[llvm-dev] tablegen exponential behavior

[Craig Topper via llvm-dev]

add and mul are both commutable and tablegen tries to create patterns with
every possible permutation of commutable operations.

~Craig


On Mon, Sep 9, 2019 at 9:08 PM Sebastian Pop via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> I implemented a pattern matching of the dot product for arm64
> and it seemed to work well for the basic case, i.e.,
>
> class mulB<SDPatternOperator ldop> :
>   PatFrag<(ops node:$Rn, node:$Rm, node:$offset),
>           (mul (ldop (add node:$Rn, node:$offset)),
>                (ldop (add node:$Rm, node:$offset)))>;
> class mulBz<SDPatternOperator ldop> :
>   PatFrag<(ops node:$Rn, node:$Rm),
>           (mul (ldop node:$Rn), (ldop node:$Rm))>;
>
> class DotProductI32<Instruction DOT, SDPatternOperator ldop> :
>   Pat<(i32 (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 3)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 2)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 1)),
>                 (mulBz<ldop> GPR64sp:$Rn, GPR64sp:$Rm))))),
>       (EXTRACT_SUBREG
>        (i64 (DOT (DUPv2i32gpr WZR),
>                  (v8i8 (LD1Onev8b GPR64sp:$Rn)),
>                  (v8i8 (LD1Onev8b GPR64sp:$Rm)))),
>        sub_32)>, Requires<[HasDotProd]>;
>
>   def : DotProductI32<SDOTv8i8, sextloadi8>;
>   def : DotProductI32<UDOTv8i8, zextloadi8>;
>
> Then when I extended it to 8 element vectors, the time spent by tblgen
> exploded:
> from under 7 seconds (on A-72) on the AArch64 td files and the above patch
> to more than half an hour when I decided to terminate the processes.
>
> Here are the additional def'pats that produce the exponential behavior:
>
> def VADDV_32 : OutPatFrag<(ops node:$R), (ADDPv2i32 node:$R, node:$R)>;
>
> class DotProduct2I32<Instruction DOT, SDPatternOperator ldop> :
>   Pat<(i32 (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 7)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 6)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 5)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 4)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 3)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 2)),
>            (add (mulB<ldop> GPR64sp:$Rn, GPR64sp:$Rm, (i64 1)),
>                 (mulBz<ldop> GPR64sp:$Rn, GPR64sp:$Rm))))))))),
>       (EXTRACT_SUBREG
>        (VADDV_32
>         (i64 (DOT (DUPv2i32gpr WZR),
>                   (v8i8 (LD1Onev8b GPR64sp:$Rn)),
>                   (v8i8 (LD1Onev8b GPR64sp:$Rm))))),
>        sub_32)>, Requires<[HasDotProd]>;
>
>   def : DotProduct2I32<SDOTv8i8, sextloadi8>;
>   def : DotProduct2I32<UDOTv8i8, zextloadi8>;
>
> linux-perf profile for the first minute executing llvm-tblgen shows
> that most of the time is spent in isIsomorphicTo:
>
>   28.25%  llvm-tblgen  llvm-tblgen          [.]
> llvm::TreePatternNode::isIsomorphicTo
>   21.62%  llvm-tblgen  llvm-tblgen          [.]
> llvm::TypeSetByHwMode::operator==
>   15.25%  llvm-tblgen  libc-2.27.so         [.] memcmp
>   14.61%  llvm-tblgen  llvm-tblgen          [.]
> std::__shared_ptr<llvm::TreePatternNode,
> (__gnu_cxx::_Lock_policy)2>::__shared_ptr
>
> In call-graph mode `perf -g` points to GenerateVariants that generates
> most of the calls to isIsomorphicTo:
>
> +  100.00%     0.00%  llvm-tblgen  llvm-tblgen          [.] main
> +  100.00%     0.00%  llvm-tblgen  llvm-tblgen          [.]
> llvm::TableGenMain
> +   99.85%     0.00%  llvm-tblgen  llvm-tblgen          [.] (anonymous
> namespace)::LLVMTableGenMain
> +   99.85%     0.00%  llvm-tblgen  llvm-tblgen          [.]
> llvm::EmitDAGISel
> +   99.85%     0.00%  llvm-tblgen  llvm-tblgen          [.]
> llvm::CodeGenDAGPatterns::CodeGenDAGPatterns
> +   99.46%    98.01%  llvm-tblgen  llvm-tblgen          [.]
> llvm::CodeGenDAGPatterns::GenerateVariants
>      0.38%     0.00%  llvm-tblgen  llvm-tblgen          [.]
> GenerateVariantsOf
>
> Sebastian
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