[llvm-dev] Question about LLVM backend and TableGen

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Sheng Yan Chen via llvm-dev

Nov 20, 2021, 8:17:00 AM11/20/21
to llvm...@lists.llvm.org
Hi all,

I'm hacking LLVM TableGen file and I'm stuck with that. My problem is: I know what the records mean, but I don't know what it will become.

Here's a simple example from RISCVInstrInfo.td

def BEQ   :  BranchCC_rri<0b000, "beq">

I know it is a branch-if-equal instruction in RISCV, I know "0b000" is its encoding, I know "beq" is its assembly name. But I don't know what it will becomes after TableGen processing it.

Will it be used at Instruction pattern matching ? Will it be used at disassembler ? Will it be used at assembler ?

How can I know what this record will become after TableGen processing it ?

Thinking from another way, when I writing a new backend, how can I know what kinds of TableGen record I need to supply ?

Is hacking TableGen backend a good way to get through this ?

Thanks for reading this dumb question :)


Nemanja Ivanovic via llvm-dev

Nov 22, 2021, 8:23:09 AM11/22/21
to Sheng Yan Chen, llvm-dev
First off, not a dumb question at all. Most tools in the LLVM tree have reasonably good debug output so you usually don't have to hack the tool to see what it's doing. For llvm-tblgen for example, a very useful option is --print-detailed-records which will show you what it is consuming and the records it creates.
If you only care to see what code llvm-tblgen produces from the .td files, you can find the outputs in the build directory. They are (roughly speaking) named for the target and the type of output llvm-tblgen was asked to produce. For example:

$ grep -l BEQ $LLVM_BUILD/lib/Target/RISCV/RISCVGen*

So you can see all the different things that llvm-tblgen produces for that record (i.e. assembly/disassembly info, ISEL info, etc.).
P.S. You can also see all the options that llvm-tblgen has (or pretty much any tool) by invoking the tool with --help.

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Madhur Amilkanthwar via llvm-dev

Nov 22, 2021, 9:02:38 AM11/22/21
to Nemanja Ivanovic, llvm-dev
A record in a tablegen file has no meaning unless there is a backend which processes it. A record gets processed by one or more tablegen backends and the backend extracts what it wants. 

There are several backends to tablegen and you can find more details here  -https://llvm.org/docs/TableGen/BackEnds.html#llvm-backends.

Disclaimer: Views, concerns, thoughts, questions, ideas expressed in this mail are of my own and my employer has no take in it. 
Thank You.
Madhur D. Amilkanthwar

Sheng Yan Chen via llvm-dev

Nov 23, 2021, 3:54:50 AM11/23/21
to Madhur Amilkanthwar, llvm-dev
Very useful technique and information. It helps me a lot :)

Thank you (*‘ v`*)


Madhur Amilkanthwar <madhu...@gmail.com> 於 2021年11月22日 週一 下午10:02寫道:
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