[LLVMdev] inline asm semantics: output constraint width smaller than input

[Chris Lattner]

On Jan 27, 2009, at 11:41 PM, Duncan Sands wrote:

> Hi Chris,
>
>>> LLVM's Codegen is rejecting inline asm when input/output constraints
>>> have different bitwidths.
>>>
>>> For example in the Linux kernel calls to the various __put_user_
>>> functions take %al, %ax, %eax, %rax/ (%eax:%edx) as input parameter,
>>> and the output parameter is always an int (%eax). (hope I explained
>>> this
>>> right)
>>
>> Right, the interesting wrinkle is that I think llvm's codegen should
>> only see matching constraints of the same type.  If the front-end  
>> sees
>> an i8 tied to an i32, it should extend the i8 or do whatever else it
>> needs to do.
>
> if I understand right the two cases are (for example):
>
> (1): input (I) is i32, output (O) is i8.  In this case the asm output
> type should be changed to i32 (O'), and O is obtained by truncating
> O' to i8.
>
> (2): input (I) is i8, output (O) is i32.  In this case the asm input
> type should be changed to i32 (I'), and I' is obtained from I by
> extending I to i32.
>
> Is that it?

Yes exactly.  On big endian targets a shift is also needed.

-Chris