[LLVMdev] Types inference in tblgen: Multiple exceptions

[Eli Friedman]

On Fri, Dec 9, 2011 at 4:46 AM, Llopard Ivan <ivanllopard at gmail.com> wrote:
> Hi all,
>
> I am writing a back-end for a processor that has complex type registers.
> It has two functional units to perform complex multiplications.
>  From clang, I emulate a complex multiplication using vectors and, at
> the IR, I got this tblgen-friendly pattern (real component) :
>
> (set RARegs:$dst, (insertelt RARegs:$src,
>           (i16 (trunc (add
>            (ncmul
>             (sext (i16 (extractelt RARegs:$a, imm))),
>             (sext (i16 (extractelt RARegs:$b, imm)))
>             ),
>            (ncmul
>             (sext (i16 (extractelt RARegs:$a, imm))),
>             (sext (i16 (extractelt RARegs:$b, imm)))
>            )
>            ))),
>           imm) )
>
> where RARegs is a register class of type [i32, v2i16]. I want to match
> that pattern in order to have one instruction which takes 2 vectors
> (complex numbers) and gives me another vector. Unfortunately, I am
> running into multiple tblgen type inference exceptions. I am new to llvm
> codegen.
>
> First of all, I realized that I need to explicitly cast intermediate i16
> type results because they are not supported by the architecture, is it
> right?
> For example, if I do not cast extractelt's node type I get the following
> error when I run tblgen with -gen-instr-info:
>
> llvm[3]: Building Meph.td instruction information with tblgen
> llvm-tblgen: llvm/include/llvm/ADT/SmallVector.h:150: T&
> llvm::SmallVectorTemplateCommon<T>::operator[](unsigned int) [with T =
> llvm::MVT::SimpleValueType]: Assertion `begin() + idx < end()' failed.
>
> which comes from utils/TableGen/CodeGenDAGPatterns.cpp:450:
>
>   for (unsigned i = 1, e = Other.TypeVec.size(); i != e; ++i)
>     if (isInteger(Other.TypeVec[i]) && Other.TypeVec[i] > LargestInt)
>       LargestInt = Other.TypeVec[i];
>
> but Other.TypeVec is empty throwing an exception when accessing
> Other.TypeVec[1].
>
> As far as I know, tblgen analyzes sext node and tries to infer operand
> types by applying the specified type constraints of the node. The 2nd
> type constraint of sext enforces extractelt to be scalar and extractelt
> gets i32:v2i16 types (legal types or RARegs types). After that, it tries
> to apply the 3rd type constraint (SDTCisOpSmallerThanOp) and it reaches
> the abnormal condition I shown.
>
> I presume this is a bug in tblgen, should not it verify that TypeVec is
> empty before entering the loop ?

If TableGen crashes, it's a bug.

> Casting intermediate i16 type results, I manage to generate instruction
> information but it throws another exception when generating the
> instruction selector :-(.
>
> vtInt:     (vt:Other)<<P:Predicate_vtInt>>
> Type constraint application shouldn't fail!
>
> Looking again into the code, tblgen does not take into account the
> explicit casts when generating the instruction selector (RemoveAllTypes
> -> InferPossibleTypes) so it gets stuck earlier into the pattern
> (insertelt-trunc). insertelt enforces trunc to be scalar and I have the
> same situation as before. Do you know how can I solve this problem ?
> If it is either not possible or too hard, how should I proceed to detect
> the pattern ?

Your email doesn't really make one thing clear: does your architecture
have i16 registers?

-Eli