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

["Ni, Ray" <ray.ni@intel.com>]

I don't have better names.

Reviewed-by: Ray Ni <ray.ni@intel.com>


> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Marvin Häuser
> Sent: Monday, August 16, 2021 9:10 PM
> To: devel@edk2.groups.io; Ni, Ray <ray.ni@intel.com>
> Cc: Kinney, Michael D <michael.d.kinney@intel.com>; Liming Gao <gaoliming@byosoft.com.cn>; Liu, Zhiguang
> <zhiguang.liu@intel.com>; Vitaly Cheptsov <vit9696@protonmail.com>
> Subject: Re: [edk2-devel] [PATCH V2 2/3] MdePkg/Base.h: Introduce various alignment-related macros
> 
> 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.
> 
> >
> >
> >
> >
> >
> 
> 
> 
> 
>