[LLVMdev] Problem in llvm gcc back-end

[Chris Lattner]

On Mon, 28 May 2007, quho wrote:
> While I testing some code, I found some problem on Union handling.
> I've wrte following test code, and it has union assignment.

BTW, this is fixed in mainline now.  Thanks for the report.

-Chris

> The code's output is
>
> from pointerToUnion: chars mystring, length 64
> from original: chars mystring, length 8000
>
> It's caused by second char member(charlength) of LongestMember.
>
> For union assignment, llvm-backend seems generates assigning each
> member of largest union member. (I've checked it on llvm assembly
> level..)
> But sometimes there is padding between members and it's different for members.
>
> So in my opinion you should handle union assignment as pseudo array of
> word(or byte).
> (it's same for argument passing..)
>
>
> =====================================================
> #include <stdio.h>
>
> struct MyString {
>    char *chars;
>    int length;
> };
>
> struct LongestMember {
>    char *chars;
>    char charlength;
>    int empty1;
>    int empty2;
> };
>
> typedef union {
>    MyString string;
>    LongestMember mt;
> } UnionType;
>
>
> int main() {
>    UnionType original;
>    UnionType *pointerToUnion;
>    UnionType buffer;
>
>    pointerToUnion = &buffer;
>
>    original.string.chars = "mystring";
>    original.string.length = 8000;
>
>    *pointerToUnion = original;
>
>    printf("from pointerToUnion: chars %s, length %d\n",
> pointerToUnion->string.chars, pointerToUnion->string.length);
>    printf("from original: chars %s, length %d\n",
> original.string.chars, original.string.length);
> }
> =====================================================
>
>
> Thanks,
> Quho.
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>

-Chris

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