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[ebp + 6] instead of +8 in a JIT compiler

- by David Titarenco

I'm implementing a simplistic JIT compiler in a VM I'm writing for fun (mostly to learn more about language design) and I'm getting some weird behavior, maybe someone can tell me why. First I define a JIT "prototype" both for C and C++: #ifdef __cplusplus typedef void* (*_JIT_METHOD) (...); #else typedef (*_JIT_METHOD) (); #endif I have a compile() function that will compile stuff into ASM and stick it somewhere in memory: void* compile (void* something) { // grab some memory unsigned char* buffer = (unsigned char*) malloc (1024); // xor eax, eax // inc eax // inc eax // inc eax // ret -> eax should be 3 /* WORKS! buffer[0] = 0x67; buffer[1] = 0x31; buffer[2] = 0xC0; buffer[3] = 0x67; buffer[4] = 0x40; buffer[5] = 0x67; buffer[6] = 0x40; buffer[7] = 0x67; buffer[8] = 0x40; buffer[9] = 0xC3; */ // xor eax, eax // mov eax, 9 // ret 4 -> eax should be 9 /* WORKS! buffer[0] = 0x67; buffer[1] = 0x31; buffer[2] = 0xC0; buffer[3] = 0x67; buffer[4] = 0xB8; buffer[5] = 0x09; buffer[6] = 0x00; buffer[7] = 0x00; buffer[8] = 0x00; buffer[9] = 0xC3; */ // push ebp // mov ebp, esp // mov eax, [ebp + 6] ; wtf? shouldn't this be [ebp + 8]!? // mov esp, ebp // pop ebp // ret -> eax should be the first value sent to the function /* WORKS! */ buffer[0] = 0x66; buffer[1] = 0x55; buffer[2] = 0x66; buffer[3] = 0x89; buffer[4] = 0xE5; buffer[5] = 0x66; buffer[6] = 0x66; buffer[7] = 0x8B; buffer[8] = 0x45; buffer[9] = 0x06; buffer[10] = 0x66; buffer[11] = 0x89; buffer[12] = 0xEC; buffer[13] = 0x66; buffer[14] = 0x5D; buffer[15] = 0xC3; // mov eax, 5 // add eax, ecx // ret -> eax should be 50 /* WORKS! buffer[0] = 0x67; buffer[1] = 0xB8; buffer[2] = 0x05; buffer[3] = 0x00; buffer[4] = 0x00; buffer[5] = 0x00; buffer[6] = 0x66; buffer[7] = 0x01; buffer[8] = 0xC8; buffer[9] = 0xC3; */ return buffer; } And finally I have the main chunk of the program: void main (int argc, char **args) { DWORD oldProtect = (DWORD) NULL; int i = 667, j = 1, k = 5, l = 0; // generate some arbitrary function _JIT_METHOD someFunc = (_JIT_METHOD) compile(NULL); // windows only #if defined _WIN64 || defined _WIN32 // set memory permissions and flush CPU code cache VirtualProtect(someFunc,1024,PAGE_EXECUTE_READWRITE, &oldProtect); FlushInstructionCache(GetCurrentProcess(), someFunc, 1024); #endif // this asm just for some debugging/testing purposes __asm mov ecx, i // run compiled function (from wherever *someFunc is pointing to) l = (int)someFunc(i, k); // did it work? printf("result: %d", l); free (someFunc); _getch(); } As you can see, the compile() function has a couple of tests I ran to make sure I get expected results, and pretty much everything works but I have a question... On most tutorials or documentation resources, to get the first value of a function passed (in the case of ints) you do [ebp+8], the second [ebp+12] and so forth. For some reason, I have to do [ebp+6] then [ebp+10] and so forth. Could anyone tell me why?

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