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

[Friedman, Eli via llvm-dev]

On 12/5/2018 1:05 PM, Stefan Pintilie wrote:
> 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?

That's normally something you'd handle in code generation. There's a 
target-independent pass in lib/CodeGen/BranchRelaxation.cpp, but it 
looks like PowerPC uses its own implementation in 
lib/Target/PowerPC/PPCBranchSelector.cpp .  Either way, the key is to 
ensure that the size of each instruction is computed conservatively (so 
it should treat your "aligned" instruction as a 64-byte instruction, 
since there will be 60 bytes of padding in the worst case).

-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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