[llvm-dev] Questions about relaxation in MC

[Kai Wang via llvm-dev]

Hi Anton,

Thanks for your information. I will look into MSP430.

 -Kai

On Tue, Oct 6, 2020 at 5:46 PM Anton Korobeynikov <anton at korobeynikov.info>
wrote:

> And... MSP430 has the similar code :)
>
> On Tue, Oct 6, 2020 at 11:14 AM Hal Finkel via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > This sounds very similar to what PowerPC does, see:
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/Target/PowerPC/PPCBranchSelector.cpp
> >
> > However, PowerPC is doing this in the compiler backend, not in the
> assembler. One issue is that the process must be iterative, because
> branches can go both forward and backward, and replacing some branch
> instructions with the branch pairs can then cause other branches to go out
> of range.
> >
> >  -Hal
> >
> > On 10/5/20 10:20 PM, Kai Wang via llvm-dev wrote:
> >
> > Correct the title.
> >
> > On Tue, Oct 6, 2020 at 11:11 AM Kai Wang <kai.wang at sifive.com> wrote:
> >>
> >> Hi all,
> >>
> >> In RISC-V ISA, the range of conditional branches is within 4KiB. In
> current implementation, if the branch target is out of range, LLVM MC will
> issue an error message to tell users it could not resolve the fixup record.
> I have compared the result with the GNU assembler. GNU assembler will
> convert the branch to inverted one plus jump to make the branch possible.
> The range of unconditional jump is 1MiB. It looks like
> >>
> >> ##########################
> >> bne a0, a1, FAR_BRANCH
> >> …
> >> FAR_BRANCH:
> >>
> >> converted to
> >>
> >> ##########################
> >> beq a0, a1, SKIP_J
> >> j FAR_BRANCH
> >> SKIP_J:
> >> …
> >> FAR_BRANCH:
> >>
> >> I found there is a target hook, relaxInstruction, that tries to achieve
> the similar goal. However, the target hook only replaces one MCInst with
> another one with a larger branch range. For example, c.beqz will be
> converted to beq in the RISC-V backend if the fixup value is out of range.
> There seems no target hook to convert one MCInst to a complex pattern in
> LLVM MC. Do I miss something obvious?
> >>
> >> I found there is a target hook, finishLayout, to manipulate the code
> generated. Does it make sense to implement the feature in finishLayout? Or
> is there any better idea to achieve the conversion? Thanks a lot.
> >>
> >> Best,
> >> Kai
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> > --
> > Hal Finkel
> > Lead, Compiler Technology and Programming Languages
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> --
> With best regards, Anton Korobeynikov
> Department of Statistical Modelling, Saint Petersburg State University
>
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