IL offset 0 vs. Native offset 0
Within a function, offset 0 into the native code stream corresponds to the very first native instruction in that function. Since the function is ultimately executed via native code (and not via interpreted IL), it's safe to say that native offset 0 corresponds to the very start of the function. When native-debugging, if you place a function-level breakpoint, it is placed at native offset 0.
Likewise, offset 0 into the IL stream corresponds to the very first IL instruction in that function. However, since the IL doesn't describe the prolog, IL offset 0 starts after the prolog. (There's no IL for the epilog either).
Thus a breakpoint placed at IL offset 0 will skip the prolog. In practice, this only matter if you want to debug the prolog. Since the prolog has only one exit point, IL offset 0 is always guaranteed to be hit.
Take a trivial function that compiles to IL:
int Add(int x, int y)
{
int z = x+y;
return z;
}
Here's a merged view of IL (in red), Native x86 (in normal font) and the source (in bold).
[update:] Note that this is specifically full unoptimized, debuggable code. That way nothing gets inlined, breakpoints all work, you can inspect all locals, etc. Once you enable optimizations, everything gets folded into a single add instruction (see comments for details).
int Add(int x, int y)
{
00000000 push edi <-- start of prolog, Native Offset 0
00000001 push esi
00000002 push ebx
00000003 push ebp
00000004 mov ebx,ecx
00000006 mov esi,edx
00000008 cmp dword ptr ds:[001AA30Ch],0
0000000f je 00000016
00000011 call 769AF339 <-- End of prolog
00000016 xor edi,edi <-- zero out local #0
00000018 xor ebp,ebp <-- zero local #1 int z = x+y;
IL_0000: ldarg.0
IL_0001: ldarg.1
IL_0002: add
0000001a lea eax,[ebx+esi] <-- Here's the native code for IL offset 0.
IL_0003: stloc.1
0000001d mov ebp,eax
return z;
IL_0004: ldloc.1
IL_0005: stloc.0
0000001f mov edi,ebp
}
IL_0006: ldloc.0
IL_0007: ret
00000021 mov eax,edi
00000023 pop ebp <-- epilog and return (return value is in eax).
00000024 pop ebx
00000025 pop esi
00000026 pop edi
00000027 ret
I often find this 3-way view convenient. As another pet project, I'd love to add a debugger tool window that automatically stitches these 3 views together.
