BasicBlock>(VMap[Header])->getInstList().erase(NewPHI);
} else {
unsigned idx = NewPHI->getBasicBlockIndex(Preheader);
NewPHI->setIncomingBlock(idx, InsertTop);
BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
idx = NewPHI->getBasicBlockIndex(Latch);
Value *InVal = NewPHI->getIncomingValue(idx);
NewPHI->setIncomingBlock(idx, NewLatch);
if (VMap[InVal])
NewPHI->setIncomingValue(idx, VMap[InVal]);
}
}
if (NewLoop) {
// Add unroll disable metadata to disable future unrolling for this loop.
SmallVector<Value *, 4> Vals;
// Reserve first location for self reference to the LoopID metadata node.
Vals.push_back(nullptr);
MDNode *LoopID = NewLoop->getLoopID();
if (LoopID) {
// First remove any existing loop unrolling metadata.
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
bool IsUnrollMetadata = false;
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (MD) {
const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
}
if (!IsUnrollMetadata) Vals.push_back(LoopID->getOperand(i));
}
}
if (Latch == *BB) {
VMap.erase((*BB)->getTerminator());
NewBB->getTerminator()->eraseFromParent();
BranchInst::Create(InsertBot, NewBB);
}
}
// LastValueMap is updated with the values for the current loop
// which are used the next time this function is called.
for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
VI != VE; ++VI) {
LVMap[VI->first] = VI->second;
LLVMContext &Context = NewLoop->getHeader()->getContext();
SmallVector<Value *, 1> DisableOperands;
DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
MDNode *DisableNode = MDNode::get(Context, DisableOperands);
Vals.push_back(DisableNode);
MDNode *NewLoopID = MDNode::get(Context, Vals);
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
NewLoop->setLoopID(NewLoopID);
}
}
@ -212,18 +259,16 @@ static void CloneLoopBlocks(Loop *L,
/// instruction in SimplifyCFG.cpp. Then, the backend decides how code for
/// the switch instruction is generated.
///
/// extraiters = tripcount % loopfactor
/// if (extraiters == 0) jump Loop:
/// if (extraiters == loopfactor) jump L1
/// if (extraiters == loopfactor-1) jump L2
/// ...
/// L1: LoopBody;
/// L2: LoopBody;
/// ...
/// if tripcount < loopfactor jump End
/// Loop:
/// ...
/// End:
/// extraiters = tripcount % loopfactor
/// if (extraiters == 0) jump Loop:
/// else jump Prol
/// Prol: LoopBody;
/// extraiters -= 1 // Omitted if unroll factor is 2.
/// if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
/// if (tripcount < loopfactor) jump End
/// Loop:
/// ...
/// End:
///
bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
LPPassManager *LPM) {
@ -284,26 +329,21 @@ bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
IRBuilder<> B(PreHeaderBR);
Value *ModVal = B.CreateAnd(TripCount, Count - 1, "xtraiter");
// Check if for no extra iterations, then jump to unrolled loop. We have to
// check that the trip count computation didn't overflow when adding one to
// the backedge taken count.
// Check if for no extra iterations, then jump to cloned/unrolled loop.
// We have to check that the trip count computation didn't overflow when
// adding one to the backedge taken count.
Value *LCmp = B.CreateIsNotNull(ModVal, "lcmp.mod");
Value *OverflowCheck = B.CreateIsNull(TripCount, "lcmp.overflow");
Value *BranchVal = B.CreateOr(OverflowCheck, LCmp, "lcmp.or");
// Branch to either the extra iterations or the unrolled loop
// Branch to either the extra iterations or the cloned/unrolled loop
// We will fix up the true branch label when adding loop body copies
BranchInst::Create(PEnd, PEnd, BranchVal, PreHeaderBR);
assert(PreHeaderBR->isUnconditional() &&
PreHeaderBR->getSuccessor(0) == PEnd &&
"CFG edges in Preheader are not correct");
PreHeaderBR->eraseFromParent();
ValueToValueMapTy LVMap;
Function *F = Header->getParent();
// These variables are used to update the CFG links in each iteration
BasicBlock *CompareBB = nullptr;
BasicBlock *LastLoopBB = PH;
// Get an ordered list of blocks in the loop to help with the ordering of the
// cloned blocks in the prolog code
LoopBlocksDFS LoopBlocks(L);
@ -314,62 +354,34 @@ bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
// and generate a condition that branches to the copy depending on the
// number of 'left over' iterations.
//
for (unsigned leftOverIters = Count-1; leftOverIters > 0; --leftOverIters) {
std::vector<BasicBlock*> NewBlocks;
ValueToValueMapTy VMap;
std::vector<BasicBlock *> NewBlocks;
ValueToValueMapTy VMap;
// Clone all the basic blocks in the loop, but we don't clone the loop
// This function adds the appropriate CFG connections.
CloneLoopBlocks(L, (leftOverIters == Count-1), LastLoopBB, PEnd, NewBlocks,
LoopBlocks, VMap, LVMap, LI);
LastLoopBB = cast<BasicBlock>(VMap[Latch]);
// Clone all the basic blocks in the loop. If Count is 2, we don't clone
// the loop, otherwise we create a cloned loop to execute the extra
// iterations. This function adds the appropriate CFG connections.
CloneLoopBlocks(L, ModVal, Count == 2, PH, PEnd, NewBlocks, LoopBlocks, VMap,
LI);
// Insert the cloned blocks into function just before the original loop
F->getBasicBlockList().splice(PEnd, F->getBasicBlockList(),
NewBlocks[0], F->end());
// Insert the cloned blocks into function just before the original loop
F->getBasicBlockList().splice(PEnd, F->getBasicBlockList(), NewBlocks[0],
F->end());
// Generate the code for the comparison which determines if the loop
// prolog code needs to be executed.
if (leftOverIters == Count-1) {
// There is no compare block for the fall-thru case when for the last
// left over iteration
CompareBB = NewBlocks[0];
} else {
// Create a new block for the comparison
BasicBlock *NewBB = BasicBlock::Create(CompareBB->getContext(), "unr.cmp",
F, CompareBB);
if (Loop *ParentLoop = L->getParentLoop()) {
// Add the new block to the parent loop, if needed
ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
}
// The comparison w/ the extra iteration value and branch
Type *CountTy = TripCount->getType();
Value *BranchVal = new ICmpInst(*NewBB, ICmpInst::ICMP_EQ, ModVal,
ConstantInt::get(CountTy, leftOverIters),
"un.tmp");
// Branch to either the extra iterations or the unrolled loop
BranchInst::Create(NewBlocks[0], CompareBB,
BranchVal, NewBB);
CompareBB = NewBB;
PH->getTerminator()->setSuccessor(0, NewBB);
VMap[NewPH] = CompareBB;
}
// Rewrite the cloned instruction operands to use the values
// created when the clone is created.
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) {
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
E = NewBlocks[i]->end(); I != E; ++I) {
RemapInstruction(I, VMap,
RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
}
// Rewrite the cloned instruction operands to use the values
// created when the clone is created.
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) {
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
E = NewBlocks[i]->end();
I != E; ++I) {
RemapInstruction(I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingEntries);
}
}
// Connect the prolog code to the original loop and update the
// PHI functions.
ConnectProlog(L, TripCount, Count, LastLoopBB, PEnd, PH, NewPH, LVMap,
BasicBlock *LastLoopBB = cast<BasicBlock>(VMap[Latch]);
ConnectProlog(L, TripCount, Count, LastLoopBB, PEnd, PH, NewPH, VMap,
LPM->getAsPass());
NumRuntimeUnrolled++;
return true;

3
test/Transforms/LoopUnroll/PowerPC/a2-unrolling.ll
View File

@ -41,8 +41,7 @@ for.end: ; preds = %for.body, %entry
}
; CHECK-LABEL: @test
; CHECK: unr.cmp{{.*}}:
; CHECK: for.body.unr{{.*}}:
; CHECK: for.body.prol{{.*}}:
; CHECK: for.body:
; CHECK: br i1 %exitcond.7, label %for.end.loopexit{{.*}}, label %for.body

27
test/Transforms/LoopUnroll/runtime-loop.ll
View File

@ -3,15 +3,16 @@
; Tests for unrolling loops with run-time trip counts
; CHECK: %xtraiter = and i32 %n
; CHECK: %lcmp.mod = icmp ne i32 %xtraiter, 0
; CHECK: %lcmp.overflow = icmp eq i32 %n, 0
; CHECK: %lcmp.or = or i1 %lcmp.overflow, %lcmp.mod
; CHECK: br i1 %lcmp.or, label %unr.cmp
; CHECK: %lcmp.mod = icmp ne i32 %xtraiter, 0
; CHECK: %lcmp.overflow = icmp eq i32 %n, 0
; CHECK: %lcmp.or = or i1 %lcmp.overflow, %lcmp.mod
; CHECK: br i1 %lcmp.or, label %for.body.prol, label %for.body.preheader.split
; CHECK: unr.cmp{{.*}}:
; CHECK: for.body.unr{{.*}}:
; CHECK: for.body:
; CHECK: br i1 %exitcond.7, label %for.end.loopexit{{.*}}, label %for.body
; CHECK: for.body.prol:
; CHECK: %indvars.iv.prol = phi i64 [ %indvars.iv.next.prol, %for.body.prol ], [ 0, %for.body.preheader ]
; CHECK: %prol.iter.sub = sub i32 %prol.iter, 1
; CHECK: %prol.iter.cmp = icmp ne i32 %prol.iter.sub, 0
; CHECK: br i1 %prol.iter.cmp, label %for.body.prol, label %for.body.preheader.split, !llvm.loop !0
define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {
entry:
@ -39,7 +40,7 @@ for.end: ; preds = %for.body, %entry
; even if the -unroll-runtime is specified
; CHECK: for.body:
; CHECK-NOT: for.body.unr:
; CHECK-NOT: for.body.prol:
define i32 @test1(i32* nocapture %a) nounwind uwtable readonly {
entry:
@ -85,8 +86,8 @@ cond_true138:
; Test run-time unrolling for a loop that counts down by -2.
; CHECK: for.body.unr:
; CHECK: br i1 %cmp.7, label %for.cond.for.end_crit_edge{{.*}}, label %for.body
; CHECK: for.body.prol:
; CHECK: br i1 %prol.iter.cmp, label %for.body.prol, label %for.body.preheader.split
define zeroext i16 @down(i16* nocapture %p, i32 %len) nounwind uwtable readonly {
entry:
@ -113,3 +114,7 @@ for.end: ; preds = %for.cond.for.end_cr
%res.0.lcssa = phi i16 [ %phitmp, %for.cond.for.end_crit_edge ], [ 0, %entry ]
ret i16 %res.0.lcssa
}
; CHECK: !0 = metadata !{metadata !0, metadata !1}
; CHECK: !1 = metadata !{metadata !"llvm.loop.unroll.disable"}

8
test/Transforms/LoopUnroll/runtime-loop1.ll
View File

@ -1,11 +1,11 @@
; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-count=4 | FileCheck %s
; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-count=2 | FileCheck %s
; This tests that setting the unroll count works
; CHECK: unr.cmp:
; CHECK: for.body.unr:
; CHECK: for.body.prol:
; CHECK: br label %for.body.preheader.split
; CHECK: for.body:
; CHECK: br i1 %exitcond.3, label %for.end.loopexit{{.*}}, label %for.body
; CHECK: br i1 %exitcond.1, label %for.end.loopexit.unr-lcssa, label %for.body
; CHECK-NOT: br i1 %exitcond.4, label %for.end.loopexit{{.*}}, label %for.body
define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {

3
test/Transforms/LoopUnroll/runtime-loop2.ll
View File

@ -3,8 +3,7 @@
; Choose a smaller, power-of-two, unroll count if the loop is too large.
; This test makes sure we're not unrolling 'odd' counts
; CHECK: unr.cmp:
; CHECK: for.body.unr:
; CHECK: for.body.prol:
; CHECK: for.body:
; CHECK: br i1 %exitcond.3, label %for.end.loopexit{{.*}}, label %for.body
; CHECK-NOT: br i1 %exitcond.4, label %for.end.loopexit{{.*}}, label %for.body