[llvm] [AMDGPU] Introduce "amdgpu-uniform-intrinsic-combine" pass to combine uniform AMDGPU lane Intrinsics. (PR #116953)

[Sameer Sahasrabuddhe via llvm-commits]

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+//===-- AMDGPUUniformIntrinsicCombine.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
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// This pass simplifies certain intrinsic calls when the arguments are uniform.
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "GCNSubtarget.h"
+#include "llvm/Analysis/DomTreeUpdater.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/UniformityAnalysis.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/InstVisitor.h"
+#include "llvm/IR/IntrinsicsAMDGPU.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+#define DEBUG_TYPE "amdgpu-uniform-intrinsic-combine"
+
+using namespace llvm;
+using namespace llvm::AMDGPU;
+using namespace llvm::PatternMatch;
+
+namespace {
+class AMDGPUUniformIntrinsicCombineLegacy : public FunctionPass {
+public:
+  static char ID;
+  AMDGPUUniformIntrinsicCombineLegacy() : FunctionPass(ID) {
+    initializeAMDGPUUniformIntrinsicCombineLegacyPass(
+        *PassRegistry::getPassRegistry());
+  }
+  bool runOnFunction(Function &F) override;
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    AU.addRequired<UniformityInfoWrapperPass>();
+    AU.addRequired<TargetPassConfig>();
+  }
+};
+
+class AMDGPUUniformIntrinsicCombineImpl
+    : public InstVisitor<AMDGPUUniformIntrinsicCombineImpl> {
+private:
+  const UniformityInfo *UI;
+  bool optimizeUniformIntrinsicInst(IntrinsicInst &II) const;
+
+public:
+  AMDGPUUniformIntrinsicCombineImpl() = delete;
+  AMDGPUUniformIntrinsicCombineImpl(const UniformityInfo *UI) : UI(UI) {}
+  bool run(Function &F);
+};
+} // namespace
+
+char AMDGPUUniformIntrinsicCombineLegacy::ID = 0;
+char &llvm::AMDGPUUniformIntrinsicCombineLegacyPassID =
+    AMDGPUUniformIntrinsicCombineLegacy::ID;
+
+bool AMDGPUUniformIntrinsicCombineLegacy::runOnFunction(Function &F) {
+  if (skipFunction(F)) {
+    return false;
+  }
+  const UniformityInfo *UI =
+      &getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
+  return AMDGPUUniformIntrinsicCombineImpl(UI).run(F);
+}
+
+PreservedAnalyses
+AMDGPUUniformIntrinsicCombinePass::run(Function &F,
+                                       FunctionAnalysisManager &AM) {
+  const auto *UI = &AM.getResult<UniformityInfoAnalysis>(F);
+  bool IsChanged = AMDGPUUniformIntrinsicCombineImpl(UI).run(F);
+
+  if (!IsChanged) {
+    return PreservedAnalyses::all();
+  }
+  PreservedAnalyses PA;
+  PA.preserve<DominatorTreeAnalysis>();
+  PA.preserve<LoopAnalysis>();
+  PA.preserve<ScalarEvolutionAnalysis>();
+  PA.preserve<UniformityInfoAnalysis>();
+  PA.preserve<TargetLibraryAnalysis>();
+  return PA;
+}
+
+bool AMDGPUUniformIntrinsicCombineImpl::run(Function &F) {
+  bool IsChanged{false};
+  Module *M = F.getParent();
+  llvm::LLVMContext &Ctx = M->getContext();
+  llvm::Type *IntrinsicTy = llvm::Type::getInt32Ty(Ctx);
+
+  if (!Intrinsic::getDeclarationIfExists(M, Intrinsic::amdgcn_permlane64,
+                                         {IntrinsicTy}) &&
+      !Intrinsic::getDeclarationIfExists(M, Intrinsic::amdgcn_readfirstlane,
----------------
ssahasra wrote:

To really avoid scanning the entire function `F`, you have to do something like this:
```
if (Function *Intr = getDeclarationIfExists(...)) {
  for (each use U of Intr) {
    if (U is in function F) {
      try replacing U
    }
  }
}
```

Every time the pass is invoked on a different function `F`, it will scan every use of the intrinsic, but that is expected to be much cheaper than scanning the entire body of `F`.

https://github.com/llvm/llvm-project/pull/116953