Calling method using inline assembler in gcc - c++

Question

so as I said, I'm trying to call a method using inline asm using gcc. So, I searched how x86 works, and what are the calling convention, then I tried some easy call witch worked perfectly. Then I tried to embed v8, which was my original goal, but it didn't work so well...
Here's my code :
v8::Handle<v8::Value> V8Method::staticInternalMethodCaller(const v8::Arguments& args, int argsize, void* object, void* method)
{
int i = 0;
char* native_args;
// Move the ESP to the end of the array (argsize is the array size in byte)
asm("subl %1, %%esp;"
"movl %%esp, %0;"
: "=r"(native_args)
: "r"(argsize));
// This for loop only converts V8 type to native type,
// and puts them in the array:
for (; i < args.Length(); ++i)
{
if (args[i]->IsInt32())
{
*(int*)(native_args) = args[i]->Int32Value();
native_args += sizeof(int);
}
else if (args[i]->IsNumber())
{
*(float*)(native_args) = (float)(args[i]->NumberValue());
native_args += sizeof(float);
}
}
// Then call the method:
asm("call *%1;" : : "c"(object), "r"(method));
return v8::Null();
}
And here is the generated assembly :
__ZN3srl8V8Method26staticInternalMethodCallerERKN2v89ArgumentsEiPvS5_:
LFB1178:
.cfi_startproc
.cfi_personality 0,___gxx_personality_v0
.cfi_lsda 0,LLSDA1178
pushl %ebp
.cfi_def_cfa_offset 8
.cfi_offset 5, -8
movl %esp, %ebp
.cfi_def_cfa_register 5
pushl %ebx
subl $68, %esp
.cfi_offset 3, -12
movl $0, -12(%ebp)
movl 12(%ebp), %eax
/APP
# 64 "method.cpp" 1
subl %eax, %esp; movl %esp, %ebx; addl $4, %esp
# 0 "" 2
/NO_APP
movl %ebx, -16(%ebp)
jmp L74
L77:
movl -12(%ebp), %eax
movl %eax, (%esp)
movl 8(%ebp), %ecx
LEHB25:
call __ZNK2v89ArgumentsixEi
LEHE25:
subl $4, %esp
movl %eax, -36(%ebp)
leal -36(%ebp), %eax
movl %eax, %ecx
call __ZNK2v86HandleINS_5ValueEEptEv
movl %eax, %ecx
LEHB26:
call __ZNK2v85Value7IsInt32Ev
LEHE26:
testb %al, %al
je L75
movl -12(%ebp), %eax
movl %eax, (%esp)
movl 8(%ebp), %ecx
LEHB27:
call __ZNK2v89ArgumentsixEi
LEHE27:
subl $4, %esp
movl %eax, -32(%ebp)
leal -32(%ebp), %eax
movl %eax, %ecx
call __ZNK2v86HandleINS_5ValueEEptEv
movl %eax, %ecx
LEHB28:
call __ZNK2v85Value10Int32ValueEv
LEHE28:
movl %eax, %edx
movl -16(%ebp), %eax
movl %edx, (%eax)
movl -16(%ebp), %eax
movl (%eax), %ebx
movl $LC4, 4(%esp)
movl $__ZSt4cout, (%esp)
LEHB29:
call __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
movl -16(%ebp), %edx
movl %edx, (%esp)
movl %eax, %ecx
call __ZNSolsEPKv
subl $4, %esp
movl $LC5, 4(%esp)
movl %eax, (%esp)
call __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
movl %ebx, (%esp)
movl %eax, %ecx
call __ZNSolsEi
subl $4, %esp
movl $__ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, (%esp)
movl %eax, %ecx
call __ZNSolsEPFRSoS_E
subl $4, %esp
addl $4, -16(%ebp)
jmp L76
L75:
movl -12(%ebp), %eax
movl %eax, (%esp)
movl 8(%ebp), %ecx
call __ZNK2v89ArgumentsixEi
LEHE29:
subl $4, %esp
movl %eax, -28(%ebp)
leal -28(%ebp), %eax
movl %eax, %ecx
call __ZNK2v86HandleINS_5ValueEEptEv
movl %eax, %ecx
LEHB30:
call __ZNK2v85Value8IsNumberEv
LEHE30:
testb %al, %al
je L76
movl -12(%ebp), %eax
movl %eax, (%esp)
movl 8(%ebp), %ecx
LEHB31:
call __ZNK2v89ArgumentsixEi
LEHE31:
subl $4, %esp
movl %eax, -24(%ebp)
leal -24(%ebp), %eax
movl %eax, %ecx
call __ZNK2v86HandleINS_5ValueEEptEv
movl %eax, %ecx
LEHB32:
call __ZNK2v85Value11NumberValueEv
LEHE32:
fstps -44(%ebp)
flds -44(%ebp)
movl -16(%ebp), %eax
fstps (%eax)
movl -16(%ebp), %eax
movl (%eax), %ebx
movl $LC4, 4(%esp)
movl $__ZSt4cout, (%esp)
LEHB33:
call __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
movl -16(%ebp), %edx
movl %edx, (%esp)
movl %eax, %ecx
call __ZNSolsEPKv
subl $4, %esp
movl $LC5, 4(%esp)
movl %eax, (%esp)
call __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
movl %ebx, (%esp)
movl %eax, %ecx
call __ZNSolsEf
subl $4, %esp
movl $__ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, (%esp)
movl %eax, %ecx
call __ZNSolsEPFRSoS_E
subl $4, %esp
addl $4, -16(%ebp)
L76:
incl -12(%ebp)
L74:
movl 8(%ebp), %ecx
call __ZNK2v89Arguments6LengthEv
cmpl -12(%ebp), %eax
setg %al
testb %al, %al
jne L77
movl 16(%ebp), %eax
movl 20(%ebp), %edx
movl %eax, %ecx
/APP
# 69 "method.cpp" 1
call *%edx;
# 0 "" 2
/NO_APP
call __ZN2v84NullEv
leal -20(%ebp), %edx
movl %eax, (%esp)
movl %edx, %ecx
call __ZN2v86HandleINS_5ValueEEC1INS_9PrimitiveEEENS0_IT_EE
subl $4, %esp
movl -20(%ebp), %eax
jmp L87
L83:
movl %eax, (%esp)
call __Unwind_Resume
L84:
movl %eax, (%esp)
call __Unwind_Resume
L85:
movl %eax, (%esp)
call __Unwind_Resume
L86:
movl %eax, (%esp)
call __Unwind_Resume
LEHE33:
L87:
movl -4(%ebp), %ebx
leave
.cfi_restore 5
.cfi_restore 3
.cfi_def_cfa 4, 4
ret
.cfi_endproc
So, this static method is a callback (I do some signature checking before) witch is supposed to call the specific method providing valid C++ native args. In order to speed up a little bit and avoid copies of args, I'm trying to load all param in an local array, and then modify the ESP to make this array an argument.
The method call works well, but I don't get correct arguments... I've done lots of research about function call, calling convention, and lots of test (which were all successful), but I don't understand what is going on... Is there something I missed ?
Basically, the callee is supposed to get its arguments at the top of the esp, in my case, the array... (I precise that the array is valid)
I use GCC.

Answer 1

There are many problems with what you are attempting.
You cannot modify %esp using inline assembly, because the compiler
is probably using %esp to reference its local variables and arguments. This may work if the compiler uses %ebp instead, but there is no guarantee.
You never undo the %esp modification before returning.
In your inline assembly, you need to declare that %esp is side-effected.
You probably need to pass object as a silent first argument. method is an instance method, not a static method?
all of this depends on what calling convention you're using: cdecl, stdcall, etc.

Answer 2

I'd recommend not trying to do this yourself, there are a lot of annoying little details that have to be gotten exactly right. I'd suggest instead using the FFCALL library, specifically the avcall set of methods, to do this.
I imagine that something like this would do what you want:
v8::Handle<v8::Value> V8Method::staticInternalMethodCaller(const v8::Arguments& args, int argsize, void* object, void* method)
{
// Set up the argument list with the function pointer, return type, and
// pointer to value storing the return value (assuming int, change if
// necessary)
int return_value;
av_alist alist;
av_start_int(alist, method, &return_value);
for(int i = args.Length() - 1; i >= 0; i--)
{
// Push the arguments onto the argument list
if (args[i]->IsInt32())
{
av_int(alist, args[i]->Int32Value());
}
else if (args[i]->IsNumber())
{
av_double(alist, (float)(args[i]->NumberValue());
}
}
av_call(alist); // Call the function
return v8::Null();
}