In A64DOpcode.h, we use '%llx' for uint64_t's. On 64-bit Linux, uint64_t is a long unsigned, not a long long unsigned. This may occur on other platforms. I'm not sure if this actually matters, but I'm trying to get rid of build warnings in WebKitGTK. The PRIx64 macro should expand to the correct format for each platform. I think it doesn't work on Visual Studio, but it's defined in Source/JavaScriptCore/os-win32/inttypes.h. I'm hoping the build will automatically pick that up, but I can add a special import for it if necessary. Build output: CXX Source/JavaScriptCore/disassembler/libjavascriptcoregtk_3_0_la-ARM64Disassembler.lo In file included from ../../Source/JavaScriptCore/disassembler/ARM64/A64DOpcode.cpp:27:0: ../../Source/JavaScriptCore/disassembler/ARM64/A64DOpcode.h: In member function 'void JSC::ARM64Disassembler::A64DOpcode::appendUnsignedImmediate64(uint64_t)': ../../Source/JavaScriptCore/disassembler/ARM64/A64DOpcode.h:175:42: error: format '%llx' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t {aka long unsigned int}' [-Werror=format=] bufferPrintf("#0x%llx", immediate); ^ ../../Source/JavaScriptCore/disassembler/ARM64/A64DOpcode.h: In member function 'void JSC::ARM64Disassembler::A64DOpcode::appendPCRelativeOffset(uint32_t*, int32_t)': ../../Source/JavaScriptCore/disassembler/ARM64/A64DOpcode.h:180:74: error: format '%llx' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t {aka long unsigned int}' [-Werror=format=] bufferPrintf("0x%llx", reinterpret_cast<uint64_t>(pc + immediate));