In D101538#2769796, @qcolombet wrote:

I wouldn't be so sure about that. One particular usecase I need this for is a bitcast between pointer types:

%1 = bitcast i8* %0 to %struct.ST*
In our target they're both of p0 LLTs, so the default method eliminates this instruction while in our target it's necessary to keep it.

That sounds like you're abusing the IR here. Shouldn't you have a different addrspace here?

FWIW, when we discussed the design of GISel internally originally we explicitly chose not to allow virtual overloading of the IR translation steps because that step follow what is represented in the IR.
To make a parallel with SDISel, this is as if we would allow SDISelBuilder to be target specific.

Bottom line, for just this one use case I feel that it doesn't make sense to allow virtual method everywhere yet.

You could workaround that with introducing a pseudo/intrisinc in codegen prepare for instance.

My 2 cents.

Cheers,
-Quentin

In D101538#2768105, @foad wrote:

Won't this problem go away when we introduce FP LLTs?

I wouldn't be so sure about that. One particular usecase I need this for is a bitcast between pointer types:

%1 = bitcast i8* %0 to %struct.ST*

In our target they're both of p0 LLTs, so the default method eliminates this instruction while in our target it's necessary to keep it.

What are you going to do when IR switches to opaque pointers? https://llvm.org/docs/OpaquePointers.html

In D101538#2764529, @arsenm wrote:

In D101538#2751818, @aemerson wrote:

In D101538#2750853, @zuban32 wrote:

In D101538#2745606, @aemerson wrote:

Is this really necessary? Our preference is to use custom combines/lowering passes, or custom legalization, to do this.

Well, our usecase might not be that canonical, let me briefly explain. For example, translating a bitcast - default IRTranslator implementation turns it into a COPY when LLTs of src and dst are the same. In our target (SPIR-V) LLTs do not matter that much, the typeinfo is represented with some pseudo instructions, so we run into this situation quite often. And losing a bitcast in the translator doesn't work for us as it'd obviously be quite difficult to recover it.

Won't this problem go away when we introduce FP LLTs?

@aemerson @foad if no one has any idea how to make CRTP approach feasible here, then I'd suggest to merge this, I have no write access though

In D101538#2753323, @foad wrote:

In D101538#2751818, @aemerson wrote:

In D101538#2750853, @zuban32 wrote:

In D101538#2745606, @aemerson wrote:

Is this really necessary? Our preference is to use custom combines/lowering passes, or custom legalization, to do this.

Well, our usecase might not be that canonical, let me briefly explain. For example, translating a bitcast - default IRTranslator implementation turns it into a COPY when LLTs of src and dst are the same. In our target (SPIR-V) LLTs do not matter that much, the typeinfo is represented with some pseudo instructions, so we run into this situation quite often. And losing a bitcast in the translator doesn't work for us as it'd obviously be quite difficult to recover it.

I see. Is this a widespread issue with a lot of the translation functions or just bitcasts? If it's just bitcast you could just make that virtual so we don't have to pay the virtual function call overhead every translate. If not, then I'm ok with doing this.

How about doing it with https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern#Static_polymorphism to avoid the virtual function call overhead?

In D101538#2751818, @aemerson wrote:

In D101538#2750853, @zuban32 wrote:

In D101538#2745606, @aemerson wrote:

Is this really necessary? Our preference is to use custom combines/lowering passes, or custom legalization, to do this.

Well, our usecase might not be that canonical, let me briefly explain. For example, translating a bitcast - default IRTranslator implementation turns it into a COPY when LLTs of src and dst are the same. In our target (SPIR-V) LLTs do not matter that much, the typeinfo is represented with some pseudo instructions, so we run into this situation quite often. And losing a bitcast in the translator doesn't work for us as it'd obviously be quite difficult to recover it.

I see. Is this a widespread issue with a lot of the translation functions or just bitcasts? If it's just bitcast you could just make that virtual so we don't have to pay the virtual function call overhead every translate. If not, then I'm ok with doing this.

In D101538#2745606, @aemerson wrote:

Is this really necessary? Our preference is to use custom combines/lowering passes, or custom legalization, to do this.

Gentle ping

Rebased