[LLVMdev] [PATCH] - Union types, attempt 2

Dan Gohman gohman at apple.com
Tue Jan 12 17:46:14 PST 2010

On Jan 12, 2010, at 5:01 PM, Talin wrote:

> Here is the LangRef part of the patch.

> +<p>The union type is used to represent a set of possible data types which can
> +   exist at a given location in memory (also known as an "untagged"
> +   union).

This wording is somewhat misleading; memory in LLVM has no types.
How about:

"A union type describes an object with size and alignment suitable for
an object of any one of a given set of types."

Also, is it really useful to support
insertvalue/extractvalue/getelementptr on unions? The benefit of unions
that I'm aware of is it allows target-independent IR to work with
appropriately sized and aligned memory. This doesn't require any special
support for accessing union members; for example:

  %p = alloca union { i32, double }
  %q = bitcast union { i32, double }* %p to double*
  store i32 2.0, double* %q

Would this be a reasonable approach?


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