[llvm] [RISCV] Introduce pass to promote double constants to a global array (PR #160536)

[Philip Reames via llvm-commits]

================
@@ -0,0 +1,192 @@
+//==- RISCVPromoteConstant.cpp - Promote constant fp to global for RISC-V --==//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "RISCV.h"
+#include "RISCVSubtarget.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Type.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "riscv-promote-const"
+#define RISCV_PROMOTE_CONSTANT_NAME "RISC-V Promote Constants"
+
+STATISTIC(NumPromoted, "Number of constant literals promoted to globals");
+STATISTIC(NumPromotedUses, "Number of uses of promoted literal constants");
+
+namespace {
+
+class RISCVPromoteConstant : public ModulePass {
+public:
+  static char ID;
+  RISCVPromoteConstant() : ModulePass(ID) {}
+
+  StringRef getPassName() const override { return RISCV_PROMOTE_CONSTANT_NAME; }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<TargetPassConfig>();
+    AU.setPreservesCFG();
+  }
+
+  /// Iterate over the functions and promote the double fp constants that
+  /// would otherwise go into the constant pool to a constant array.
+  bool runOnModule(Module &M) override {
+    if (skipModule(M))
+      return false;
+    // TargetMachine and Subtarget are needed to query isFPImmlegal.
+    const TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
+    const TargetMachine &TM = TPC.getTM<TargetMachine>();
+    bool Changed = false;
+    for (Function &F : M) {
+      const RISCVSubtarget &ST = TM.getSubtarget<RISCVSubtarget>(F);
+      const RISCVTargetLowering *TLI = ST.getTargetLowering();
+      Changed |= runOnFunction(F, TLI);
+    }
+    return Changed;
+  }
+
+private:
+  bool runOnFunction(Function &F, const RISCVTargetLowering *TLI);
+};
+} // end anonymous namespace
+
+char RISCVPromoteConstant::ID = 0;
+
+INITIALIZE_PASS(RISCVPromoteConstant, DEBUG_TYPE, RISCV_PROMOTE_CONSTANT_NAME,
+                false, false)
+
+ModulePass *llvm::createRISCVPromoteConstantPass() {
+  return new RISCVPromoteConstant();
+}
+
+bool RISCVPromoteConstant::runOnFunction(Function &F,
+                                         const RISCVTargetLowering *TLI) {
+  if (F.hasOptNone() || F.hasOptSize())
+    return false;
+
+  // Bail out and make no transformation if the target doesn't support
+  // doubles, or if we're not targeting RV64 as we currently see some
+  // regressions for those targets.
+  if (!TLI->isTypeLegal(MVT::f64) || !TLI->isTypeLegal(MVT::i64))
+    return false;
+
+  // Collect all unique double constants and their uses in the function. Use
+  // MapVector to preserve insertion order.
+  MapVector<ConstantFP *, SmallVector<Use *, 8>> ConstUsesMap;
+
+  for (Instruction &I : instructions(F)) {
+    for (Use &U : I.operands()) {
+      auto *C = dyn_cast<ConstantFP>(U.get());
+      if (!C || !C->getType()->isDoubleTy())
+        continue;
+      if (TLI->isFPImmLegal(C->getValueAPF(), MVT::f64,
+                            /*ForCodeSize=*/false))
+        continue;
+      ConstUsesMap[C].push_back(&U);
+    }
+  }
+
+  int PromotableConstants = ConstUsesMap.size();
+  LLVM_DEBUG(dbgs() << "Found " << PromotableConstants
+                    << " promotable constants in " << F.getName() << "\n");
+  // Bail out if no promotable constants found, or if only one is found.
+  if (PromotableConstants < 2) {
+    LLVM_DEBUG(dbgs() << "Performing no promotions as insufficient promotable "
+                         "constants found\n");
+    return false;
+  }
+
+  NumPromoted += PromotableConstants;
+
+  // Create a global array containing the promoted constants.
+  Module *M = F.getParent();
+  Type *DoubleTy = Type::getDoubleTy(M->getContext());
+
+  SmallVector<Constant *, 16> ConstantVector;
+  for (auto const &Pair : ConstUsesMap)
+    ConstantVector.push_back(Pair.first);
+
+  ArrayType *ArrayTy = ArrayType::get(DoubleTy, ConstantVector.size());
+  Constant *GlobalArrayInitializer =
+      ConstantArray::get(ArrayTy, ConstantVector);
+
+  auto *GlobalArray = new GlobalVariable(
+      *M, ArrayTy,
+      /*isConstant=*/true, GlobalValue::InternalLinkage, GlobalArrayInitializer,
+      ".promoted_doubles." + F.getName());
+
+  // A cache to hold the loaded value for a given constant within a basic block.
+  DenseMap<std::pair<ConstantFP *, BasicBlock *>, Value *> LocalLoads;
+
+  // Replace all uses with the loaded value.
+  unsigned Idx = 0;
+  for (auto const &Pair : ConstUsesMap) {
+    ConstantFP *Const = Pair.first;
+    const SmallVector<Use *, 8> &Uses = Pair.second;
+
+    for (Use *U : Uses) {
+      Instruction *UserInst = cast<Instruction>(U->getUser());
+      BasicBlock *InsertionBB;
+      BasicBlock::iterator InsertionPt;
+
+      // If the user is a PHI node, we must insert the load in the
+      // corresponding predecessor basic block. Otherwise, it's inserted into
+      // the same block as the use.
+      if (auto *PN = dyn_cast<PHINode>(UserInst))
+        InsertionBB = PN->getIncomingBlock(*U);
+      else
+        InsertionBB = UserInst->getParent();
+
+      // It is always safe to insert in the first insertion point in the BB,
+      // so do that and let other passes reorder.
+      InsertionPt = InsertionBB->getFirstInsertionPt();
----------------
preames wrote:

Looks like this is only used in the else block below.

https://github.com/llvm/llvm-project/pull/160536