[LLVMdev] Union type, is it really used or necessary?

[Erik de Castro Lopo]

David Given wrote:

> On 07/09/10 14:22, Erik de Castro Lopo wrote:
> [...]
> > When generating 32 bit code the struct looks like:
> > 
> >      <{ i32, pointer }>
> > 
> > and for 64 bit code:
> > 
> >      <{ union { i32, i64 }, pointer }>
> 
> Surely LLVM will cause the first structure to be correctly aligned on
> 64-bit platforms by automatically inserting padding? Is explicit
> alignment by the user really necessary?

You missed the point. a struct defined with just parentheses like

     { i32, pointer }

may be padded. A struct defined with angle brackets and parentheses

     <{ i32, pointer }>

is a packed struct, guaranteed not to have padding.

See:

    http://llvm.org/docs/LangRef.html#t_pstruct

Erik
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Erik de Castro Lopo
http://www.mega-nerd.com/