[llvm] Add a pass to convert jump tables to switches. (PR #77709)

[Nikita Popov via llvm-commits]

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@@ -0,0 +1,194 @@
+//===- JumpTableToSwitch.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
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/JumpTableToSwitch.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/DomTreeUpdater.h"
+#include "llvm/Analysis/PostDominators.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/Local.h"
+
+using namespace llvm;
+using namespace PatternMatch;
+
+static cl::opt<unsigned>
+    JumpTableSizeThreshold("jump-table-to-switch-size-threshold", cl::Hidden,
+                           cl::desc("Only split jump tables with size less or "
+                                    "equal than JumpTableSizeThreshold."),
+                           cl::init(10));
+
+#define DEBUG_TYPE "jump-table-to-switch"
+
+namespace {
+struct JumpTableTy {
+  Value *Index;
+  SmallVector<Function *, 5> Funcs;
+};
+} // anonymous namespace
+
+static std::optional<JumpTableTy> parseJumpTable(GetElementPtrInst *GEP) {
+  if (!GEP || !GEP->isInBounds())
+    return std::nullopt;
+  ArrayType *ArrayTy = dyn_cast<ArrayType>(GEP->getSourceElementType());
+  if (!ArrayTy || ArrayTy->getArrayNumElements() > JumpTableSizeThreshold)
+    return std::nullopt;
+  const uint64_t N = ArrayTy->getArrayNumElements();
+  Constant *Ptr = dyn_cast<Constant>(GEP->getPointerOperand());
+  if (!Ptr || !Ptr->getNumOperands())
+    return std::nullopt;
+
+  GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr);
+  if (!GV || !GV->isConstant())
+    return std::nullopt;
+
+  Function &F = *GEP->getParent()->getParent();
+
+  const DataLayout &DL = F.getParent()->getDataLayout();
+  const unsigned BitWidth =
+      DL.getIndexSizeInBits(GEP->getPointerAddressSpace());
+  MapVector<Value *, APInt> VariableOffsets;
+  APInt ConstantOffset(BitWidth, 0);
+  if (!GEP->collectOffset(DL, BitWidth, VariableOffsets, ConstantOffset))
+    return std::nullopt;
+  if (VariableOffsets.empty() || VariableOffsets.size() > 1)
+    return std::nullopt;
+  unsigned Offset = ConstantOffset.getZExtValue();
+  // TODO: consider supporting more general patterns
+  if (Offset != 0)
+    return std::nullopt;
+
+  JumpTableTy JumpTable;
+  JumpTable.Index = VariableOffsets.front().first;
+  JumpTable.Funcs.assign(N, nullptr);
+  PointerType *PtrTy =
+      PointerType::get(F.getContext(), DL.getProgramAddressSpace());
+  const unsigned PtrSizeBits = DL.getPointerTypeSizeInBits(PtrTy);
+  const unsigned PtrSizeBytes = DL.getPointerTypeSize(PtrTy);
+  for (uint64_t Index = 0; Index < N; ++Index) {
+    APInt Offset(PtrSizeBits, Index * PtrSizeBytes);
+    Constant *C = ConstantFoldLoadFromConst(
+        cast<Constant>(GV->getOperand(0)),
+        PointerType::get(F.getContext(), DL.getProgramAddressSpace()), Offset,
----------------
nikic wrote:

This should be using the actual load type -- please test the case where we're not using the program address space. (Which is just a default, not a requirement.)

https://github.com/llvm/llvm-project/pull/77709