[llvm] r229966 - [PowerPC] Loop Data Prefetching for the BG/Q
Hal Finkel
hfinkel at anl.gov
Thu Feb 19 21:08:22 PST 2015
Author: hfinkel
Date: Thu Feb 19 23:08:21 2015
New Revision: 229966
URL: http://llvm.org/viewvc/llvm-project?rev=229966&view=rev
Log:
[PowerPC] Loop Data Prefetching for the BG/Q
The IBM BG/Q supercomputer's A2 cores have a hardware prefetching unit, the
L1P, but it does not prefetch directly into the A2's L1 cache. Instead, it
prefetches into its own L1P buffer, and the latency to access that buffer is
significantly higher than that to the L1 cache (although smaller than the
latency to the L2 cache). As a result, especially when multiple hardware
threads are not actively busy, explicitly prefetching data into the L1 cache is
advantageous.
I've been using this pass out-of-tree for data prefetching on the BG/Q for well
over a year, and it has worked quite well. It is enabled by default only for
the BG/Q, but can be enabled for other cores as well via a command-line option.
Eventually, we might want to add some TTI interfaces and move this into
Transforms/Scalar (there is nothing particularly target dependent about it,
although only machines like the BG/Q will benefit from its simplistic
strategy).
Added:
llvm/trunk/lib/Target/PowerPC/PPCLoopDataPrefetch.cpp
llvm/trunk/test/CodeGen/PowerPC/loop-data-prefetch.ll
Modified:
llvm/trunk/lib/Target/PowerPC/CMakeLists.txt
llvm/trunk/lib/Target/PowerPC/PPC.h
llvm/trunk/lib/Target/PowerPC/PPCTargetMachine.cpp
Modified: llvm/trunk/lib/Target/PowerPC/CMakeLists.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/CMakeLists.txt?rev=229966&r1=229965&r2=229966&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/CMakeLists.txt (original)
+++ llvm/trunk/lib/Target/PowerPC/CMakeLists.txt Thu Feb 19 23:08:21 2015
@@ -23,6 +23,7 @@ add_llvm_target(PowerPCCodeGen
PPCEarlyReturn.cpp
PPCFastISel.cpp
PPCFrameLowering.cpp
+ PPCLoopDataPrefetch.cpp
PPCLoopPreIncPrep.cpp
PPCMCInstLower.cpp
PPCMachineFunctionInfo.cpp
Modified: llvm/trunk/lib/Target/PowerPC/PPC.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPC.h?rev=229966&r1=229965&r2=229966&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPC.h (original)
+++ llvm/trunk/lib/Target/PowerPC/PPC.h Thu Feb 19 23:08:21 2015
@@ -34,6 +34,7 @@ namespace llvm {
#ifndef NDEBUG
FunctionPass *createPPCCTRLoopsVerify();
#endif
+ FunctionPass *createPPCLoopDataPrefetchPass();
FunctionPass *createPPCLoopPreIncPrepPass(PPCTargetMachine &TM);
FunctionPass *createPPCEarlyReturnPass();
FunctionPass *createPPCVSXCopyPass();
Added: llvm/trunk/lib/Target/PowerPC/PPCLoopDataPrefetch.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCLoopDataPrefetch.cpp?rev=229966&view=auto
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPCLoopDataPrefetch.cpp (added)
+++ llvm/trunk/lib/Target/PowerPC/PPCLoopDataPrefetch.cpp Thu Feb 19 23:08:21 2015
@@ -0,0 +1,231 @@
+//===-------- PPCLoopDataPrefetch.cpp - Loop Data Prefetching Pass --------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a Loop Data Prefetching Pass.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "ppc-loop-data-prefetch"
+#include "PPC.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+using namespace llvm;
+
+// By default, we limit this to creating 16 PHIs (which is a little over half
+// of the allocatable register set).
+static cl::opt<bool>
+PrefetchWrites("ppc-loop-prefetch-writes", cl::Hidden, cl::init(false),
+ cl::desc("Prefetch write addresses"));
+
+// This seems like a reasonable default for the BG/Q (this pass is enabled, by
+// default, only on the BG/Q).
+static cl::opt<unsigned>
+PrefDist("ppc-loop-prefetch-distance", cl::Hidden, cl::init(300),
+ cl::desc("The loop prefetch distance"));
+
+static cl::opt<unsigned>
+CacheLineSize("ppc-loop-prefetch-cache-line", cl::Hidden, cl::init(64),
+ cl::desc("The loop prefetch cache line size"));
+
+namespace llvm {
+ void initializePPCLoopDataPrefetchPass(PassRegistry&);
+}
+
+namespace {
+
+ class PPCLoopDataPrefetch : public FunctionPass {
+ public:
+ static char ID; // Pass ID, replacement for typeid
+ PPCLoopDataPrefetch() : FunctionPass(ID) {
+ initializePPCLoopDataPrefetchPass(*PassRegistry::getPassRegistry());
+ }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addRequired<AssumptionCacheTracker>();
+ AU.addPreserved<DominatorTreeWrapperPass>();
+ AU.addRequired<LoopInfoWrapperPass>();
+ AU.addPreserved<LoopInfoWrapperPass>();
+ AU.addRequired<ScalarEvolution>();
+ // FIXME: For some reason, preserving SE here breaks LSR (even if
+ // this pass changes nothing).
+ // AU.addPreserved<ScalarEvolution>();
+ AU.addRequired<TargetTransformInfoWrapperPass>();
+ }
+
+ bool runOnFunction(Function &F) override;
+ bool runOnLoop(Loop *L);
+
+ private:
+ AssumptionCache *AC;
+ LoopInfo *LI;
+ ScalarEvolution *SE;
+ const TargetTransformInfo *TTI;
+ const DataLayout *DL;
+ };
+}
+
+char PPCLoopDataPrefetch::ID = 0;
+INITIALIZE_PASS_BEGIN(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
+ "PPC Loop Data Prefetch", false, false)
+INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_END(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
+ "PPC Loop Data Prefetch", false, false)
+
+FunctionPass *llvm::createPPCLoopDataPrefetchPass() { return new PPCLoopDataPrefetch(); }
+
+bool PPCLoopDataPrefetch::runOnFunction(Function &F) {
+ LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ SE = &getAnalysis<ScalarEvolution>();
+ DL = F.getParent()->getDataLayout();
+ AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+ TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
+
+ bool MadeChange = false;
+
+ for (LoopInfo::iterator I = LI->begin(), E = LI->end();
+ I != E; ++I) {
+ Loop *L = *I;
+ MadeChange |= runOnLoop(L);
+ }
+
+ return MadeChange;
+}
+
+bool PPCLoopDataPrefetch::runOnLoop(Loop *L) {
+ bool MadeChange = false;
+
+ // Only prefetch in the inner-most loop
+ if (!L->empty())
+ return MadeChange;
+
+ SmallPtrSet<const Value *, 32> EphValues;
+ CodeMetrics::collectEphemeralValues(L, AC, EphValues);
+
+ // Calculate the number of iterations ahead to prefetch
+ CodeMetrics Metrics;
+ for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
+ I != IE; ++I) {
+
+ // If the loop already has prefetches, then assume that the user knows
+ // what he or she is doing and don't add any more.
+ for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
+ J != JE; ++J)
+ if (CallInst *CI = dyn_cast<CallInst>(J))
+ if (Function *F = CI->getCalledFunction())
+ if (F->getIntrinsicID() == Intrinsic::prefetch)
+ return MadeChange;
+
+ Metrics.analyzeBasicBlock(*I, *TTI, EphValues);
+ }
+ unsigned LoopSize = Metrics.NumInsts;
+ if (!LoopSize)
+ LoopSize = 1;
+
+ unsigned ItersAhead = PrefDist/LoopSize;
+ if (!ItersAhead)
+ ItersAhead = 1;
+
+ SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
+ for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
+ I != IE; ++I) {
+ for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
+ J != JE; ++J) {
+ Value *PtrValue;
+ Instruction *MemI;
+
+ if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
+ MemI = LMemI;
+ PtrValue = LMemI->getPointerOperand();
+ } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
+ if (!PrefetchWrites) continue;
+ MemI = SMemI;
+ PtrValue = SMemI->getPointerOperand();
+ } else continue;
+
+ unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
+ if (PtrAddrSpace)
+ continue;
+
+ if (L->isLoopInvariant(PtrValue))
+ continue;
+
+ const SCEV *LSCEV = SE->getSCEV(PtrValue);
+ const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
+ if (!LSCEVAddRec)
+ continue;
+
+ // We don't want to double prefetch individual cache lines. If this load
+ // is known to be within one cache line of some other load that has
+ // already been prefetched, then don't prefetch this one as well.
+ bool DupPref = false;
+ for (SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>,
+ 16>::iterator K = PrefLoads.begin(), KE = PrefLoads.end();
+ K != KE; ++K) {
+ const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, K->second);
+ if (const SCEVConstant *ConstPtrDiff =
+ dyn_cast<SCEVConstant>(PtrDiff)) {
+ int64_t PD = abs64(ConstPtrDiff->getValue()->getSExtValue());
+ if (PD < (int64_t) CacheLineSize) {
+ DupPref = true;
+ break;
+ }
+ }
+ }
+ if (DupPref)
+ continue;
+
+ const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
+ SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
+ LSCEVAddRec->getStepRecurrence(*SE)));
+ if (!isSafeToExpand(NextLSCEV, *SE))
+ continue;
+
+ PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
+
+ Type *I8Ptr = Type::getInt8PtrTy((*I)->getContext(), PtrAddrSpace);
+ SCEVExpander SCEVE(*SE, "prefaddr");
+ Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
+
+ IRBuilder<> Builder(MemI);
+ Module *M = (*I)->getParent()->getParent();
+ Type *I32 = Type::getInt32Ty((*I)->getContext());
+ Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
+ Builder.CreateCall4(PrefetchFunc, PrefPtrValue,
+ ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
+ ConstantInt::get(I32, 3), ConstantInt::get(I32, 1));
+
+ MadeChange = true;
+ }
+ }
+
+ return MadeChange;
+}
+
Modified: llvm/trunk/lib/Target/PowerPC/PPCTargetMachine.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCTargetMachine.cpp?rev=229966&r1=229965&r2=229966&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPCTargetMachine.cpp (original)
+++ llvm/trunk/lib/Target/PowerPC/PPCTargetMachine.cpp Thu Feb 19 23:08:21 2015
@@ -43,6 +43,11 @@ EnableGEPOpt("ppc-gep-opt", cl::Hidden,
cl::desc("Enable optimizations on complex GEPs"),
cl::init(true));
+static cl::opt<bool>
+EnablePrefetch("enable-ppc-prefetching",
+ cl::desc("disable software prefetching on PPC"),
+ cl::init(false), cl::Hidden);
+
extern "C" void LLVMInitializePowerPCTarget() {
// Register the targets
RegisterTargetMachine<PPC32TargetMachine> A(ThePPC32Target);
@@ -240,6 +245,16 @@ TargetPassConfig *PPCTargetMachine::crea
void PPCPassConfig::addIRPasses() {
addPass(createAtomicExpandPass(&getPPCTargetMachine()));
+ // For the BG/Q (or if explicitly requested), add explicit data prefetch
+ // intrinsics.
+ bool UsePrefetching =
+ Triple(TM->getTargetTriple()).getVendor() == Triple::BGQ &&
+ getOptLevel() != CodeGenOpt::None;
+ if (EnablePrefetch.getNumOccurrences() > 0)
+ UsePrefetching = EnablePrefetch;
+ if (UsePrefetching)
+ addPass(createPPCLoopDataPrefetchPass());
+
if (TM->getOptLevel() == CodeGenOpt::Aggressive && EnableGEPOpt) {
// Call SeparateConstOffsetFromGEP pass to extract constants within indices
// and lower a GEP with multiple indices to either arithmetic operations or
Added: llvm/trunk/test/CodeGen/PowerPC/loop-data-prefetch.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/loop-data-prefetch.ll?rev=229966&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/loop-data-prefetch.ll (added)
+++ llvm/trunk/test/CodeGen/PowerPC/loop-data-prefetch.ll Thu Feb 19 23:08:21 2015
@@ -0,0 +1,29 @@
+; RUN: llc -mcpu=a2 < %s | FileCheck %s
+target datalayout = "E-m:e-i64:64-n32:64"
+target triple = "powerpc64-bgq-linux"
+
+; Function Attrs: nounwind
+define void @foo(double* nocapture %a, double* nocapture readonly %b) #0 {
+entry:
+ br label %for.body
+
+for.body: ; preds = %for.body, %entry
+ %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+ %arrayidx = getelementptr inbounds double* %b, i64 %indvars.iv
+ %0 = load double* %arrayidx, align 8
+ %add = fadd double %0, 1.000000e+00
+ %arrayidx2 = getelementptr inbounds double* %a, i64 %indvars.iv
+ store double %add, double* %arrayidx2, align 8
+ %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+ %exitcond = icmp eq i64 %indvars.iv.next, 1600
+ br i1 %exitcond, label %for.end, label %for.body
+
+for.end: ; preds = %for.body
+ ret void
+
+; CHECK-LABEL: @foo
+; CHECK: dcbt
+}
+
+attributes #0 = { nounwind }
+
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