GCC "thiscall" calling convention [Linux/Win32 MinGW]

[Cypher]

ZOMBIE! Just had to comment on this nearly year-old thread. As someone who used to work on the MSVC team, I should point out that both internal calling conventions and structure packing/layout are, technically, implementation-defined. So it's not that MSVC is "wrong" and GCC is "right" (or vice-versa). Just different.

Of course, this implementation-dependent thing may have changed since FWIK C++0x has done some work to define backwards "binary compatibility" w/C, but back when we were doing the MSVC compiler (circa VS2k3... yes, I'm old), the ONLY thing that guided us in this respect was the ANSI/ISO standards (which left the implementation up to the compiler writer) and the Intel ABI, which just gave hints and a few hard-coded specifications (specifically calling conventions).

<end defensive rant for a product that I no longer work around>

While you're here... Why does MSVC pad the object if you use an __int64?

[MaiN]

While you're here... Why does MSVC pad the object if you use an __int64?

Wouldn't that be to make sure they're always 8-byte aligned?

[Cypher]

Wouldn't that be to make sure they're always 8-byte aligned?

Objects in MSVC are 8-byte aligned even when compiling for x86? I always just assumed it would be 4-byte, i.e. sizeof(void*). (Serious question.. I've never actually bothered to look into the internals of all that.)

[MaiN]

Objects in MSVC are 8-byte aligned even when compiling for x86? I always just assumed it would be 4-byte, i.e. sizeof(void*). (Serious question.. I've never actually bothered to look into the internals of all that.)

It does make sense to 8-byte align 64-bit datatypes (like __int64) because there are certain instructions that require aligned 8-byte memory instructions, for instance almost all of the SSE instruction set requires aligned operands.

[Cypher]

It does make sense to 8-byte align 64-bit datatypes (like __int64) because there are certain instructions that require aligned 8-byte memory instructions, for instance almost all of the SSE instruction set requires aligned operands.

Good point. Thanks.