[LLVMdev] Calls to functions with signext/zeroext return values

[Cameron Zwarich]

On Mar 16, 2011, at 11:43 AM, Ken Dyck wrote:

> On Wed, Mar 16, 2011 at 12:35 PM, Cameron Zwarich <zwarich at apple.com> wrote:
>> On Mar 16, 2011, at 9:31 AM, Cameron Zwarich wrote:
>> 
>>> Promoting the return value is unsafe for bool returns on x86-64, which
>>> in the latest revision of the ABI only guarantees that the top 7 bits of
>>> the 8-bit register are 0.
>> 
>> My comment is a bit off, because the question of what type to make
>> the return value is somewhat orthogonal to the question of which zext
>> assert we should add.
> 
> I'm not sure I follow. Won't a zeroext attribute on a bool return
> value ensure that it will be zero-extended to 32 bits by the callee?
> Or does the X86 backend consider such functions unlowerable (via
> TargetLowering::CanLowerReturn()) and thereby bypass the extension to
> 32 bits in SelectionDAGBuilder::visitRet() making a promotion in the
> caller unnecessary?

The X86 backend currently zero-extends them to 32 bits, but according to the ABI it need only zero-extend them to 8 bits. I'm going to change this in a few minutes to expose some additional optimization opportunities.

Cameron