[llvm-dev] [RISCV][PIC] Lowering pseudo instructions in MCCodeEmitter vs AsmPrinter

[Friedman, Eli via llvm-dev]

On 7/10/2018 9:51 AM, Roger Ferrer Ibáñez via llvm-dev wrote:
> H all,
>
> I'm looking at generating PIC code for RISC-V in the context of Linux. 
> Not sure if anyone is working on this already, any inputs are very 
> welcome.
>
> I'm now looking at function calls which in the RISCV backend are 
> represented via two pseudoinstructions RISCV::TAIL and RISCV::CALL.
>
> Currently those pseudos are lowered in MCCodeEmitter. They are 
> expanded into AUIPC and JALR instructions and the first one needs a 
> relocation, which for a static reloc model is R_RISCV_CALL but for PIC 
> code should be R_RISCV_CALL_PLT.

This is not really correct.  A direct call is represented using 
R_RISCV_CALL; a call through a PLT is represented with 
R_RISCV_CALL_PLT.  In a static relocation model, no calls use the PLT; 
with a PIC relocation model, some, but not all, calls use the PLT.

In assembly, this is represented by changing the operand of the call 
instruction: a direct call is "call f", a call through the PLT is "call 
f at plt". In the MC layer, this is represented using 
MCSymbolRefExpr::VK_PLT.  In the LLVM backend, this is represented with 
a target-specifc flag (for example, X86II::MO_PLT).

-Eli

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