llvm dev - Feb 2009 - [LLVMdev] Question about variable sized arrays

I'd like to represent strings "Pascal-style" with a length field
followed by the actual bytes. Reading the reference manual, I think
using a zero-length array should work in a struct like this:

      { i32, [0 x int8] }
      
However, I get a problem with referencing constants. The following
code doesn't compie:

    @foo = constant { i32, [3 x i8] } { i32 3, [3 x i8] c"foo" }

    define void @main( { i32, [0 x i8] }* %ptr) {
        store { i32, [3 x i8] }* @foo, { i32, [0 x i8] }** %ptr
    }
         
llvm-as reports: 

    Can't store '{ i32, [3 x i8] } *' into space of type '{ i32,
[0 x i8] } *'

I'm wondering what the right way to do this is? I suppose I could
cast the pointer but is that really necessary? I'm pretty new to
LLVM so please forgive me if I'm missing something obvious ...

Thanks a lot!

Robin

You should bitcast the storage type of the constant (which includes  
the actual array bound) to the logical type (which includes the  
flexible array member instead). That is, bitcast { i32, [3 x i8] }*  
@foo to { i32, [0 x i8] }* before storing to %ptr.

This is somewhat analogous to C array type decay which allows you to  
pass "abc" (type const char[4]) to strlen(const char*). (However, in  
the case of array -> pointer conversions, a 'gep 0, 0' is sufficient.
That technique doesn't work for flexible array constants.)

On 2009-02-25, at 21:09, Robin Sommer wrote:
> I'd like to represent strings "Pascal-style" with a length
field
> followed by the actual bytes. Reading the reference manual, I think
> using a zero-length array should work in a struct like this:
>
>      { i32, [0 x int8] }
>
> However, I get a problem with referencing constants. The following
> code doesn't compie:
>
>    @foo = constant { i32, [3 x i8] } { i32 3, [3 x i8] c"foo" }
>
>    define void @main( { i32, [0 x i8] }* %ptr) {
>        store { i32, [3 x i8] }* @foo, { i32, [0 x i8] }** %ptr
>    }
>
> llvm-as reports:
>
>    Can't store '{ i32, [3 x i8] } *' into space of type '{
i32, [0 x
> i8] } *'
>
> I'm wondering what the right way to do this is? I suppose I could
> cast the pointer but is that really necessary? I'm pretty new to
> LLVM so please forgive me if I'm missing something obvious ...
>
> Thanks a lot!
>
> Robin
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> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


— Gordon