[GitHub] [incubator-tvm] FrozenGene edited a comment on issue #4657: [CodeGen] Generate blob use LLVM directly

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FrozenGene edited a comment on issue #4657: [CodeGen] Generate blob use LLVM directly
URL: https://github.com/apache/incubator-tvm/pull/4657#issuecomment-572199993
 
 
   @tqchen @zhiics About the LLVM target part, I also think current way maybe is not good. I also think of it and cost my some time. So I want to discuss it too. 
   
   Firstly Let me describe why we need llvm target host. 
   
   When we use PackImportToLLVM, we will need to use LLVM to create LLVM module. 
   
   However, we must need one target machine, this is answered to @tqchen 's question. We can not create one generic target. Because we have platform's specific feature to handle. See:https://github.com/apache/incubator-tvm/blob/4f1772c0598ae708477f457a993f5be52fa71eb9/src/codegen/llvm/codegen_blob.cc#L107 and https://github.com/apache/incubator-tvm/blob/4f1772c0598ae708477f457a993f5be52fa71eb9/src/codegen/llvm/codegen_blob.cc#L167. Another thing is if we leave the target is empty, i.e. no module->setTargetTriple(target_triple.str()); On llvm 6.0 of Mac will report problem : assert error Target-incompatible DataLayout.
   
   More import thing is we need to consider the target host is not x86 cpu. For example, target host is llvm -target aarch64, if we leave it empty and build it on x86 server, we will generate devc.o into x86 format, but in fact we want aarch64 of devc.o. So in the codegen_blob part, we should know the correct llvm target host to generate correct file.
   
   Current way is a little ugly I think too. Current way will create target host based on LLVM when we have detected target host finally (because tvm.build / relay.build's api could leave `target_host` be None, we have some logic to handle this situation). One simple way of course is let users specific the target host of llvm representation, but I use this ugly way is just to detect it automatically and don't want to let users do. I also want to listen to your advices. Thanks in advance.

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