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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