[llvm-dev] Custom Instruction Cost Model to LLVM RISC-V Backend

[Bandhav Veluri via llvm-dev]

Thanks a lot Henrik!

I figured following would mark a pointer to a specific address space:

#define __myaddrspace __attribute__((address_space(1)))

__myaddrspace int* data;

And, I was able to verify loads being annotated to be from addrspace 1 in
the generated IR. Would this work for automatic variables as well?

In regards to using this in the backend, do I have to just modify the
source, or is it possible to load a library at runtime, like we load a
frontend pass?

On Tue, May 26, 2020 at 12:37 PM Henrik Olsson <hnrklssn at gmail.com> wrote:

> Hi Bandhav,
>
> While I'm unfamiliar with the details of codegen and cost modelling in
> LLVM, it is possible to declare multiple address spaces for a target, mark
> pointers as belonging to a specific address space, and cast between them.
> To quote the language reference "For targets that support them, address
> spaces may affect how optimizations are performed and/or what target
> instructions are used to access the variable.", which sounds quite hopeful
> for your usecase. If nothing else, you could probably model the loads from
> different address spaces as being different instructions, even if they are
> in the end the same instruction in the output machine code, and tag them
> with different costs.
>
> Hope this helps,
> Henrik
>
> On Tue, May 26, 2020 at 4:55 AM Bandhav Veluri via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> I’m working on a RISC-V architecture that has instruction costs different
>> from those in the default cost model. Is there an out-of-source way to
>> provide llc with custom cost model? Or, does this need a change in LLVM
>> backend?
>>
>> Also, the cost model is not totally static. For example, loads from
>> 0x1000-0x1ffc take 1 cycle, whereas loads from address > 0x80000000, take
>> 10-100 cycles. Is it possible to model these costs in the scheduler?
>> Requiring programmer to define custom pointer types to mark pointers, to
>> one or the other address space, is okay. Is there a way to model different
>> costs for different pointer types?
>>
>> Thank you,
>> Bandhav
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
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>>
>
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