[BOLT][AArch64] Inlining of Memcpy (llvm#154929)

The pass for inlining memcpy in BOLT was currently X86-specific and was
using the instruction `rep movsb`.

This patch implements a static size analysis system for AArch64 memcpy
inlining that extracts copy sizes from preceding instructions to then
use it to generate the optimal width-specific load/store sequences.

Author: yafet-a

bolt/docs/CommandLineArgumentReference.md

@@ -637,7 +637,7 @@
 
 - `--inline-memcpy`
 
-  Inline memcpy using 'rep movsb' instruction (X86-only)
+  Inline memcpy using optimized instruction sequences (X86: 'rep movsb', AArch64: width-optimized register operations)
 
 - `--inline-small-functions`
 

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -14,6 +14,7 @@
 #ifndef BOLT_CORE_MCPLUSBUILDER_H
 #define BOLT_CORE_MCPLUSBUILDER_H
 
+#include "bolt/Core/BinaryBasicBlock.h"
 #include "bolt/Core/MCPlus.h"
 #include "bolt/Core/Relocation.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -1902,13 +1903,38 @@ class MCPlusBuilder {
     return {};
   }
 
+  /// Find memcpy size in bytes by using preceding instructions.
+  /// Returns std::nullopt if size cannot be determined (no-op for most
+  /// targets).
+  virtual std::optional<uint64_t>
+  findMemcpySizeInBytes(const BinaryBasicBlock &BB,
+                        BinaryBasicBlock::iterator CallInst) const {
+    return std::nullopt;
+  }
+
   /// Creates inline memcpy instruction. If \p ReturnEnd is true, then return
   /// (dest + n) instead of dest.
   virtual InstructionListType createInlineMemcpy(bool ReturnEnd) const {
     llvm_unreachable("not implemented");
     return {};
   }
 
+  /// Creates size-aware inline memcpy instruction. If \p KnownSize is provided,
+  /// generates optimized code for that specific size. Falls back to regular
+  /// createInlineMemcpy if size is unknown or not needed (e.g. with X86).
+  virtual InstructionListType
+  createInlineMemcpy(bool ReturnEnd, std::optional<uint64_t> KnownSize) const {
+    return createInlineMemcpy(ReturnEnd);
+  }
+
+  /// Extract immediate value from move instruction that sets the given
+  /// register. Returns the immediate value if the instruction is a
+  /// move-immediate to TargetReg.
+  virtual std::optional<uint64_t>
+  extractMoveImmediate(const MCInst &Inst, MCPhysReg TargetReg) const {
+    return std::nullopt;
+  }
+
   /// Create a target-specific relocation out of the \p Fixup.
   /// Note that not every fixup could be converted into a relocation.
   virtual std::optional<Relocation>

bolt/lib/Passes/BinaryPasses.cpp

@@ -1843,7 +1843,7 @@ Error StripRepRet::runOnFunctions(BinaryContext &BC) {
 }
 
 Error InlineMemcpy::runOnFunctions(BinaryContext &BC) {
-  if (!BC.isX86())
+  if (!BC.isX86() && !BC.isAArch64())
     return Error::success();
 
   uint64_t NumInlined = 0;
@@ -1866,8 +1866,16 @@ Error InlineMemcpy::runOnFunctions(BinaryContext &BC) {
         const bool IsMemcpy8 = (CalleeSymbol->getName() == "_memcpy8");
         const bool IsTailCall = BC.MIB->isTailCall(Inst);
 
+        // Extract size from preceding instructions (AArch64 only).
+        // Pattern: MOV X2, #nb-bytes; BL memcpy src, dest, X2.
+        std::optional<uint64_t> KnownSize =
+            BC.MIB->findMemcpySizeInBytes(BB, II);
+
+        if (BC.isAArch64() && (!KnownSize.has_value() || *KnownSize > 64))
+          continue;
+
         const InstructionListType NewCode =
-            BC.MIB->createInlineMemcpy(IsMemcpy8);
+            BC.MIB->createInlineMemcpy(IsMemcpy8, KnownSize);
         II = BB.replaceInstruction(II, NewCode);
         std::advance(II, NewCode.size() - 1);
         if (IsTailCall) {

bolt/lib/Rewrite/BinaryPassManager.cpp

@@ -248,7 +248,9 @@ static cl::opt<bool> Stoke("stoke", cl::desc("turn on the stoke analysis"),
 
 static cl::opt<bool> StringOps(
     "inline-memcpy",
-    cl::desc("inline memcpy using 'rep movsb' instruction (X86-only)"),
+    cl::desc(
+        "inline memcpy using size-specific optimized instructions "
+        "(X86: 'rep movsb', AArch64: width-optimized register operations)"),
     cl::cat(BoltOptCategory));
 
 static cl::opt<bool> StripRepRet(

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -2620,6 +2620,122 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
   getInstructionSize(const MCInst &Inst) const override {
     return 4;
   }
+
+  std::optional<uint64_t>
+  extractMoveImmediate(const MCInst &Inst, MCPhysReg TargetReg) const override {
+    // Match MOVZ instructions (both X and W register variants) with no shift.
+    if ((Inst.getOpcode() == AArch64::MOVZXi ||
+         Inst.getOpcode() == AArch64::MOVZWi) &&
+        Inst.getOperand(2).getImm() == 0 &&
+        getAliases(TargetReg)[Inst.getOperand(0).getReg()])
+      return Inst.getOperand(1).getImm();
+    return std::nullopt;
+  }
+
+  std::optional<uint64_t>
+  findMemcpySizeInBytes(const BinaryBasicBlock &BB,
+                        BinaryBasicBlock::iterator CallInst) const override {
+    MCPhysReg SizeReg = getIntArgRegister(2);
+    if (SizeReg == getNoRegister())
+      return std::nullopt;
+
+    BitVector WrittenRegs(RegInfo->getNumRegs());
+    const BitVector &SizeRegAliases = getAliases(SizeReg);
+
+    for (auto InstIt = BB.begin(); InstIt != CallInst; ++InstIt) {
+      const MCInst &Inst = *InstIt;
+      WrittenRegs.reset();
+      getWrittenRegs(Inst, WrittenRegs);
+
+      if (WrittenRegs.anyCommon(SizeRegAliases))
+        return extractMoveImmediate(Inst, SizeReg);
+    }
+    return std::nullopt;
+  }
+
+  InstructionListType
+  createInlineMemcpy(bool ReturnEnd,
+                     std::optional<uint64_t> KnownSize) const override {
+    assert(KnownSize.has_value() &&
+           "AArch64 memcpy inlining requires known size");
+    InstructionListType Code;
+    uint64_t Size = *KnownSize;
+
+    generateSizeSpecificMemcpy(Code, Size);
+
+    // If _memcpy8, adjust X0 to return dest+size instead of dest.
+    if (ReturnEnd)
+      Code.emplace_back(MCInstBuilder(AArch64::ADDXri)
+                            .addReg(AArch64::X0)
+                            .addReg(AArch64::X0)
+                            .addImm(Size)
+                            .addImm(0));
+    return Code;
+  }
+
+  InstructionListType generateSizeSpecificMemcpy(InstructionListType &Code,
+                                                 uint64_t Size) const {
+    auto AddLoadStorePair = [&](unsigned LoadOpc, unsigned StoreOpc,
+                                unsigned Reg, unsigned Offset = 0) {
+      Code.emplace_back(MCInstBuilder(LoadOpc)
+                            .addReg(Reg)
+                            .addReg(AArch64::X1)
+                            .addImm(Offset));
+      Code.emplace_back(MCInstBuilder(StoreOpc)
+                            .addReg(Reg)
+                            .addReg(AArch64::X0)
+                            .addImm(Offset));
+    };
+
+    // Generate optimal instruction sequences based on exact size.
+    switch (Size) {
+    case 1:
+      AddLoadStorePair(AArch64::LDRBBui, AArch64::STRBBui, AArch64::W9);
+      break;
+    case 2:
+      AddLoadStorePair(AArch64::LDRHHui, AArch64::STRHHui, AArch64::W9);
+      break;
+    case 4:
+      AddLoadStorePair(AArch64::LDRWui, AArch64::STRWui, AArch64::W9);
+      break;
+    case 8:
+      AddLoadStorePair(AArch64::LDRXui, AArch64::STRXui, AArch64::X9);
+      break;
+    case 16:
+      AddLoadStorePair(AArch64::LDRQui, AArch64::STRQui, AArch64::Q16);
+      break;
+    case 32:
+      AddLoadStorePair(AArch64::LDRQui, AArch64::STRQui, AArch64::Q16, 0);
+      AddLoadStorePair(AArch64::LDRQui, AArch64::STRQui, AArch64::Q17, 1);
+      break;
+
+    default:
+      // For sizes up to 64 bytes, greedily use the largest possible loads.
+      // Caller should have already filtered out sizes > 64 bytes.
+      assert(Size <= 64 &&
+             "Size should be <= 64 bytes for AArch64 memcpy inlining");
+
+      uint64_t Remaining = Size;
+      uint64_t Offset = 0;
+
+      const std::array<std::tuple<uint64_t, unsigned, unsigned, unsigned>, 5>
+          LoadStoreOps = {
+              {{16, AArch64::LDRQui, AArch64::STRQui, AArch64::Q16},
+               {8, AArch64::LDRXui, AArch64::STRXui, AArch64::X9},
+               {4, AArch64::LDRWui, AArch64::STRWui, AArch64::W9},
+               {2, AArch64::LDRHHui, AArch64::STRHHui, AArch64::W9},
+               {1, AArch64::LDRBBui, AArch64::STRBBui, AArch64::W9}}};
+
+      for (const auto &[OpSize, LoadOp, StoreOp, TempReg] : LoadStoreOps)
+        while (Remaining >= OpSize) {
+          AddLoadStorePair(LoadOp, StoreOp, TempReg, Offset / OpSize);
+          Remaining -= OpSize;
+          Offset += OpSize;
+        }
+      break;
+    }
+    return Code;
+  }
 };
 
 } // end anonymous namespace