forked from OSchip/llvm-project
399 lines
15 KiB
C++
399 lines
15 KiB
C++
//===- DivRemPairs.cpp - Hoist/[dr]ecompose division and remainder --------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass hoists and/or decomposes/recomposes integer division and remainder
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// instructions to enable CFG improvements and better codegen.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/DivRemPairs.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/MapVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/DebugCounter.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BypassSlowDivision.h"
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using namespace llvm;
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using namespace llvm::PatternMatch;
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#define DEBUG_TYPE "div-rem-pairs"
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STATISTIC(NumPairs, "Number of div/rem pairs");
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STATISTIC(NumRecomposed, "Number of instructions recomposed");
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STATISTIC(NumHoisted, "Number of instructions hoisted");
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STATISTIC(NumDecomposed, "Number of instructions decomposed");
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DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform",
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"Controls transformations in div-rem-pairs pass");
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namespace {
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struct ExpandedMatch {
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DivRemMapKey Key;
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Instruction *Value;
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};
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} // namespace
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/// See if we can match: (which is the form we expand into)
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/// X - ((X ?/ Y) * Y)
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/// which is equivalent to:
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/// X ?% Y
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static llvm::Optional<ExpandedMatch> matchExpandedRem(Instruction &I) {
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Value *Dividend, *XroundedDownToMultipleOfY;
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if (!match(&I, m_Sub(m_Value(Dividend), m_Value(XroundedDownToMultipleOfY))))
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return llvm::None;
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Value *Divisor;
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Instruction *Div;
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// Look for ((X / Y) * Y)
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if (!match(
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XroundedDownToMultipleOfY,
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m_c_Mul(m_CombineAnd(m_IDiv(m_Specific(Dividend), m_Value(Divisor)),
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m_Instruction(Div)),
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m_Deferred(Divisor))))
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return llvm::None;
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ExpandedMatch M;
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M.Key.SignedOp = Div->getOpcode() == Instruction::SDiv;
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M.Key.Dividend = Dividend;
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M.Key.Divisor = Divisor;
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M.Value = &I;
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return M;
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}
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namespace {
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/// A thin wrapper to store two values that we matched as div-rem pair.
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/// We want this extra indirection to avoid dealing with RAUW'ing the map keys.
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struct DivRemPairWorklistEntry {
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/// The actual udiv/sdiv instruction. Source of truth.
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AssertingVH<Instruction> DivInst;
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/// The instruction that we have matched as a remainder instruction.
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/// Should only be used as Value, don't introspect it.
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AssertingVH<Instruction> RemInst;
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DivRemPairWorklistEntry(Instruction *DivInst_, Instruction *RemInst_)
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: DivInst(DivInst_), RemInst(RemInst_) {
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assert((DivInst->getOpcode() == Instruction::UDiv ||
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DivInst->getOpcode() == Instruction::SDiv) &&
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"Not a division.");
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assert(DivInst->getType() == RemInst->getType() && "Types should match.");
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// We can't check anything else about remainder instruction,
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// it's not strictly required to be a urem/srem.
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}
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/// The type for this pair, identical for both the div and rem.
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Type *getType() const { return DivInst->getType(); }
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/// Is this pair signed or unsigned?
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bool isSigned() const { return DivInst->getOpcode() == Instruction::SDiv; }
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/// In this pair, what are the divident and divisor?
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Value *getDividend() const { return DivInst->getOperand(0); }
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Value *getDivisor() const { return DivInst->getOperand(1); }
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bool isRemExpanded() const {
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switch (RemInst->getOpcode()) {
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case Instruction::SRem:
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case Instruction::URem:
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return false; // single 'rem' instruction - unexpanded form.
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default:
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return true; // anything else means we have remainder in expanded form.
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}
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}
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};
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} // namespace
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using DivRemWorklistTy = SmallVector<DivRemPairWorklistEntry, 4>;
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/// Find matching pairs of integer div/rem ops (they have the same numerator,
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/// denominator, and signedness). Place those pairs into a worklist for further
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/// processing. This indirection is needed because we have to use TrackingVH<>
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/// because we will be doing RAUW, and if one of the rem instructions we change
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/// happens to be an input to another div/rem in the maps, we'd have problems.
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static DivRemWorklistTy getWorklist(Function &F) {
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// Insert all divide and remainder instructions into maps keyed by their
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// operands and opcode (signed or unsigned).
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DenseMap<DivRemMapKey, Instruction *> DivMap;
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// Use a MapVector for RemMap so that instructions are moved/inserted in a
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// deterministic order.
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MapVector<DivRemMapKey, Instruction *> RemMap;
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for (auto &BB : F) {
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for (auto &I : BB) {
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if (I.getOpcode() == Instruction::SDiv)
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DivMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
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else if (I.getOpcode() == Instruction::UDiv)
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DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
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else if (I.getOpcode() == Instruction::SRem)
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RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
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else if (I.getOpcode() == Instruction::URem)
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RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
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else if (auto Match = matchExpandedRem(I))
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RemMap[Match->Key] = Match->Value;
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}
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}
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// We'll accumulate the matching pairs of div-rem instructions here.
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DivRemWorklistTy Worklist;
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// We can iterate over either map because we are only looking for matched
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// pairs. Choose remainders for efficiency because they are usually even more
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// rare than division.
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for (auto &RemPair : RemMap) {
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// Find the matching division instruction from the division map.
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auto It = DivMap.find(RemPair.first);
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if (It == DivMap.end())
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continue;
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// We have a matching pair of div/rem instructions.
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NumPairs++;
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Instruction *RemInst = RemPair.second;
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// Place it in the worklist.
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Worklist.emplace_back(It->second, RemInst);
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}
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return Worklist;
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}
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/// Find matching pairs of integer div/rem ops (they have the same numerator,
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/// denominator, and signedness). If they exist in different basic blocks, bring
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/// them together by hoisting or replace the common division operation that is
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/// implicit in the remainder:
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/// X % Y <--> X - ((X / Y) * Y).
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///
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/// We can largely ignore the normal safety and cost constraints on speculation
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/// of these ops when we find a matching pair. This is because we are already
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/// guaranteed that any exceptions and most cost are already incurred by the
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/// first member of the pair.
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///
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/// Note: This transform could be an oddball enhancement to EarlyCSE, GVN, or
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/// SimplifyCFG, but it's split off on its own because it's different enough
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/// that it doesn't quite match the stated objectives of those passes.
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static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
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const DominatorTree &DT) {
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bool Changed = false;
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// Get the matching pairs of div-rem instructions. We want this extra
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// indirection to avoid dealing with having to RAUW the keys of the maps.
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DivRemWorklistTy Worklist = getWorklist(F);
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// Process each entry in the worklist.
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for (DivRemPairWorklistEntry &E : Worklist) {
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if (!DebugCounter::shouldExecute(DRPCounter))
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continue;
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bool HasDivRemOp = TTI.hasDivRemOp(E.getType(), E.isSigned());
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auto &DivInst = E.DivInst;
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auto &RemInst = E.RemInst;
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const bool RemOriginallyWasInExpandedForm = E.isRemExpanded();
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(void)RemOriginallyWasInExpandedForm; // suppress unused variable warning
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if (HasDivRemOp && E.isRemExpanded()) {
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// The target supports div+rem but the rem is expanded.
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// We should recompose it first.
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Value *X = E.getDividend();
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Value *Y = E.getDivisor();
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Instruction *RealRem = E.isSigned() ? BinaryOperator::CreateSRem(X, Y)
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: BinaryOperator::CreateURem(X, Y);
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// Note that we place it right next to the original expanded instruction,
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// and letting further handling to move it if needed.
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RealRem->setName(RemInst->getName() + ".recomposed");
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RealRem->insertAfter(RemInst);
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Instruction *OrigRemInst = RemInst;
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// Update AssertingVH<> with new instruction so it doesn't assert.
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RemInst = RealRem;
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// And replace the original instruction with the new one.
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OrigRemInst->replaceAllUsesWith(RealRem);
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OrigRemInst->eraseFromParent();
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NumRecomposed++;
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// Note that we have left ((X / Y) * Y) around.
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// If it had other uses we could rewrite it as X - X % Y
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Changed = true;
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}
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assert((!E.isRemExpanded() || !HasDivRemOp) &&
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"*If* the target supports div-rem, then by now the RemInst *is* "
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"Instruction::[US]Rem.");
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// If the target supports div+rem and the instructions are in the same block
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// already, there's nothing to do. The backend should handle this. If the
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// target does not support div+rem, then we will decompose the rem.
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if (HasDivRemOp && RemInst->getParent() == DivInst->getParent())
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continue;
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bool DivDominates = DT.dominates(DivInst, RemInst);
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if (!DivDominates && !DT.dominates(RemInst, DivInst)) {
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// We have matching div-rem pair, but they are in two different blocks,
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// neither of which dominates one another.
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// FIXME: We could hoist both ops to the common predecessor block?
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continue;
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}
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// The target does not have a single div/rem operation,
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// and the rem is already in expanded form. Nothing to do.
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if (!HasDivRemOp && E.isRemExpanded())
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continue;
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if (HasDivRemOp) {
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// The target has a single div/rem operation. Hoist the lower instruction
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// to make the matched pair visible to the backend.
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if (DivDominates)
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RemInst->moveAfter(DivInst);
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else
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DivInst->moveAfter(RemInst);
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NumHoisted++;
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} else {
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// The target does not have a single div/rem operation,
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// and the rem is *not* in a already-expanded form.
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// Decompose the remainder calculation as:
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// X % Y --> X - ((X / Y) * Y).
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assert(!RemOriginallyWasInExpandedForm &&
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"We should not be expanding if the rem was in expanded form to "
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"begin with.");
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Value *X = E.getDividend();
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Value *Y = E.getDivisor();
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Instruction *Mul = BinaryOperator::CreateMul(DivInst, Y);
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Instruction *Sub = BinaryOperator::CreateSub(X, Mul);
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// If the remainder dominates, then hoist the division up to that block:
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//
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// bb1:
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// %rem = srem %x, %y
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// bb2:
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// %div = sdiv %x, %y
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// -->
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// bb1:
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// %div = sdiv %x, %y
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// %mul = mul %div, %y
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// %rem = sub %x, %mul
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//
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// If the division dominates, it's already in the right place. The mul+sub
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// will be in a different block because we don't assume that they are
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// cheap to speculatively execute:
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//
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// bb1:
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// %div = sdiv %x, %y
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// bb2:
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// %rem = srem %x, %y
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// -->
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// bb1:
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// %div = sdiv %x, %y
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// bb2:
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// %mul = mul %div, %y
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// %rem = sub %x, %mul
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//
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// If the div and rem are in the same block, we do the same transform,
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// but any code movement would be within the same block.
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if (!DivDominates)
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DivInst->moveBefore(RemInst);
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Mul->insertAfter(RemInst);
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Sub->insertAfter(Mul);
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// If X can be undef, X should be frozen first.
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// For example, let's assume that Y = 1 & X = undef:
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// %div = sdiv undef, 1 // %div = undef
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// %rem = srem undef, 1 // %rem = 0
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// =>
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// %div = sdiv undef, 1 // %div = undef
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// %mul = mul %div, 1 // %mul = undef
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// %rem = sub %x, %mul // %rem = undef - undef = undef
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// If X is not frozen, %rem becomes undef after transformation.
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// TODO: We need a undef-specific checking function in ValueTracking
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if (!isGuaranteedNotToBeUndefOrPoison(X, nullptr, DivInst, &DT)) {
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auto *FrX = new FreezeInst(X, X->getName() + ".frozen", DivInst);
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DivInst->setOperand(0, FrX);
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Sub->setOperand(0, FrX);
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}
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// Same for Y. If X = 1 and Y = (undef | 1), %rem in src is either 1 or 0,
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// but %rem in tgt can be one of many integer values.
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if (!isGuaranteedNotToBeUndefOrPoison(Y, nullptr, DivInst, &DT)) {
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auto *FrY = new FreezeInst(Y, Y->getName() + ".frozen", DivInst);
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DivInst->setOperand(1, FrY);
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Mul->setOperand(1, FrY);
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}
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// Now kill the explicit remainder. We have replaced it with:
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// (sub X, (mul (div X, Y), Y)
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Sub->setName(RemInst->getName() + ".decomposed");
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Instruction *OrigRemInst = RemInst;
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// Update AssertingVH<> with new instruction so it doesn't assert.
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RemInst = Sub;
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// And replace the original instruction with the new one.
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OrigRemInst->replaceAllUsesWith(Sub);
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OrigRemInst->eraseFromParent();
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NumDecomposed++;
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}
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Changed = true;
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}
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return Changed;
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}
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// Pass manager boilerplate below here.
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namespace {
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struct DivRemPairsLegacyPass : public FunctionPass {
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static char ID;
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DivRemPairsLegacyPass() : FunctionPass(ID) {
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initializeDivRemPairsLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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AU.setPreservesCFG();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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FunctionPass::getAnalysisUsage(AU);
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}
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bool runOnFunction(Function &F) override {
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if (skipFunction(F))
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return false;
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auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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return optimizeDivRem(F, TTI, DT);
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}
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};
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} // namespace
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char DivRemPairsLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(DivRemPairsLegacyPass, "div-rem-pairs",
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"Hoist/decompose integer division and remainder", false,
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false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_END(DivRemPairsLegacyPass, "div-rem-pairs",
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"Hoist/decompose integer division and remainder", false,
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false)
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FunctionPass *llvm::createDivRemPairsPass() {
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return new DivRemPairsLegacyPass();
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}
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PreservedAnalyses DivRemPairsPass::run(Function &F,
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FunctionAnalysisManager &FAM) {
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TargetTransformInfo &TTI = FAM.getResult<TargetIRAnalysis>(F);
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DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
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if (!optimizeDivRem(F, TTI, DT))
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return PreservedAnalyses::all();
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// TODO: This pass just hoists/replaces math ops - all analyses are preserved?
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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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return PA;
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}
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