[squeak-dev] FFI | Byte alignment
Eliot Miranda
eliot.miranda at gmail.com
Sat May 30 23:58:33 UTC 2020
Hi Nicolas, Hi Marcel,
> On May 29, 2020, at 8:02 AM, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com> wrote:
>
>
> Try with this:
>
> cat > test_align.c <<END
> #include <stdio.h>
>
> struct foo1 {int a; long long b; int d;};
> struct foo2 {int a; double c; int d;};
Here’s a suggestion. We add a primitive To the ThreadedFFIPlugin that answers structure alignment thusly:
- the primitive takes as it’s argument the numeric code for the integer or float type or a structure as used in the compiled type specs
- the primitive answers the alignment of the data type in a struct that looks like this:
struct alignment_of_longlong {
char pad_to_force_alignment;
long long element;
}
return &((struct alignment_of_longlong *)0)-> element);
struct alignment_of_double {
char pad_to_force_alignment;
long long element;
}
return &((struct alignment_of_double *)0)-> element);
struct alignment_of_struct {
char pad_to_force_alignment;
struct { char element; } element;
}
return &((struct alignment_of_struct *)0)-> element);
etc
Then we can remember these alignments in class vars of, say, ExternalStructure, check them on start-up, and resize/update ExternalStructure if any of the values change.
> int main() {
> struct foo1 x;
> struct foo2 y;
> printf("size of x = %d\n",sizeof(x));
> printf("size of y = %d\n",sizeof(y));
> return 0;
> }
> END
> i686-w64-mingw32-gcc test_align.c
> ./a.exe
>
> size of x = 24
> size of y = 24
>
> so at leat in windows (mingw) the alignment is 8 bytes for both long long and double.
> I have no 32bits linux handy, but it should be easy enough to pass the same test...
>
>> Le ven. 29 mai 2020 à 16:45, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com> a écrit :
>> Good catch!
>> Wikipedia page does not even agree on longlong... But it's not the ultimate normative reference, better check by ourselves!
>> https://en.wikipedia.org/wiki/Data_structure_alignment
>>
>>> Le ven. 29 mai 2020 à 15:53, Marcel Taeumel <marcel.taeumel at hpi.de> a écrit :
>>> Hi, there.
>>>
>>> In ExternalType class >> #initializeAtomicTypes, there is an interesting claim and a piece of dead code:
>>>
>>> "On 32 bits Windows and MacOS, double and long have an alignment of 8. But on Linux, their alignment is 4"
>>> (Smalltalk wordSize = 4 and: [Smalltalk platformName = 'unix']) ifTrue: [
>>> (#('double longlong ulonglong') includes: typeName) ifTrue: [
>>> byteAlignment := 4
>>> ]
>>> ].
>>>
>>> As you can see, there are single quotes missing and so will the path "byteAlignment := 4" never be reached.
>>>
>>> I tried to figure out whether one should either fix the conditional or remove the entire passage. Maybe this got long fixed inside the FFI plugin?
>>>
>>> Best,
>>> Marcel
>>>
>
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