[llvm-dev] Branch relaxation at assembler level (RISCV)

[Stefan Pintilie via llvm-dev]

Hi,

This discussion caught my eye because we are also looking at a very 
similar problem on PowerPC. 
We have a situation where we want to align a given instruction to a 64 
byte boundary. If it's not already aligned we just add nops until it is 
aligned (We plan to do this in a custom PPCStreamer). If the branch and 
the target of that branch are far enough away adding a few nops in-between 
may actually overflow the 14 bits we have to represent the offset in the 
branch instruction and in that case we have to do something special and 
replace the branch with something else that, once again, is not a single 
instruction.

If mayNeedRelaxation  and relaxInstruction are not the way to do this is 
there any other better way?

Thanks, 
Stefan




From:   "Friedman, Eli via llvm-dev" <llvm-dev at lists.llvm.org>
To:     Paolo <paolo.savini at embecosm.com>, llvm-dev at lists.llvm.org
Date:   2018/12/04 03:15 PM
Subject:        Re: [llvm-dev] Branch relaxation at assembler level 
(RISCV)
Sent by:        "llvm-dev" <llvm-dev-bounces at lists.llvm.org>



On 12/3/2018 2:45 PM, Paolo via llvm-dev wrote:
> Hi all,
>
> I'm trying to implement the same branch relaxation mechanism implemented
> in CodeGen in the MC layer of RISCV.
>
>    beqz t1, L1
>
>    =>
>
>    bnez t1, L2
>
>    j L1
>
> That's because LLVM does not apply the CodeGen optimizations when
> compiling directly from assembly code.
>
> What I'd like to do would be to add a pass that does that on the MC
> instructions or at least to find a way to implement this relaxation in
> the MC assembler.
>
> Any suggestions on where/how to do it? Or any existing fixes?

The RISCV assembler already has code for similar transforms; see 
RISCVAsmBackend::mayNeedRelaxation and 
RISCVAsmBackend::relaxInstruction.  The only tricky bit is that the 
relaxation interface doesn't expect one instruction to be relaxed to two 
instructions... probably not too hard to change, though, if necessary.

That said, I'm a little skeptical this is actually a good idea; the more 
"smart" the assembler is, the harder it becomes to understand what it's 
doing. No other in-tree target does this sort of transform.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux 
Foundation Collaborative Project

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