llvm dev - Mar 2021 - Prevent Instruction Fixup from using relocations

Hi:


I'm trying to build a toy backend that produces code without relocation for
my toy ELF loader. For now, my InstructionSelection works as intended and for
MCInstLowering, I just lower MO_GlobalAddress operands with the following code:


```
MCOperand::createExpr(MCSymbolRefExpr::create(getSymbol(MO.getGlobal())))
```


However the backend still tries to use relocation even when I've set
RelocationModel to Reloc::Static. Further debugging shows that in
MCAssembler::evaluateFixup, which in turn calls
MCExpr::evaluateAsRelocatableImpl, the evaluation failed due to the MCSymbol is
not variable. 


That's where I'm lost with what does variable MCSymbol means, where does
it come from and what should I do to fix it. Any help would be appreciated.


Zhang
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On Thu, Mar 11, 2021 at 06:43:16PM +0800, Zhang via llvm-dev
wrote:
> I'm trying to build a toy backend that produces code without
> relocation for my toy ELF loader.
The MC layer is not meant to produce ELF executables, but to produce ELF
relocatable object files. You are still supposed to run a linker on
them. As such, the MC layer will emit relocations for things like symbol
references (e.g. the name of a function) as it is the job of the linker
to resolve those. There are some complications like turning symbol
references into section references, but the same idea applies to those.

Joerg