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

[edA-qa mort-ora-y via llvm-dev]

I think I've figured it out now. The setup of my triple/datalayout for
the module was apparently not correct. It likely worked mainly to this
point since the default dataLayout is close to the host ABI, but not quite.

I have the following setup, which appears to work:

    module = new llvm::Module( "test", *llvm_context );
    module->setTargetTriple( platform::target->triple );
//x86_64-pc-linux-gnu
   
    std::string err_str;
    auto triple_str = llvm::Triple::normalize(platform::target->triple);
    llvm::Target const *  target = llvm::TargetRegistry::lookupTarget(
triple_str, err_str );
    STATE_CHECK( target, err_str ); //assert target != nullptr
    auto targetMachine = target->createTargetMachine(triple_str,
"generic", "", llvm::TargetOptions{},
        llvm::Optional<llvm::Reloc::Model>());
   
    module->setDataLayout( targetMachine->createDataLayout() );

I'll be testing on Mac later today, but I suspect it has the same data
layout.



On 19/04/18 11:37, mats petersson wrote:
> 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 <mailto: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 <mailto: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
>     >
>     >> _______________________________________________
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>
>
>     -- 
>     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
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>
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-- 
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
	

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