[LLVMdev] question on instruction selection

[Nuno Lopes]

> On Jan 26, 2010, at 3:16 PM, Nuno Lopes wrote:
>>
>> For now, I'm just trying to understand what's the main contribution of 
>> this
>> paper towards simplifying the retargeting of a compiler. Don't get me 
>> wrong;
>> I do not want to bash the paper; I just feel that something is escaping 
>> me.
>> The approach proposed seems to be fairly similar to what gcc and LLVM do.
>> What makes me wondering is why their algorithm needs heavy reasoning to 
>> do
>> semantic equivalence checking, while gcc & LLVM only need simple pattern
>> matching. That's why I've been scratching my head the whole day :)
>> Do you have any insight that can enlighten me, please?
>
> I took a very quick look at the paper.  I think the idea is to 
> declaratively specify
> the semantics of each IR operation and each machine instruction, and then
> automatically figure out how to select the IR operations to machine 
> instructions.
>  GCC and LLVM require that mapping to be specified manually.

I believe it is not that automated. The IR->RTL pass is not automated at 
all.
I think I'll just talk with the author once again (now more knowledgeable 
about llvm) :)

Thanks,
Nuno