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

[Daniel Sanders via llvm-dev]

> On May 20, 2019, at 10:04, Quentin Colombet <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.

> 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 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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