How tvm codegen arm instruction

Questions
#1

Hi all,

I look into tvm source code and wonder how tvm codegen specific target machine code.

with transform.PassContext(opt_level=3):
    graph, lib, params = relay.build_module.build(mod, target, params=params)
lib.export_library("./testnet.tar")

Take arm as an example, relay.build_module.build returns lib as module, I think it is the container of tvm module. And lib.export_library interpret the Module into machine code as shared library and compress it as *.tar file.

  1. How export_library codegen the arm machine code?

I tried to read the source code and find the calling stack as follows:

export_library -->PackImportsToLLVM -->CodeGenBlob

However I don’t find where CodeGenBlob map LLVM IR into corresponding machine code

  1. how export_library codegen shared library?

Does the LLVM do anything for us?

Thanks for any reply!

#2

hi @chiuchiu,

export_library links modules with type llvm or c (termed “DSO-exportable”) using either the system linker or one specified with fcompile. CodeGenBlob handles the rest–those which are not DSO-exportable. The other types of Module, we expect there to be a subclass of Module available at load_library time. That subclass is expected to read a portion of the binary data from CodeGenBlob and return a copy of the original Module.

LLVM is used to generate code for llvm modules.

Hope this helps, Andrew

#3

Hi @areusch,

thanks for your reply!

Can you say more about “DSO-exportable” which I don’t understand what it means? I think, take ‘arm’ as an example, tvm relay IR will transform to llvm IR, but I don’t know how can the llvm IR compile to binary code since I don’t find any *td file, tableGen or anything like llvm backend.

Can you point me some clue? Thanks.

#4

The compiler output is a tree of runtime.Module. DSO-exportable means a runtime.Module in that tree whose type_key is c or llvm. TVM links directly against LLVM and invokes the LLVM APIs to generate code.

When you call export_library, TVM traverses the tree of runtime.Module. When it encounters one with type_key of llvm, it uses LLVM APIs to produce binary .o for the targeted platform, then passes all such encountered modules to the system linker to produce a .so.

#5

Hi @areusch ,

Thanks a lot. Your reply makes it clear.