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llvm-simdloop.cpp
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llvm-simdloop.cpp
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// This file is a part of Julia. License is MIT: https://julialang.org/license
#define DEBUG_TYPE "lower_simd_loop"
#undef DEBUG
// This file defines two entry points:
// global function annotateSimdLoop: mark a loop as a SIMD loop.
// createLowerSimdLoopPass: construct LLVM for lowering a marked loop later.
#include "llvm-version.h"
#include "support/dtypes.h"
#include <llvm/Analysis/LoopPass.h>
#include <llvm/IR/Instructions.h>
#include <llvm/IR/LLVMContext.h>
#include <llvm/IR/Metadata.h>
#include <llvm/Support/Debug.h>
#include <cstdio>
#include "julia_assert.h"
namespace llvm {
// simd loop
static unsigned simd_loop_mdkind = 0;
static MDNode *simd_loop_md = NULL;
/// Mark loop as a SIMD loop. Return false if loop cannot be marked.
/// incr should be the basic block that increments the loop counter.
bool annotateSimdLoop(BasicBlock *incr)
{
DEBUG(dbgs() << "LSL: annotating simd_loop\n");
// Lazy initialization
if (!simd_loop_mdkind) {
simd_loop_mdkind = incr->getContext().getMDKindID("simd_loop");
simd_loop_md = MDNode::get(incr->getContext(), ArrayRef<Metadata*>());
}
// Ideally, the decoration would go on the block itself, but LLVM 3.3 does not
// support putting metadata on blocks. So instead, put the decoration on the last
// Add instruction, which (somewhat riskily) is assumed to be the loop increment.
for (BasicBlock::reverse_iterator ri = incr->rbegin(); ri!=incr->rend(); ++ri) {
Instruction& i = *ri;
unsigned op = i.getOpcode();
if (op==Instruction::Add) {
if (i.getType()->isIntegerTy()) {
DEBUG(dbgs() << "LSL: setting simd_loop metadata\n");
i.setMetadata(simd_loop_mdkind, simd_loop_md);
return true;
}
else {
return false;
}
}
}
return false;
}
/// 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.
struct LowerSIMDLoop : public ModulePass {
static char ID;
LowerSIMDLoop() : ModulePass(ID)
{
}
protected:
void getAnalysisUsage(AnalysisUsage &AU) const override
{
ModulePass::getAnalysisUsage(AU);
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.setPreservesCFG();
}
private:
bool runOnModule(Module &M) override;
bool markSIMDLoop(Module &M, Function *marker, bool ivdep);
/// Check if loop has "simd_loop" annotation.
/// If present, the annotation is an MDNode attached to an instruction in the loop's latch.
bool hasSIMDLoopMetadata( Loop *L) const;
/// 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
void enableUnsafeAlgebraIfReduction(PHINode *Phi, Loop *L) const;
};
bool LowerSIMDLoop::hasSIMDLoopMetadata(Loop *L) const
{
// Note: If a loop has 0 or multiple latch blocks, it's probably not a simd_loop anyway.
if (BasicBlock *latch = L->getLoopLatch())
for (BasicBlock::iterator II = latch->begin(), EE = latch->end(); II!=EE; ++II)
if (II->getMetadata(simd_loop_mdkind))
return true;
return false;
}
static unsigned getReduceOpcode(Instruction *J, Instruction *operand)
{
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;
}
}
void LowerSIMDLoop::enableUnsafeAlgebraIfReduction(PHINode *Phi, Loop *L) const
{
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) {
DEBUG(dbgs() << "LSL: not a reduction var because op has two internal uses: " << *I << "\n");
return;
}
J = U;
}
}
if (!J) {
DEBUG(dbgs() << "LSL: chain prematurely terminated at " << *I << "\n");
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) {
DEBUG(dbgs() << "LSL: chain broke at " << *J << " because of wrong opcode\n");
return;
}
}
else {
// First arithmetic op in the chain.
opcode = getReduceOpcode(J, I);
if (!opcode) {
DEBUG(dbgs() << "LSL: first arithmetic op in chain is uninteresting" << *J << "\n");
return;
}
}
chain.push_back(J);
}
for (chainVector::const_iterator K=chain.begin(); K!=chain.end(); ++K) {
DEBUG(dbgs() << "LSL: marking " << **K << "\n");
#if JL_LLVM_VERSION >= 60000
(*K)->setFast(true);
#else
(*K)->setHasUnsafeAlgebra(true);
#endif
}
}
bool LowerSIMDLoop::runOnModule(Module &M)
{
Function *simdloop_marker = M.getFunction("julia.simdloop_marker");
Function *simdivdep_marker = M.getFunction("julia.simdivdep_marker");
bool Changed = false;
if (simdloop_marker)
Changed |= markSIMDLoop(M, simdloop_marker, false);
if (simdivdep_marker)
Changed |= markSIMDLoop(M, simdivdep_marker, true);
return Changed;
}
bool LowerSIMDLoop::markSIMDLoop(Module &M, Function *marker, bool ivdep)
{
bool Changed = false;
std::vector<Instruction*> ToDelete;
for (User *U : marker->users()) {
Instruction *I = cast<Instruction>(U);
ToDelete.push_back(I);
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>(*I->getParent()->getParent()).getLoopInfo();
Loop *L = LI.getLoopFor(I->getParent());
I->removeFromParent();
if (!L)
continue;
DEBUG(dbgs() << "LSL: simd_loop found\n");
DEBUG(dbgs() << "LSL: ivdep is: " << ivdep << "\n");
BasicBlock *Lh = L->getHeader();
DEBUG(dbgs() << "LSL: loop header: " << *Lh << "\n");
MDNode *n = L->getLoopID();
if (!n) {
// Loop does not have a LoopID yet, so give it one.
n = MDNode::get(Lh->getContext(), ArrayRef<Metadata *>(NULL));
n->replaceOperandWith(0, n);
L->setLoopID(n);
}
assert(L->getLoopID());
MDNode *m = MDNode::get(Lh->getContext(), ArrayRef<Metadata *>(n));
// 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) {
// 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()) {
I.setMetadata(LLVMContext::MD_mem_parallel_loop_access, m);
}
}
}
assert(L->isAnnotatedParallel());
}
// 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);
else
break;
}
Changed = true;
}
for (Instruction *I : ToDelete)
I->deleteValue();
marker->eraseFromParent();
return Changed;
}
char LowerSIMDLoop::ID = 0;
static RegisterPass<LowerSIMDLoop> X("LowerSIMDLoop", "LowerSIMDLoop Pass",
false /* Only looks at CFG */,
false /* Analysis Pass */);
JL_DLLEXPORT Pass *createLowerSimdLoopPass()
{
return new LowerSIMDLoop();
}
} // namespace llvm