[LLVMdev] Header in bitcode format 3.0?

[Stuart Hastings]

On May 12, 2011, at 12:30 AM, Sandeep Patel wrote:

> On Wed, May 11, 2011 at 11:42 PM, Stuart Hastings <stuart at apple.com> wrote:
>> 
>> On May 9, 2011, at 1:41 PM, Renato Golin wrote:
>> 
>> [snip]
>> 
>>> One example is the struct byval. The ARM back-end still doesn't
>>> support struct byval (maybe now it does, I was away for a while),
>> 
>> The ARM backend now supports struct byval for APCS.  Extending it to support AAPCS shouldn't be too difficult.  Alas, I won't have time to revisit this in the near future.
> 
> It's slightly unclear to me what byval means for an ABI that passes
> some structs in registers, such as AAPCS-VFP.

IIUC, the byval attribute on a pointer to a structure means "this struct should really be passed by value."  If you ignore the byval attribute, the IR mis-represents what the developer wrote.

The target ISel is supposed to notice the byval attribute and replace the pointer argument with a copy of the structure in the generated code.

I'm not familiar with AAPCS-VFP, but I'd assume that any byval struct should be passed by value, as if byval didn't exist.

The ABI should not be aware that byval is in use.

stuart