[llvm-dev] Question about LLVM backend and TableGen

[Nemanja Ivanovic via 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*
$LLVM_BUILD/lib/Target/RISCV/RISCVGenAsmMatcher.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenAsmWriter.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenCompressInstEmitter.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenDAGISel.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenDisassemblerTables.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenInstrInfo.inc
$LLVM_BUILD/lib/Target/RISCV/RISCVGenMCCodeEmitter.inc

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.


On Sat, Nov 20, 2021 at 8:17 AM Sheng Yan Chen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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 :)
>
> Sheng.
>
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