Revision 96b96c8d5195b968656e786ded8f6a1a96168427 authored by Jishnu Bhattacharya on 20 March 2024, 18:20:09 UTC, committed by Jishnu Bhattacharya on 20 March 2024, 18:20:09 UTC
1 parent bc7ba3d
llvm-simdloop.cpp
// This file is a part of Julia. License is MIT: https://julialang.org/license
#include "llvm-version.h"
#include "passes.h"
// This file defines a LLVM pass that:
// 1. Set's loop information in form of metadata
// 2. If the metadata contains `julia.simdloop` finds reduction chains and marks
// floating-point operations as fast-math. `See enableUnsafeAlgebraIfReduction`.
// 3. If the metadata contains `julia.ivdep` marks all memory accesses in the loop
// as independent of each other.
//
// The pass hinges on a call to a marker function that has metadata attached to it.
#include "support/dtypes.h"
#include <llvm-c/Core.h>
#include <llvm-c/Types.h>
#include <llvm/ADT/Statistic.h>
#include <llvm/Analysis/LoopPass.h>
#include <llvm/Analysis/OptimizationRemarkEmitter.h>
#include <llvm/Analysis/MemorySSA.h>
#include <llvm/IR/Instructions.h>
#include <llvm/IR/Metadata.h>
#include <llvm/IR/Verifier.h>
#include <llvm/Support/Debug.h>
#include "julia_assert.h"
#define DEBUG_TYPE "lower_simd_loop"
using namespace llvm;
STATISTIC(TotalMarkedLoops, "Total number of loops marked with simdloop");
STATISTIC(IVDepLoops, "Number of loops with no loop-carried dependencies");
STATISTIC(SimdLoops, "Number of loops with SIMD instructions");
STATISTIC(IVDepInstructions, "Number of instructions marked ivdep");
STATISTIC(ReductionChains, "Number of reduction chains folded");
STATISTIC(ReductionChainLength, "Total sum of instructions folded from reduction chain");
STATISTIC(MaxChainLength, "Max length of reduction chain");
STATISTIC(AddChains, "Addition reduction chains");
STATISTIC(MulChains, "Multiply reduction chains");
#ifndef __clang_gcanalyzer__
#define REMARK(remark) ORE.emit(remark)
#else
#define REMARK(remark) (void) 0;
#endif
namespace {
static unsigned getReduceOpcode(Instruction *J, Instruction *operand) JL_NOTSAFEPOINT
{
switch (J->getOpcode()) {
case Instruction::FSub:
if (J->getOperand(0) != operand)
return 0;
JL_FALLTHROUGH;
case Instruction::FAdd:
return Instruction::FAdd;
case Instruction::FDiv:
if (J->getOperand(0) != operand)
return 0;
JL_FALLTHROUGH;
case Instruction::FMul:
return Instruction::FMul;
default:
return 0;
}
}
/// If Phi is part of a reduction cycle of FAdd, FSub, FMul or FDiv,
/// mark the ops as permitting reassociation/commuting.
/// As of LLVM 4.0, FDiv is not handled by the loop vectorizer
static void enableUnsafeAlgebraIfReduction(PHINode *Phi, Loop &L, OptimizationRemarkEmitter &ORE, ScalarEvolution *SE) JL_NOTSAFEPOINT
{
typedef SmallVector<Instruction*, 8> chainVector;
chainVector chain;
Instruction *J;
unsigned opcode = 0;
for (Instruction *I = Phi; ; I=J) {
J = NULL;
// Find the user of instruction I that is within loop L.
for (User *UI : I->users()) { /*}*/
Instruction *U = cast<Instruction>(UI);
if (L.contains(U)) {
if (J) {
LLVM_DEBUG(dbgs() << "LSL: not a reduction var because op has two internal uses: " << *I << "\n");
REMARK([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NotReductionVar", U)
<< "not a reduction variable because operation has two internal uses";
});
return;
}
J = U;
}
}
if (!J) {
LLVM_DEBUG(dbgs() << "LSL: chain prematurely terminated at " << *I << "\n");
REMARK([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "ChainPrematurelyTerminated", I)
<< "chain prematurely terminated at " << ore::NV("Instruction", I);
});
return;
}
if (J == Phi) {
// Found the entire chain.
break;
}
if (opcode) {
// Check that arithmetic op matches prior arithmetic ops in the chain.
if (getReduceOpcode(J, I) != opcode) {
LLVM_DEBUG(dbgs() << "LSL: chain broke at " << *J << " because of wrong opcode\n");
REMARK([&](){
return OptimizationRemarkMissed(DEBUG_TYPE, "ChainBroke", J)
<< "chain broke at " << ore::NV("Instruction", J) << " because of wrong opcode";
});
return;
}
}
else {
// First arithmetic op in the chain.
opcode = getReduceOpcode(J, I);
if (!opcode) {
LLVM_DEBUG(dbgs() << "LSL: first arithmetic op in chain is uninteresting" << *J << "\n");
REMARK([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "FirstArithmeticOpInChainIsUninteresting", J)
<< "first arithmetic op in chain is uninteresting";
});
return;
}
}
chain.push_back(J);
}
switch (opcode) {
case Instruction::FAdd:
++AddChains;
break;
case Instruction::FMul:
++MulChains;
break;
}
++ReductionChains;
int length = 0;
for (chainVector::const_iterator K=chain.begin(); K!=chain.end(); ++K) {
LLVM_DEBUG(dbgs() << "LSL: marking " << **K << "\n");
REMARK([&]() {
return OptimizationRemark(DEBUG_TYPE, "MarkedUnsafeAlgebra", *K)
<< "marked unsafe algebra on " << ore::NV("Instruction", *K);
});
(*K)->setHasAllowReassoc(true);
(*K)->setHasAllowContract(true);
if (SE)
SE->forgetValue(*K);
++length;
}
ReductionChainLength += length;
MaxChainLength.updateMax(length);
}
static bool processLoop(Loop &L, OptimizationRemarkEmitter &ORE, ScalarEvolution *SE) JL_NOTSAFEPOINT
{
MDNode *LoopID = L.getLoopID();
if (!LoopID)
return false;
bool simd = false;
bool ivdep = false;
BasicBlock *Lh = L.getHeader();
LLVM_DEBUG(dbgs() << "LSL: loop header: " << *Lh << "\n");
SmallVector<Metadata*, 4> MDs(1);
// First Operand is self-reference
// Drop `julia.` prefixes
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
Metadata *Op = LoopID->getOperand(i);
const MDString *S = dyn_cast<MDString>(Op);
if (S) {
LLVM_DEBUG(dbgs() << "LSL: found " << S->getString() << "\n");
if (S->getString().startswith("julia")) {
if (S->getString().equals("julia.simdloop"))
simd = true;
if (S->getString().equals("julia.ivdep"))
ivdep = true;
continue;
}
}
MDs.push_back(Op);
}
LLVM_DEBUG(dbgs() << "LSL: simd: " << simd << " ivdep: " << ivdep << "\n");
if (!simd && !ivdep)
return false;
++TotalMarkedLoops;
LLVMContext &Context = L.getHeader()->getContext();
LoopID = MDNode::get(Context, MDs);
// Set operand 0 to refer to the loop id itself
LoopID->replaceOperandWith(0, LoopID);
L.setLoopID(LoopID);
REMARK([&]() {
return OptimizationRemarkAnalysis(DEBUG_TYPE, "Loop SIMD Flags", L.getStartLoc(), L.getHeader())
<< "Loop marked for SIMD vectorization with flags { \"simd\": " << (simd ? "true" : "false") << ", \"ivdep\": " << (ivdep ? "true" : "false") << " }";
});
// If ivdep is true we assume that there is no memory dependency between loop iterations
// This is a fairly strong assumption and does often not hold true for generic code.
if (ivdep) {
++IVDepLoops;
MDNode *m = MDNode::get(Lh->getContext(), ArrayRef<Metadata *>(LoopID));
// Mark memory references so that Loop::isAnnotatedParallel will return true for this loop.
for (BasicBlock *BB : L.blocks()) {
for (Instruction &I : *BB) {
if (I.mayReadOrWriteMemory()) {
++IVDepInstructions;
I.setMetadata(LLVMContext::MD_mem_parallel_loop_access, m);
}
}
}
assert(L.isAnnotatedParallel());
}
if (simd) {
++SimdLoops;
// Mark floating-point reductions as okay to reassociate/commute.
for (BasicBlock::iterator I = Lh->begin(), E = Lh->end(); I != E; ++I) {
if (PHINode *Phi = dyn_cast<PHINode>(I))
enableUnsafeAlgebraIfReduction(Phi, L, ORE, SE);
else
break;
}
if (SE)
#if JL_LLVM_VERSION >= 160000
SE->forgetLoopDispositions();
#else
SE->forgetLoopDispositions(&L);
#endif
}
#ifdef JL_VERIFY_PASSES
assert(!verifyLLVMIR(L));
#endif
return true;
}
} // end anonymous namespace
/// This pass should run after reduction variables have been converted to phi nodes,
/// otherwise floating-point reductions might not be recognized as such and
/// prevent SIMDization.
PreservedAnalyses LowerSIMDLoopPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR, LPMUpdater &U)
{
OptimizationRemarkEmitter ORE(L.getHeader()->getParent());
if (processLoop(L, ORE, &AR.SE)) {
#ifdef JL_DEBUG_BUILD
if (AR.MSSA)
AR.MSSA->verifyMemorySSA();
#endif
auto preserved = getLoopPassPreservedAnalyses();
preserved.preserveSet<CFGAnalyses>();
preserved.preserve<MemorySSAAnalysis>();
return preserved;
}
return PreservedAnalyses::all();
}
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