[llvm-dev] GlobalISel: Very limited pattern matching?

Mon May 20 13:17:01 PDT 2019

> On May 20, 2019, at 12:54, Quentin Colombet <qcolombet at apple.com> wrote:
> 
> 
> 
>> On May 20, 2019, at 11:58 AM, Daniel Sanders <daniel_l_sanders at apple.com <mailto:daniel_l_sanders at apple.com>> wrote:
>> 
>> 
>> 
>>> On May 20, 2019, at 10:04, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>>> 
>>> +gisel folks
>>> 
>>> Hi Alex,
>>> 
>>> You’re doing the right thing.
>>> That’s a known limitation that we’ve discussed in https://reviews.llvm.org/D59227 <https://reviews.llvm.org/D59227> but we didn’t really reach a conclusion back them.
>>> Short term, I believe you’re right, we should patch up the GISel emitter to use getConstantVRegVal instead of looking directly for G_CONSTANT.
>> 
>> That's not as easy as you make it sound :-) but let's suppose we can recognize enough of the underlying pattern to be able to use getConstantVRegVal() instead of needing to do a direct translation of the SelectionDAG pattern. In the SelectionDAG patterns an ImmLeaf is an `imm` node with arbitrary C++ predicate(s) attached. There's no practical way to programatically fold a sext/zext/trunc node into the arbitrary C++ code to allow it to match a plain G_CONSTANT, as well as a zexted/sexted/trunced G_CONSTANT. For example, `isUInt<16>(Imm)` will fail if you want it to match (sext (imm):<<Predicate>>) as if it were (sext (imm)):<<Predicate>> because there's no way to understand that we need to replace the isUInt<16> with isInt<16> when we hoist the predicate up.
> 
> I was naively thinking we could replace any check of G_CONSTANT with getConstantVRegValWithLookThrough, but indeed, I have no idea what it would take :).

Mutating the C++ is the only insurmountable problem that I can think of. The others are just tricky. The biggest of the tricky problems is probably linking the operand renderers up to the matched operands. We currently do it by recording the visited instructions and then referencing the matched operands by instruction id and operand index but that only works if the state machine is the one that visited the node. If getConstantVRegValWithLookThrough() visits it outside the view of the state machine then the instruction recorded in the matcher state isn't the right one and we'd need to resolve that somehow.

>>> Long term, this is something we need to discuss. I personally think that we shouldn’t consider G_CONSTANT as true instructions and we should always extend them in place, but this is not a generally belief.
>>> 
>>> Cheers,
>>> -Quentin
>>> 
>>>> On May 20, 2019, at 5:49 AM, via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> I'm trying to get GlobalISel up and running on an off-tree architecture and am thinking I must be doing something wrong, given by how few things actually work.
>>>> 
>>>> Namely, any ImmLeaf<> pattern will fail to match if there is a (TRUNC/ZEXT/SEXT) applied to the constant operand, all of which are commonly created through Legalization. This is due to G_CONSTANT being explicitly looked for by the tablegened code, rather than code that makes use of getConstantVRegVal.
>> 
>> As Quentin notes, getConstantVRegVal() can't look through trunc/zext/sext
> 
> But getConstantVRegValWithLookThrough can (with the right arguments, IIRC).

Ah, I didn't know about that version. It doesn't look like it's being used much but it looks like it does get used for simple immediates in tablegen patterns where there's no arbitrary C++ involved.

>> either so that change by itself wouldn't resolve the issue by itself. The main issue here is that the the SelectionDAG pattern importer is following the semantics of SelectionDAG patterns which in this case is that each node in the pattern (after expanding PatFrag) corresponds to a SelectionDAG node. If the patterns rely on optimizations happening prior to instruction selection then those either same optimizations need implementing or the patterns need extending to support the additional cases. At the moment, it's fairly common to have some C++ instruction selection or additional patterns (usually C++ as many of the targets pre-date the SelectionDAG importer) filling in the gaps in the imported rules.
>> 
>> There's fairly limited support for constant folding at the moment. There's a ConstantFoldingMIRBuilder which folds when instructions are created and a CSEMIRBuilder which also constant folds at the same time as CSE'ing. I just had a quick look at them and it seems we only constant fold binary operations. I just asked Aditya (who added it) about this and the main issue is identifying that a new G_CONSTANT is legal for unary operations that change the type of the constant like G_ZEXT/G_SEXT/G_TRUNC.
>> 
>>>> Is there supposed to be a constant folding pass before Instruction Selection? CSE does not fold past unaries applied to operands, I'm surely missing a pass somewhere...
>>>> 
>>>> Thanks,
>>>> - Alex Davies
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