[llvm-dev] [LLVMdev] TableGen Register Class not matching for MI in 3.6

Wed Aug 19 10:32:08 PDT 2015

> On Aug 19, 2015, at 9:42 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> 
> It seems the problem arises from using multiple reg classes for one MI in the td file, I guess.

Probably, that does not sound something used widely :).

> I'm not sure it takes first available, if I swap the reg classes in the list it does not change and if I replace the GPR reg class with something different than it picks the base reg class fine, potentially it is using the reg class with most available? idk.

My guess is it would take the register class that come first in the register class IDs (not the list on the instruction itself), but I am just guessing.

> 
> I just need to create MIs for every possible case I guess.
> Thanks for the help! :)
> 
> On Wed, Aug 19, 2015 at 12:04 PM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
> Hi Ryan,
> 
>> On Aug 19, 2015, at 6:35 AM, Ryan Taylor via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Essentially it doesn't appear that the reg class assignment is based on uses and is instead inserting an extra COPY for this. Is this accurate? If so, why?
> 
> We match the instructions bottom-up and I believe that copy are automatically inserted when the register classes do not match.
> That seems strange to me that the isel logs are exactly the same but still you are seeing a different result of instruction selection.
> 
>> 
>> In this above example, I'm getting an extra "mov %r0, $b1" (this is an MI::COPY) even though "mov @a, %b1" (this is an MI::MOV) is entirely acceptable since both GPRRegs and BaseRegs are in the reg class list..
>> 
>> If the heuristic is simply picking the first available reg class or the reg class with the most available, for example, instead of picking the reg class based on uses, I will have to change this heuristic to reduce code bloat, or we'll have to add backend passes to reduce the code bloat.
> 
> I think the current approach is rather simple and we just match the type for the first available reg class (assuming your selection dag patterns are not choosing something different). There are not per say passes to reduce this "code bloat” since we never encounter this problem in practice. The peephole optimizer has some logic to avoid this move and CodeGenPrepare also does a few transformation to avoid some of that.
> 
> Anyhow, I’d say debug the isel process (probably the InstrEmitter part) to see where the problem arise.
> 
> Cheers,
> -Quentin
> 
>> 
>> Thanks.
>> 
>> On Mon, Aug 17, 2015 at 7:00 PM, Ryan Taylor <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>> wrote:
>> 
>> ---------- Forwarded message ----------
>> From: Ryan Taylor <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>>
>> Date: Mon, Aug 17, 2015 at 6:59 PM
>> Subject: Re: [LLVMdev] TableGen Register Class not matching for MI in 3.6
>> To: Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>>
>> Cc: "llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>" <llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>>
>> 
>> 
>> The isel logs are exactly the same, nothing changed, it's matching the same instructions just the reg classes are different.
>> 
>> Where in the SelectionDAG is the code that adds an extra MI COPY? I'll have to set a breakpoint and debug backwards from there to see what's going on. It's only happening with the GPRRegs class, for example if I use another reg class instead along with the base regs than it matches the base reg just fine, it's like GPR is overriding any other reg class matching.
>> 
>> Thanks.
>> 
>> On Fri, Jul 31, 2015 at 1:21 PM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>> 
>>> On Jul 31, 2015, at 10:14 AM, Ryan Taylor <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>> wrote:
>>> 
>>> Quentin,
>>> 
>>>  It's in the instruction selection, sorry I forgot to mention that. The Vreg class is GPR and an extra COPY is generated to copy from the GPR to the Base Reg, even though my 'mov' instruction has Base in the Register class list.
>> 
>> Then, I suggest you use -debug-only=isel to check what changed in your selection instruction process. I do not see off-hand what could have caused.
>> 
>> Sorry for not being more helpful here.
>> 
>> Q.
>> 
>>> 
>>> 
>>> On Fri, Jul 31, 2015 at 12:50 PM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>>> Hi Ryan,
>>> 
>>> Could you check where those moves come from?
>>> 
>>> In particular, is this the product of the instruction selection process?
>>> 
>>> You use -print-machineinstrs to see when it is inserted.
>>> 
>>> Thanks,
>>> -Quentin
>>> 
>>> > On Jul 30, 2015, at 2:02 PM, Ryan Taylor <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>> wrote:
>>> >
>>> > In LLVM 3.6,
>>> >
>>> >  We have an instruction that uses a register class that is defined of several different reg classes. In 3.4 this works fine but in 3.6 this is broken.
>>> >
>>> > For example, I have a mov instruction. mov can be executed between different register types (ie gpr, index, base, etc..)
>>> >
>>> > In 3.4, we would get something like this:
>>> >
>>> > mov @a, %b1 // moving this immediate to a base register, which is what we want
>>> >
>>> > In 3.6, we now get this:
>>> >
>>> > mov @a, %r0  // r0 = gpr
>>> > mov %r0, %b1 // b1 = base reg
>>> >
>>> > The register class looks like this:
>>> >
>>> > def ARegs : RegisterClass<"us", [i16], i16, (add GPRRegs, IndexRegs, BaseRegs)>;
>>> >
>>> > I have absolutely no idea why this is not matching any longer?
>>> >
>>> > The fix here is to define an MI with explicit single register class (ie it only allows PTRRegs as the destination).
>>> >
>>> > This must be an issue with something else and not the tablegen but if that was the case I'm not sure. Anyway help would be great, what should I be looking at here?
>>> >
>>> > Thanks.
>>> > _______________________________________________
>>> > LLVM Developers mailing list
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>> 
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