lib/Analysis/CodeMetrics.cpp

   1 //===- CodeMetrics.cpp - Code cost measurements ---------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements code cost measurement utilities.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Analysis/CodeMetrics.h"
  14 #include "llvm/Analysis/AssumptionCache.h"
  15 #include "llvm/Analysis/LoopInfo.h"
  16 #include "llvm/Analysis/TargetTransformInfo.h"
  17 #include "llvm/Analysis/ValueTracking.h"
  18 #include "llvm/IR/DataLayout.h"
  19 #include "llvm/IR/Function.h"
  20 #include "llvm/Support/Debug.h"
  21 #include "llvm/Support/raw_ostream.h"
  22
  23 #define DEBUG_TYPE "code-metrics"
  24
  25 using namespace llvm;
  26
  27 static void
  28 appendSpeculatableOperands(const Value *V,
  29                            SmallPtrSetImpl<const Value *> &Visited,
  30                            SmallVectorImpl<const Value *> &Worklist) {
  31   const User *U = dyn_cast<User>(V);
  32   if (!U)
  33     return;
  34
  35   for (const Value *Operand : U->operands())
  36     if (Visited.insert(Operand).second)
  37       if (isSafeToSpeculativelyExecute(Operand))
  38         Worklist.push_back(Operand);
  39 }
  40
  41 static void completeEphemeralValues(SmallPtrSetImpl<const Value *> &Visited,
  42                                     SmallVectorImpl<const Value *> &Worklist,
  43                                     SmallPtrSetImpl<const Value *> &EphValues) {
  44   // Note: We don't speculate PHIs here, so we'll miss instruction chains kept
  45   // alive only by ephemeral values.
  46
  47   // Walk the worklist using an index but without caching the size so we can
  48   // append more entries as we process the worklist. This forms a queue without
  49   // quadratic behavior by just leaving processed nodes at the head of the
  50   // worklist forever.
  51   for (int i = 0; i < (int)Worklist.size(); ++i) {
  52     const Value *V = Worklist[i];
  53
  54     assert(Visited.count(V) &&
  55            "Failed to add a worklist entry to our visited set!");
  56
  57     // If all uses of this value are ephemeral, then so is this value.
  58     if (!all_of(V->users(), [&](const User *U) { return EphValues.count(U); }))
  59       continue;
  60
  61     EphValues.insert(V);
  62     LLVM_DEBUG(dbgs() << "Ephemeral Value: " << *V << "\n");
  63
  64     // Append any more operands to consider.
  65     appendSpeculatableOperands(V, Visited, Worklist);
  66   }
  67 }
  68
  69 // Find all ephemeral values.
  70 void CodeMetrics::collectEphemeralValues(
  71     const Loop *L, AssumptionCache *AC,
  72     SmallPtrSetImpl<const Value *> &EphValues) {
  73   SmallPtrSet<const Value *, 32> Visited;
  74   SmallVector<const Value *, 16> Worklist;
  75
  76   for (auto &AssumeVH : AC->assumptions()) {
  77     if (!AssumeVH)
  78       continue;
  79     Instruction *I = cast<Instruction>(AssumeVH);
  80
  81     // Filter out call sites outside of the loop so we don't do a function's
  82     // worth of work for each of its loops (and, in the common case, ephemeral
  83     // values in the loop are likely due to @llvm.assume calls in the loop).
  84     if (!L->contains(I->getParent()))
  85       continue;
  86
  87     if (EphValues.insert(I).second)
  88       appendSpeculatableOperands(I, Visited, Worklist);
  89   }
  90
  91   completeEphemeralValues(Visited, Worklist, EphValues);
  92 }
  93
  94 void CodeMetrics::collectEphemeralValues(
  95     const Function *F, AssumptionCache *AC,
  96     SmallPtrSetImpl<const Value *> &EphValues) {
  97   SmallPtrSet<const Value *, 32> Visited;
  98   SmallVector<const Value *, 16> Worklist;
  99
 100   for (auto &AssumeVH : AC->assumptions()) {
 101     if (!AssumeVH)
 102       continue;
 103     Instruction *I = cast<Instruction>(AssumeVH);
 104     assert(I->getParent()->getParent() == F &&
 105            "Found assumption for the wrong function!");
 106
 107     if (EphValues.insert(I).second)
 108       appendSpeculatableOperands(I, Visited, Worklist);
 109   }
 110
 111   completeEphemeralValues(Visited, Worklist, EphValues);
 112 }
 113
 114 /// Fill in the current structure with information gleaned from the specified
 115 /// block.
 116 void CodeMetrics::analyzeBasicBlock(const BasicBlock *BB,
 117                                     const TargetTransformInfo &TTI,
 118                                     const SmallPtrSetImpl<const Value*> &EphValues) {
 119   ++NumBlocks;
 120   unsigned NumInstsBeforeThisBB = NumInsts;
 121   for (const Instruction &I : *BB) {
 122     // Skip ephemeral values.
 123     if (EphValues.count(&I))
 124       continue;
 125
 126     // Special handling for calls.
 127     if (const auto *Call = dyn_cast<CallBase>(&I)) {
 128       if (const Function *F = Call->getCalledFunction()) {
 129         // If a function is both internal and has a single use, then it is
 130         // extremely likely to get inlined in the future (it was probably
 131         // exposed by an interleaved devirtualization pass).
 132         if (!Call->isNoInline() && F->hasInternalLinkage() && F->hasOneUse())
 133           ++NumInlineCandidates;
 134
 135         // If this call is to function itself, then the function is recursive.
 136         // Inlining it into other functions is a bad idea, because this is
 137         // basically just a form of loop peeling, and our metrics aren't useful
 138         // for that case.
 139         if (F == BB->getParent())
 140           isRecursive = true;
 141
 142         if (TTI.isLoweredToCall(F))
 143           ++NumCalls;
 144       } else {
 145         // We don't want inline asm to count as a call - that would prevent loop
 146         // unrolling. The argument setup cost is still real, though.
 147         if (!Call->isInlineAsm())
 148           ++NumCalls;
 149       }
 150     }
 151
 152     if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
 153       if (!AI->isStaticAlloca())
 154         this->usesDynamicAlloca = true;
 155     }
 156
 157     if (isa<ExtractElementInst>(I) || I.getType()->isVectorTy())
 158       ++NumVectorInsts;
 159
 160     if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
 161       notDuplicatable = true;
 162
 163     if (const CallInst *CI = dyn_cast<CallInst>(&I)) {
 164       if (CI->cannotDuplicate())
 165         notDuplicatable = true;
 166       if (CI->isConvergent())
 167         convergent = true;
 168     }
 169
 170     if (const InvokeInst *InvI = dyn_cast<InvokeInst>(&I))
 171       if (InvI->cannotDuplicate())
 172         notDuplicatable = true;
 173
 174     NumInsts += TTI.getUserCost(&I);
 175   }
 176
 177   if (isa<ReturnInst>(BB->getTerminator()))
 178     ++NumRets;
 179
 180   // We never want to inline functions that contain an indirectbr.  This is
 181   // incorrect because all the blockaddress's (in static global initializers
 182   // for example) would be referring to the original function, and this indirect
 183   // jump would jump from the inlined copy of the function into the original
 184   // function which is extremely undefined behavior.
 185   // FIXME: This logic isn't really right; we can safely inline functions
 186   // with indirectbr's as long as no other function or global references the
 187   // blockaddress of a block within the current function.  And as a QOI issue,
 188   // if someone is using a blockaddress without an indirectbr, and that
 189   // reference somehow ends up in another function or global, we probably
 190   // don't want to inline this function.
 191   notDuplicatable |= isa<IndirectBrInst>(BB->getTerminator());
 192
 193   // Remember NumInsts for this BB.
 194   NumBBInsts[BB] = NumInsts - NumInstsBeforeThisBB;
 195 }