1 //===- AMDGPURewriteUndefForPHI.cpp ---------------------------------------===//
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 // This file implements the idea to rewrite undef incoming operand for certain
9 // PHIs in structurized CFG. This pass only works on IR that has gone through
10 // StructurizedCFG pass, and this pass has some additional limitation that make
11 // it can only run after SIAnnotateControlFlow.
12 //
13 // To achieve optimal code generation for AMDGPU, we assume that uniformity
14 // analysis reports the PHI in join block of divergent branch as uniform if
15 // it has one unique uniform value plus additional undefined/poisoned incoming
16 // value. That is to say the later compiler pipeline will ensure such PHI always
17 // return uniform value and ensure it work correctly. Let's take a look at two
18 // typical patterns in structured CFG that need to be taken care: (In both
19 // patterns, block %if terminate with divergent branch.)
20 //
21 // Pattern A: Block with undefined incoming value dominates defined predecessor
22 //  %if
23 //  | \
24 //  | %then
25 //  | /
26 //  %endif: %phi = phi [%undef, %if], [%uniform, %then]
27 //
28 //  Pattern B: Block with defined incoming value dominates undefined predecessor
29 //  %if
30 //  | \
31 //  | %then
32 //  | /
33 //  %endif: %phi = phi [%uniform, %if], [%undef, %then]
34 //
35 // For pattern A, by reporting %phi as uniform, the later pipeline need to make
36 // sure it be handled correctly. The backend usually allocates a scalar register
37 // and if any thread in a wave takes %then path, the scalar register will get
38 // the %uniform value.
39 //
40 // For pattern B, we will replace the undef operand with the other defined value
41 // in this pass. So the scalar register allocated for such PHI will get correct
42 // liveness. Without this transformation, the scalar register may be overwritten
43 // in the %then block.
44 //
45 // Limitation note:
46 // If the join block of divergent threads is a loop header, the pass cannot
47 // handle it correctly right now. For below case, the undef in %phi should also
48 // be rewritten. Currently we depend on SIAnnotateControlFlow to split %header
49 // block to get a separate join block, then we can rewrite the undef correctly.
50 //     %if
51 //     | \
52 //     | %then
53 //     | /
54 // -> %header: %phi = phi [%uniform, %if], [%undef, %then], [%uniform2, %header]
55 // |   |
56 // \---
57 
58 #include "AMDGPU.h"
59 #include "llvm/Analysis/UniformityAnalysis.h"
60 #include "llvm/IR/BasicBlock.h"
61 #include "llvm/IR/Constants.h"
62 #include "llvm/IR/Dominators.h"
63 #include "llvm/IR/Instructions.h"
64 #include "llvm/InitializePasses.h"
65 
66 using namespace llvm;
67 
68 #define DEBUG_TYPE "amdgpu-rewrite-undef-for-phi"
69 
70 namespace {
71 
72 class AMDGPURewriteUndefForPHI : public FunctionPass {
73 public:
74   static char ID;
75   AMDGPURewriteUndefForPHI() : FunctionPass(ID) {
76     initializeAMDGPURewriteUndefForPHIPass(*PassRegistry::getPassRegistry());
77   }
78   bool runOnFunction(Function &F) override;
79   StringRef getPassName() const override {
80     return "AMDGPU Rewrite Undef for PHI";
81   }
82 
83   void getAnalysisUsage(AnalysisUsage &AU) const override {
84     AU.addRequired<UniformityInfoWrapperPass>();
85     AU.addRequired<DominatorTreeWrapperPass>();
86 
87     AU.addPreserved<DominatorTreeWrapperPass>();
88     AU.addPreserved<UniformityInfoWrapperPass>();
89     AU.setPreservesCFG();
90   }
91 };
92 
93 } // end anonymous namespace
94 char AMDGPURewriteUndefForPHI::ID = 0;
95 
96 INITIALIZE_PASS_BEGIN(AMDGPURewriteUndefForPHI, DEBUG_TYPE,
97                       "Rewrite undef for PHI", false, false)
98 INITIALIZE_PASS_DEPENDENCY(UniformityInfoWrapperPass)
99 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
100 INITIALIZE_PASS_END(AMDGPURewriteUndefForPHI, DEBUG_TYPE,
101                     "Rewrite undef for PHI", false, false)
102 
103 bool rewritePHIs(Function &F, UniformityInfo &UA, DominatorTree *DT) {
104   bool Changed = false;
105   SmallVector<PHINode *> ToBeDeleted;
106   for (auto &BB : F) {
107     for (auto &PHI : BB.phis()) {
108       if (UA.isDivergent(&PHI))
109         continue;
110 
111       // The unique incoming value except undef/poison for the PHI node.
112       Value *UniqueDefinedIncoming = nullptr;
113       // The divergent block with defined incoming value that dominates all
114       // other block with the same incoming value.
115       BasicBlock *DominateBB = nullptr;
116       // Predecessors with undefined incoming value (excluding loop backedge).
117       SmallVector<BasicBlock *> Undefs;
118 
119       for (unsigned i = 0; i < PHI.getNumIncomingValues(); i++) {
120         Value *Incoming = PHI.getIncomingValue(i);
121         BasicBlock *IncomingBB = PHI.getIncomingBlock(i);
122 
123         if (Incoming == &PHI)
124           continue;
125 
126         if (isa<UndefValue>(Incoming)) {
127           // Undef from loop backedge will not be replaced.
128           if (!DT->dominates(&BB, IncomingBB))
129             Undefs.push_back(IncomingBB);
130           continue;
131         }
132 
133         if (!UniqueDefinedIncoming) {
134           UniqueDefinedIncoming = Incoming;
135           DominateBB = IncomingBB;
136         } else if (Incoming == UniqueDefinedIncoming) {
137           // Update DominateBB if necessary.
138           if (DT->dominates(IncomingBB, DominateBB))
139             DominateBB = IncomingBB;
140         } else {
141           UniqueDefinedIncoming = nullptr;
142           break;
143         }
144       }
145       // We only need to replace the undef for the PHI which is merging
146       // defined/undefined values from divergent threads.
147       // TODO: We should still be able to replace undef value if the unique
148       // value is a Constant.
149       if (!UniqueDefinedIncoming || Undefs.empty() ||
150           !UA.isDivergent(DominateBB->getTerminator()))
151         continue;
152 
153       // We only replace the undef when DominateBB truly dominates all the
154       // other predecessors with undefined incoming value. Make sure DominateBB
155       // dominates BB so that UniqueDefinedIncoming is available in BB and
156       // afterwards.
157       if (DT->dominates(DominateBB, &BB) && all_of(Undefs, [&](BasicBlock *UD) {
158             return DT->dominates(DominateBB, UD);
159           })) {
160         PHI.replaceAllUsesWith(UniqueDefinedIncoming);
161         ToBeDeleted.push_back(&PHI);
162         Changed = true;
163       }
164     }
165   }
166 
167   for (auto *PHI : ToBeDeleted)
168     PHI->eraseFromParent();
169 
170   return Changed;
171 }
172 
173 bool AMDGPURewriteUndefForPHI::runOnFunction(Function &F) {
174   UniformityInfo &UA =
175       getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
176   DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
177   return rewritePHIs(F, UA, DT);
178 }
179 
180 FunctionPass *llvm::createAMDGPURewriteUndefForPHIPass() {
181   return new AMDGPURewriteUndefForPHI();
182 }
183