[llvm-commits] [patch] Add TLS hook to MCTargetExpr (AArch64 generic change)

[Tim Northover]

Hi all,

ELF ABIs require that TLS symbols should have type STT_TLS. This is 
implemented via the function MCELFStreamer::fixSymbolsInTLSFixups. However, 
this function cannot delve into an MCTargetExpr at the moment.

In practice this means that existing targets put their TLS operand variants 
into the global namespace (VK_ARM_TLSGD, VK_Mips_DTPREL_HI, ...). However, the 
AArch64 target has more of these modifiers than all other targets combined and 
I felt it was inappropriate to pollute the global space for them.

This patch adds a virtual callback to MCTargetExpr so that analysis doesn't 
stop at this barrier and the work can be delegated to the target.

I believe this can be improved further; the implementatoin of 
MCTargetExpr::fixELFSymbolsInTLSFixups involves more duplication than I'd 
like. However, I would prefer to delay this further refactoring until AArch64 
is committed since it would provide an example use to reason from more easily.

Can I commit it?

Cheers.

Tim.
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