Hi Eliot,

those values are constant on a platform, right? So, we could make an external-shared-pool for it... :-)

I am almost done with the next iteration on FFI-Pools package. I am just cleaning up, adding some class comments, ...

Best,
Marcel
Am 31.05.2020 01:58:46 schrieb Eliot Miranda <eliot.miranda at gmail.com>:
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 [mailto: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 [https://en.wikipedia.org/wiki/Data_structure_alignment]

Le ven. 29 mai 2020 à 15:53, Marcel Taeumel <marcel.taeumel at hpi.de [mailto: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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