[llvm-dev] Questions about relaxation in MC

[Konstantin Schwarz via llvm-dev]

Hi Kai,

did you make any progress on this topic?
We have a similar case in our backend, where conditional branches have 
only a limited range.
So for hand-written assembly, we would like to relax using the scheme 
you describe: invert the branch condition and add an unconditional jump.

As you've already discovered, the relaxInstruction callback doesn't 
allow to insert additional instructions, but requires the relaxed 
instruction to be returned.
We could work around that by relaxing to a pseudo instruction whose size 
is the combined size of the conditional branch + jump and expand it 
later to the actual instructions, but that seems unnecessarily complex.

Did you come up with a better solution?

Konstantin

On 07.10.2020 17:20, Philip Reames via llvm-dev wrote:
>
> If done in the assembler, this is branch relaxation.  You need to 
> implement the calls backs mayNeedRelaxation, and relaxInstruction on 
> your target MCAsmBackend.  The iteration relaxation logic already 
> exists, see the generic MC code.
>
> Philip
>
> On 10/5/20 8:20 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 
>> <mailto: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
>>
>>
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