[cfe-dev] ABIArgInfo question for RISC-V and hard float abi

[Roger Ferrer Ibáñez via cfe-dev]

Hi John,

thanks. I'll look into CoerceAndExpand and see how can I fit the
information I need there.

Kind regards,
Roger

2018-06-21 20:00 GMT+02:00 John McCall <rjmccall at apple.com>:
>> On Jun 21, 2018, at 12:48 PM, Roger Ferrer Ibáñez via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>> Dear all,
>>
>> in RISC-V 32-bit, the base calling convention states that integers
>> smaller than 32-bit are sign or zero extended according to their
>> "signedness". So something like the definition below
>>
>> void foo(short s16, unsigned short u16) { }
>>
>> is emitted in IR as
>>
>> void @foo(i16 signext %s16, i16 zeroext %u16)
>>
>> Now, looking at the RISC-V hard float ABI it states that a case like
>> the one below
>>
>> struct float_and_short_t {
>>   float a;
>>   short b;
>> }
>>
>> where there are enough available registers, 'a' goes into a
>> floating-point register and 'b' goes into an integer register and the
>> appropriate sign/zero extension happens. Using ABIArgInfo::getExpand
>> seems to do exactly what I want except there is no way to state that
>> some of the expanded types may need to be sign/zero extended.
>>
>> So, a case like the following
>>
>> struct float_and_short_t
>> {
>>    float a;
>>    short b;
>> };
>> void bar(struct float_and_short_t fs, short i16) { }
>>
>> the emitted IR is
>>
>> void @bar(float %fs.0, i16 %fs.1, i16 signext %i16)
>>
>> where %fs.1 lacks the attribute signext (%i16 does correctly have it).
>>
>> My understanding is that signext/zeroext are attributes of the
>> arguments not llvm;:Type. This would exclude
>> ABIArgInfo::getCoerceAndExpand. Reading the code in CodeGen/CGCall.cpp
>> apparently only ABIArgInfo::getExtend (not to be confused with
>> getExpand) is able to set signext/zeroext. I haven't found any example
>> in CodeGen/TargetInfo.cpp for some other target where a similar case
>> happens.
>>
>> I wonder what would be the best way to achieve this. In this case I
>> know exactly the size of the expansion so I could define a "companion"
>> sequence (i.e. a smallvector or similar) that relates to every element
>> expanded and allows me to set the proper attribute and set it as an
>> attribute of the ABIArgInfo. Perhaps there is a better way.
>
> I think it would be reasonable to generalize what can be done by
> CoerceAndExpand to include possibly adding attributes to the components.
> We might have to think about how to do that.
>
> I would like to remove Expand entirely in favor of CoerceAndExpand, which
> is strictly more flexible, so please don't add anything to it.
>
> John.
>



-- 
Roger Ferrer Ibáñez