1 //===- CodeMetrics.cpp - Code cost measurements ---------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements code cost measurement utilities.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Analysis/AssumptionCache.h"
15 #include "llvm/Analysis/CodeMetrics.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/Analysis/TargetTransformInfo.h"
18 #include "llvm/Analysis/ValueTracking.h"
19 #include "llvm/IR/CallSite.h"
20 #include "llvm/IR/DataLayout.h"
21 #include "llvm/IR/Function.h"
22 #include "llvm/IR/IntrinsicInst.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 
26 #define DEBUG_TYPE "code-metrics"
27 
28 using namespace llvm;
29 
30 static void
31 appendSpeculatableOperands(const Value *V,
32                            SmallPtrSetImpl<const Value *> &Visited,
33                            SmallVectorImpl<const Value *> &Worklist) {
34   const User *U = dyn_cast<User>(V);
35   if (!U)
36     return;
37 
38   for (const Value *Operand : U->operands())
39     if (Visited.insert(Operand).second)
40       if (isSafeToSpeculativelyExecute(Operand))
41         Worklist.push_back(Operand);
42 }
43 
44 static void completeEphemeralValues(SmallPtrSetImpl<const Value *> &Visited,
45                                     SmallVectorImpl<const Value *> &Worklist,
46                                     SmallPtrSetImpl<const Value *> &EphValues) {
47   // Note: We don't speculate PHIs here, so we'll miss instruction chains kept
48   // alive only by ephemeral values.
49 
50   // Walk the worklist using an index but without caching the size so we can
51   // append more entries as we process the worklist. This forms a queue without
52   // quadratic behavior by just leaving processed nodes at the head of the
53   // worklist forever.
54   for (int i = 0; i < (int)Worklist.size(); ++i) {
55     const Value *V = Worklist[i];
56 
57     assert(Visited.count(V) &&
58            "Failed to add a worklist entry to our visited set!");
59 
60     // If all uses of this value are ephemeral, then so is this value.
61     if (!all_of(V->users(), [&](const User *U) { return EphValues.count(U); }))
62       continue;
63 
64     EphValues.insert(V);
65     DEBUG(dbgs() << "Ephemeral Value: " << *V << "\n");
66 
67     // Append any more operands to consider.
68     appendSpeculatableOperands(V, Visited, Worklist);
69   }
70 }
71 
72 // Find all ephemeral values.
73 void CodeMetrics::collectEphemeralValues(
74     const Loop *L, AssumptionCache *AC,
75     SmallPtrSetImpl<const Value *> &EphValues) {
76   SmallPtrSet<const Value *, 32> Visited;
77   SmallVector<const Value *, 16> Worklist;
78 
79   for (auto &AssumeVH : AC->assumptions()) {
80     if (!AssumeVH)
81       continue;
82     Instruction *I = cast<Instruction>(AssumeVH);
83 
84     // Filter out call sites outside of the loop so we don't do a function's
85     // worth of work for each of its loops (and, in the common case, ephemeral
86     // values in the loop are likely due to @llvm.assume calls in the loop).
87     if (!L->contains(I->getParent()))
88       continue;
89 
90     if (EphValues.insert(I).second)
91       appendSpeculatableOperands(I, Visited, Worklist);
92   }
93 
94   completeEphemeralValues(Visited, Worklist, EphValues);
95 }
96 
97 void CodeMetrics::collectEphemeralValues(
98     const Function *F, AssumptionCache *AC,
99     SmallPtrSetImpl<const Value *> &EphValues) {
100   SmallPtrSet<const Value *, 32> Visited;
101   SmallVector<const Value *, 16> Worklist;
102 
103   for (auto &AssumeVH : AC->assumptions()) {
104     if (!AssumeVH)
105       continue;
106     Instruction *I = cast<Instruction>(AssumeVH);
107     assert(I->getParent()->getParent() == F &&
108            "Found assumption for the wrong function!");
109 
110     if (EphValues.insert(I).second)
111       appendSpeculatableOperands(I, Visited, Worklist);
112   }
113 
114   completeEphemeralValues(Visited, Worklist, EphValues);
115 }
116 
117 /// Fill in the current structure with information gleaned from the specified
118 /// block.
119 void CodeMetrics::analyzeBasicBlock(const BasicBlock *BB,
120                                     const TargetTransformInfo &TTI,
121                                     const SmallPtrSetImpl<const Value*> &EphValues) {
122   ++NumBlocks;
123   unsigned NumInstsBeforeThisBB = NumInsts;
124   for (const Instruction &I : *BB) {
125     // Skip ephemeral values.
126     if (EphValues.count(&I))
127       continue;
128 
129     // Special handling for calls.
130     if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
131       ImmutableCallSite CS(&I);
132 
133       if (const Function *F = CS.getCalledFunction()) {
134         // If a function is both internal and has a single use, then it is
135         // extremely likely to get inlined in the future (it was probably
136         // exposed by an interleaved devirtualization pass).
137         if (!CS.isNoInline() && F->hasInternalLinkage() && F->hasOneUse())
138           ++NumInlineCandidates;
139 
140         // If this call is to function itself, then the function is recursive.
141         // Inlining it into other functions is a bad idea, because this is
142         // basically just a form of loop peeling, and our metrics aren't useful
143         // for that case.
144         if (F == BB->getParent())
145           isRecursive = true;
146 
147         if (TTI.isLoweredToCall(F))
148           ++NumCalls;
149       } else {
150         // We don't want inline asm to count as a call - that would prevent loop
151         // unrolling. The argument setup cost is still real, though.
152         if (!isa<InlineAsm>(CS.getCalledValue()))
153           ++NumCalls;
154       }
155     }
156 
157     if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
158       if (!AI->isStaticAlloca())
159         this->usesDynamicAlloca = true;
160     }
161 
162     if (isa<ExtractElementInst>(I) || I.getType()->isVectorTy())
163       ++NumVectorInsts;
164 
165     if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
166       notDuplicatable = true;
167 
168     if (const CallInst *CI = dyn_cast<CallInst>(&I)) {
169       if (CI->cannotDuplicate())
170         notDuplicatable = true;
171       if (CI->isConvergent())
172         convergent = true;
173     }
174 
175     if (const InvokeInst *InvI = dyn_cast<InvokeInst>(&I))
176       if (InvI->cannotDuplicate())
177         notDuplicatable = true;
178 
179     NumInsts += TTI.getUserCost(&I);
180   }
181 
182   if (isa<ReturnInst>(BB->getTerminator()))
183     ++NumRets;
184 
185   // We never want to inline functions that contain an indirectbr.  This is
186   // incorrect because all the blockaddress's (in static global initializers
187   // for example) would be referring to the original function, and this indirect
188   // jump would jump from the inlined copy of the function into the original
189   // function which is extremely undefined behavior.
190   // FIXME: This logic isn't really right; we can safely inline functions
191   // with indirectbr's as long as no other function or global references the
192   // blockaddress of a block within the current function.  And as a QOI issue,
193   // if someone is using a blockaddress without an indirectbr, and that
194   // reference somehow ends up in another function or global, we probably
195   // don't want to inline this function.
196   notDuplicatable |= isa<IndirectBrInst>(BB->getTerminator());
197 
198   // Remember NumInsts for this BB.
199   NumBBInsts[BB] = NumInsts - NumInstsBeforeThisBB;
200 }
201