llvm/llvm/lib/Target/AMDGPU/AMDGPURewriteUndefForPHI.cpp

//===- AMDGPURewriteUndefForPHI.cpp ---------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// This file implements the idea to rewrite undef incoming operand for certain
// PHIs in structurized CFG. This pass only works on IR that has gone through
// StructurizedCFG pass, and this pass has some additional limitation that make
// it can only run after SIAnnotateControlFlow.
//
// To achieve optimal code generation for AMDGPU, we assume that uniformity
// analysis reports the PHI in join block of divergent branch as uniform if
// it has one unique uniform value plus additional undefined/poisoned incoming
// value. That is to say the later compiler pipeline will ensure such PHI always
// return uniform value and ensure it work correctly. Let's take a look at two
// typical patterns in structured CFG that need to be taken care: (In both
// patterns, block %if terminate with divergent branch.)
//
// Pattern A: Block with undefined incoming value dominates defined predecessor
//  %if
//  | \
//  | %then
//  | /
//  %endif: %phi = phi [%undef, %if], [%uniform, %then]
//
//  Pattern B: Block with defined incoming value dominates undefined predecessor
//  %if
//  | \
//  | %then
//  | /
//  %endif: %phi = phi [%uniform, %if], [%undef, %then]
//
// For pattern A, by reporting %phi as uniform, the later pipeline need to make
// sure it be handled correctly. The backend usually allocates a scalar register
// and if any thread in a wave takes %then path, the scalar register will get
// the %uniform value.
//
// For pattern B, we will replace the undef operand with the other defined value
// in this pass. So the scalar register allocated for such PHI will get correct
// liveness. Without this transformation, the scalar register may be overwritten
// in the %then block.
//
// Limitation note:
// If the join block of divergent threads is a loop header, the pass cannot
// handle it correctly right now. For below case, the undef in %phi should also
// be rewritten. Currently we depend on SIAnnotateControlFlow to split %header
// block to get a separate join block, then we can rewrite the undef correctly.
//     %if
//     | \
//     | %then
//     | /
// -> %header: %phi = phi [%uniform, %if], [%undef, %then], [%uniform2, %header]
// |   |
// \---

#include "AMDGPU.h"
#include "llvm/Analysis/UniformityAnalysis.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/InitializePasses.h"

usingnamespacellvm;

#define DEBUG_TYPE …

namespace {

class AMDGPURewriteUndefForPHILegacy : public FunctionPass { … };

} // end anonymous namespace
char AMDGPURewriteUndefForPHILegacy::ID = …;

INITIALIZE_PASS_BEGIN(AMDGPURewriteUndefForPHILegacy, DEBUG_TYPE,
                      "Rewrite undef for PHI", false, false)
INITIALIZE_PASS_DEPENDENCY(UniformityInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(AMDGPURewriteUndefForPHILegacy, DEBUG_TYPE,
                    "Rewrite undef for PHI", false, false)

bool rewritePHIs(Function &F, UniformityInfo &UA, DominatorTree *DT) { … }

bool AMDGPURewriteUndefForPHILegacy::runOnFunction(Function &F) { … }

PreservedAnalyses
AMDGPURewriteUndefForPHIPass::run(Function &F, FunctionAnalysisManager &AM) { … }

FunctionPass *llvm::createAMDGPURewriteUndefForPHILegacyPass() { … }