Index: tools/llvm-exegesis/lib/BenchmarkResult.cpp
===================================================================
--- tools/llvm-exegesis/lib/BenchmarkResult.cpp
+++ tools/llvm-exegesis/lib/BenchmarkResult.cpp
@@ -17,8 +17,8 @@
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 
-static constexpr const char kIntegerFormat[] = "i_0x%" PRId64 "x";
-static constexpr const char kDoubleFormat[] = "f_%la";
+static constexpr const char kIntegerPrefix[] = "i_0x";
+static constexpr const char kDoublePrefix[] = "f_";
 static constexpr const char kInvalidOperand[] = "INVALID";
 
 // A mutable struct holding an LLVMState that can be passed through the
@@ -73,14 +73,45 @@
   }
 
 private:
+  void serializeIntegerOperand(llvm::raw_ostream &OS, int64_t Value) {
+    OS << kIntegerPrefix;
+    unsigned long long Unsigned;
+    static_assert(sizeof(unsigned long long) == sizeof(uint64_t), "");
+    std::memcpy(&Unsigned, &Value, sizeof(Unsigned));
+    OS.write_hex(Unsigned);
+  }
+
+  bool tryDeserializeIntegerOperand(llvm::StringRef String, int64_t &Value) {
+    if (!String.consume_front(kIntegerPrefix))
+      return false;
+    unsigned long long Result;
+    if (consumeUnsignedInteger(String, 16, Result))
+      return false;
+    static_assert(sizeof(unsigned long long) == sizeof(uint64_t), "");
+    std::memcpy(&Value, &Result, sizeof(Value));
+    return true;
+  }
+
+  void serializeFPOperand(llvm::raw_ostream &OS, double Value) {
+    OS << kDoublePrefix << llvm::format("%la", Value);
+  }
+
+  bool tryDeserializeFPOperand(llvm::StringRef String, double &Value) {
+    if (!String.consume_front(kDoublePrefix))
+      return false;
+    char *EndPointer = nullptr;
+    Value = strtod(String.begin(), &EndPointer);
+    return EndPointer == String.end();
+  }
+
   void serializeMCOperand(const llvm::MCOperand &MCOperand,
                           llvm::raw_ostream &OS) {
     if (MCOperand.isReg()) {
       OS << getRegName(MCOperand.getReg());
     } else if (MCOperand.isImm()) {
-      OS << llvm::format(kIntegerFormat, MCOperand.getImm());
+      serializeIntegerOperand(OS, MCOperand.getImm());
     } else if (MCOperand.isFPImm()) {
-      OS << llvm::format(kDoubleFormat, MCOperand.getFPImm());
+      serializeFPOperand(OS, MCOperand.getFPImm());
     } else {
       OS << kInvalidOperand;
     }
@@ -90,11 +121,9 @@
     assert(!String.empty());
     int64_t IntValue = 0;
     double DoubleValue = 0;
-    if (String[0] == kIntegerFormat[0] &&
-        sscanf(String.data(), kIntegerFormat, &IntValue) == 1)
+    if (tryDeserializeIntegerOperand(String, IntValue))
       return llvm::MCOperand::createImm(IntValue);
-    if (String[0] == kDoubleFormat[0] &&
-        sscanf(String.data(), kDoubleFormat, &DoubleValue) == 1)
+    if (tryDeserializeFPOperand(String, DoubleValue))
       return llvm::MCOperand::createFPImm(DoubleValue);
     if (unsigned RegNo = getRegNo(String))
       return llvm::MCOperand::createReg(RegNo);