[llvm-dev] tablegen exponential behavior

[Sebastian Pop via llvm-dev]

Thanks Craig!
I looked at the implementation of tablegen and it is not easy to come up
with some limiting factor.  The best I have found is to compute the
height of the
TreePatternNode and do not generate permutations if the height is
larger than 4 or 5.
Would there be interest in cleaning up that patch and submitting for review?

I have also found out that (on the codes that I care for) I don't need a long
def-pat matching dot products of 8 by 8 bytes, as the SLP vectorizer
transforms that code into vector add/mul that only needs a simple pattern.

Thanks,
Sebastian

On Tue, Sep 10, 2019 at 12:43 AM Craig Topper <craig.topper at gmail.com> wrote:
>
> 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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