[BOLT][AArch64] Speedup computeInstructionSize (#121106)

AArch64 instructions have a fixed size 4 bytes, no need to compute.

bolt/include/bolt/Core/BinaryContext.h

@@ -1363,6 +1363,12 @@ public:
     if (std::optional<uint32_t> Size = MIB->getSize(Inst))
       return *Size;
 
+    if (MIB->isPseudo(Inst))
+      return 0;
+
+    if (std::optional<uint32_t> Size = MIB->getInstructionSize(Inst))
+      return *Size;
+
     if (!Emitter)
       Emitter = this->MCE.get();
     SmallString<256> Code;

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -1204,6 +1204,11 @@ public:
   /// Get instruction size specified via annotation.
   std::optional<uint32_t> getSize(const MCInst &Inst) const;
 
+  /// Get target-specific instruction size.
+  virtual std::optional<uint32_t> getInstructionSize(const MCInst &Inst) const {
+    return std::nullopt;
+  }
+
   /// Set instruction size.
   void setSize(MCInst &Inst, uint32_t Size) const;
 

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -1792,6 +1792,11 @@ public:
   }
 
   uint16_t getMinFunctionAlignment() const override { return 4; }
+
+  std::optional<uint32_t>
+  getInstructionSize(const MCInst &Inst) const override {
+    return 4;
+  }
 };
 
 } // end anonymous namespace