[llvm-dev] A struct {i8, i64} has size == 12, clang says size 16

[mats petersson via llvm-dev]

What exactly is your alignment settings in your LLVM struct?

Something like this would tell you the alignment of "something".
const llvm::DataLayout dl(theModule);
size_t size = dl.getPrefTypeAlignment(something);
IIn "my" compiler, I don't do anything special to align structs, so it's
plausibly your specific data-layout that says that i64 only needs aligning
to 32-bits (and clang adjusts it because "it can", by adding extra padding
elements) - or you're using different targets (or settings for the targets)

--
Mats
On 18 April 2018 at 21:47, edA-qa mort-ora-y via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I think I see a potential issue. My ExecutionEngine setup may not be
> using the same target as my object code emitting, and in this test case
> I'm running in the ExecutionEngine.  I'll go over this code to ensure
> I'm creating the same triple and see if that helps -- I'm assuming it
> will, since I can't imagine the exact same triple with clang would
> produce a different layout.
>
>
> On 18/04/18 21:37, Reid Kleckner wrote:
> > It sounds like your DataLayout may not match clang's for x86_64-linux.
> What
> > does it say about the alignment of i64?
> > On Wed, Apr 18, 2018 at 12:05 PM edA-qa mort-ora-y via llvm-dev <
> > llvm-dev at lists.llvm.org> wrote:
> >
> >> I'm creating a struct of `{i8,i64}` and `DataLayout::getTypeAllocSize`
> >> is returning `12`. `getStructLayout` also gives an `4` offset for the
> >> second element.
> >> The native ABI, and clang, for the same type are producing a size of 16,
> >> with an alignment of 8, for the second element.
> >> This is for the system triple "x86_64-linux-gnu"
> >> What could be causing this difference in alignment and how can I fix it?
> >
> >> I'm verifying the native ABI with:
> >>      // clang -emit-llvm ll_struct_arg.c -S -o /dev/tty
> >>      #include <stddef.h>
> >>      typedef struct vpt_data {
> >>          _Bool c;
> >>          long int b;
> >>      } vpt_data;
> >>      int main() {
> >>          vpt_data v;
> >>          vpt_test(v);
> >>          auto a = sizeof(v);
> >>          auto off = offsetof(vpt_data,b);
> >>      }
> >
> >> 16 is stored to `a` and 8 to `off`.
> >> --
> >> edA-qa mort-ora-y
> >>          http://mortoray.com/
> >> Creator of the Leaf language
> >>          http://leaflang.org/
> >> Streaming algorithms, AI, and design on Twitch
> >>          https://www.twitch.tv/mortoray
> >> Twitter
> >>          edaqa
> >
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> edA-qa mort-ora-y
>         http://mortoray.com/
>
> Creator of the Leaf language
>         http://leaflang.org/
>
> Streaming algorithms, AI, and design on Twitch
>         https://www.twitch.tv/mortoray
>
> Twitter
>         edaqa
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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