Re: [edk2-devel] [PATCH V2 2/3] MdePkg/Base.h: Introduce various alignment-related macros

["Marvin Häuser" <mhaeuser@posteo.de>]

Hey Ray,

On 16/08/2021 11:42, Ni, Ray wrote:
> Marvin,
> So lucky to have you in the edk2 project looking into these fundamentals!

Thank you. :)

> +  #define ALIGNOF(TYPE) OFFSET_OF (struct { CHAR8 C; TYPE A; }, A)
>
> 1. Does struct{} inside a macro conform to C standard? How is the compatibility with different compilers?

This should work, yes. The C standard defines offsetof as such:

"The macros are [...]

         offsetof(type, member-designator)

which expands to an integer constant expression that has type size_t, 
the value of
which is the offset in bytes, to the structure member (designated by 
member-designator),
from the beginning of its structure (designated by type). The type and 
member designator
shall be such that given

         static type t;

then the expression &(t.member-designator) evaluates to an address 
constant. [...]" [1]

If we plug in t:

         static struct { CHAR8 C; TYPE A; } t;

we get a valid static storage duration variable declaration that 
satisfies the the last condition because:

"An address constant is [...], a pointer to an lvalue designating an 
object of static
storage duration, or [...]" [2]

It worked with all compilers I tinkered with at https://godbolt.org/
I sadly do not have access to any of the compilers where this may be 
used effectively (RVCT, EBC).

> +#define IS_POW2(Value)  ((Value) != 0U && ((Value) & ((Value) - 1U)) ==
> +0U)
>
> 2. Good to me. I learned this trick when implementing the MtrrLib.
>
> +#define ALIGN_VALUE_ADDEND(Value, Alignment)  (((Alignment) - (Value))
> +& ((Alignment) - 1U))
>
> 3. Is any other open source project using the same macro for the addend?
> This is actually a general question to all new macros.
> I would like the macros look familiar to developers from other open source projects.

Good question, I never really saw it. I only came up with it because for 
the new PE loader, we may align the PE memory within an underaligned 
buffer, and for that we need the addend. I initially used to align up 
and then subtract, but I saw this could be simplified with 
ALIGN_VALUE_ADDEND, which was used in ALIGN_VALUE anyway. If you have a 
better name, I'll change it.

Best regards,
Marvin


[1] ISO/IEC 9899:2011, 7.19, 3.

[2] ISO/IEC 9899:2011, 6.6, 9.

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