Re: Struct alignment vs alignment of fields
On Thursday, 7 August 2014 at 17:22:15 UTC, Marc Schütz wrote:
(Original discussion:
http://forum.dlang.org/thread/fckwpddiwxonabqaf...@forum.dlang.org#post-pskjgieddhpntzaokohj:40forum.dlang.org)
I would expect `B` to have a gap between `ex` and `mmid`. AFAIK
the outer `align(1)` only applies to the struct in its
entirety, not to the individual fields. However for both DMD
git and LDC 0.14.0-alpha1 (based on DMD 2.065), `A` and `B`
have the same size.
After some thinking, I believe this is because arrays inherit
the alignment of their element types. Is this correct? If yes,
where is this documented? I had expected `char[4]` to be
aligned at a 4-byte boundary.
I'm not sure about all the latest compiler changes, but let's
try and answer some of this.
TDPL pg. 268-269 explains this (although could be out of date
with recent changes). I'll copy what's relevant..
7.1.11.1 The align Attribute
If you want to override the compiler's choice of alignment, which
influences the padding inserted. You could use an align
modifier... etc..
class A {
char a;
align(1) int b;
char c;
}
With the specification above the fields of A are laid out
without gaps between them.
You may use align with an entire class definition:
align(1) struct S {
char a;
int b;
char c
}
...
Align is not suppose to be used with pointers and references...
Back to the question. Most fields will be aligned on 4-byte
boundaries, or 8-byte depending on if it's 32/64 bit. This is
mostly for performance reasons, but also with addresses it may
affect the GC. Overriding the compiler is mostly going to be more
useful when working against C/C++ structures where they are also
forcibly aligned for space.
So assuming we have the above struct.
S[2] s;
It's probably going to be aligned on 4's for the first one; But
it doesn't have to be. But the others? I'm not so sure... The
inner alignment and padding is 4 per, so if the struct S has a
size of 12, then it's still going to be aligned by 4's by
default... i think? Maybe the alignment has to do if it's
inserted into another object. On the stack it probably ignores
the alignment attribute...
struct B {
char a;
S s; //infers align(1) by it's definition?
}
Well regardless, unless you're overriding both, you're probably
going to get some form of alignments of 4, be it for arrays or
for speed... I hope this isn't confusing.
Re: Struct alignment vs alignment of fields
Still watching this, but the Dconf 2014 bare metal presentation gets into it a bit... http://youtu.be/qErXPomAWYI?t=37m20s
Re: Struct alignment vs alignment of fields
p.s. seems that aligning works only on ints. i.e. on types which has sizeof >= default platform align.
Re: Struct alignment vs alignment of fields
yeah, chars (and bytes, and so on) are not aligned. i.e.
align(1) struct B {
int qtim;
int bid;
int ofr;
int bidsiz;
int ofrsiz;
short mode;
char ex;
byte mmid;
char z;
}
has sizeof == 25. not sure if specs mentions this, but they
should.
Re: Struct alignment vs alignment of fields
On Friday, 8 August 2014 at 18:20:41 UTC, ketmar wrote:
yeah, chars (and bytes, and so on) are not aligned. i.e.
align(1) struct B {
int qtim;
int bid;
int ofr;
int bidsiz;
int ofrsiz;
short mode;
char ex;
byte mmid;
char z;
}
has sizeof == 25. not sure if specs mentions this, but they
should.
It's not surprising that `char` and `byte` behave like this,
because `byte.alignof == 1`. But it's not obvious that this also
applies to arrays of them. I had expected them to be treated as
opaque objects of a certain size, and therefore have an alignment
that corresponds to their size.
pragma(msg, byte.alignof);
pragma(msg, (byte[1]).alignof);
pragma(msg, (byte[2]).alignof);
pragma(msg, (byte[3]).alignof);
pragma(msg, (byte[4]).alignof);
pragma(msg, (byte[5]).alignof);
pragma(msg, (byte[6]).alignof);
struct S {
byte a;
byte b;
byte c;
byte d;
}
struct T {
byte a;
short b;
int c;
}
pragma(msg, S.alignof);
pragma(msg, T.alignof);
This outputs "1" for all types except `T`, which has 4. So this
even applies to structs, not only arrays. Which may make sense,
because each element will always be accessed with correct
alignment. However, accessing the aggregate as a whole might
result in unaligned reads/writes.
