[llvm-dev] Handling far branches with fixups or ELF relocs

[Hal Finkel via llvm-dev]

Hi, Denis,

You might also take a look at llvm/lib/Target/PowerPC/PPCBranchSelector.cpp

It does things like change:

   //   short branch:
   //     bCC MBB

into:

   //   long branch:
   //     b!CC $PC+8
   //     b MBB

  -Hal

On 7/1/20 3:47 AM, Denis Steckelmacher via llvm-dev wrote:
> Hello,
>
> I'm working on an LLVM backend for an experimental microprocessor. 
> Work is going on nicely, and I've until now found the answer to all my 
> questions directly in the LLVM source code, or in the documentation.
>
> However, I'm having problems with the AsmBackend class and the 
> handling of fixups.
>
> The processor I'm working with has a single conditional branch 
> instruction, JCC, that takes an IP-relative 9-bit immediate offset as 
> operand. A second version of the instruction takes a register as 
> operand and can therefore jump to any 32-bit address.
>
> In AsmBackend, there are methods for relaxing instructions, that I 
> wanted to use to replace "JCC imm9" instructions with a sequence of 
> instructions that jumps further. However, I have two questions:
>
> - relaxInstruction does not seem to be able to replace one instruction 
> with a sequence of instructions
> - I've looked at many other LLVM backends (AVR, ARC, ARM, MIPS and 
> RISC-V), and none of them really seem to do interesting things in 
> relaxInstruction. ARM for instance relaxes Thumb instructions to 
> normal instructions, and RISC-V relaxes compressed instructions to 
> normal ones too. But in both cases, even the normal instructions have 
> limited range, for which there is an assertion, but no "solution" for 
> when a branch exceeds the range of the instruction.
>
> It therefore seems that the problem of "conditional branches that jump 
> too far" is solved elsewhere, but I could not find that location. I 
> looked at LLD, and I've seen that RISC-V has some code there (related 
> to the PLT) that produces sequences of instructions, but not the other 
> targets.
>
> So, what would be the best way to change "JCC imm9" instructions to 
> something else when a branch has to jump further than 256 instructions 
> before or after the current one?
>
> Best regards,
> Denis Steckelmacher
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory