Re: [RFC][V2] RISC-V: Support -mcmodel=large.

[Patrick O'Neill <patrick@rivosinc.com>]

On 12/20/23 10:41, Palmer Dabbelt wrote:
> On Wed, 20 Dec 2023 10:25:00 PST (-0800), jeffreyalaw@gmail.com wrote:
>>
>>
>> On 12/20/23 11:21, Palmer Dabbelt wrote:
>>
>>>> Yea, the implementation relies largely on just pushing stuff into the
>>>> constant pool, so we're largely independent ABI stuff with the likely
>>>> exception being relocations.
>>>
>>> Ya, but I think we'd only need the relocations if we were going to try
>>> relaxing stuff.  We'd kicked around some ideas there: we could
>>> de-duplicate constant pools or inline smaller constants. That's all way
>>> to complex to try and get into this upcoming binutils release, though
>>> (doubly so with this LEB128 ABI break we're still trying to deal with).
>> Agreed.  And note that de-duplication is mostly implemented without need
>> for the target to do anything.  I was kindof amazed to see some of the
>> places it kicked in on other ports I've worked with.
>
> I think all we'd need from GCC is some way to get the "this load is a 
> constant pool address that can be messed with" relocation in there, 
> the linker would do all the heavy lifting.  That's probably just a new 
> assembler pseudo, so pretty much nothing on the compiler side of things.
>
>>>> In theory (and I did not test this), it should be possible to use 
>>>> large
>>>> code model codegen in a smaller mode and it should interoperate.  I
>>>> seriously pondered doing that as an additional test, then figured I 
>>>> had
>>>> other higher priority items on my list.
>>>
>>> IMO we should test that.  At least the common case of a medlow libc
>>> linked into medany programs should be easy.
>>>
>>> +Patrick: let's add some configs to the CI for this?
>> I was pondering a one-off by turning on the large code model by default,
>> then doing a bootstrap & regression test in QEMU.  But integrated into
>> CI is even better.
>
> OK, let's just add it to CI -- it'd be essentially the same testing, 
> just it'll stick around.
ACK :)

Patrick
>
>>
>> Jeff