[PATCH] D36749: [LLD][ELF][AArch64] Complete implementation of -fix-cortex-a53-843419

[Rafael Avila de Espindola via llvm-commits]

Peter Smith via Phabricator <reviews at reviews.llvm.org> writes:

> The alternative to changing the target of the Thunk is to modify RelExpr, such as what is done for PLT generation (when the linker sees R_PLT_PC, it knows that it needs to resolve the relocation not to the target symbol, but to the PLT entry for that symbol). Altering RelExpr does not require a new Symbol to be created as the relocation Target isn't changed. However it does mean that the relocation code and other areas need to distinguish between R_PC and R_PLT_PC. There are trade-offs for each approach, for example if Thunks were to change the RelExpr and indirect via some Thunk target then we would have to handle R_THUNK_PC and R_THUNK_PLT_PC. Redirecting the relocation Targets means that calls to Thunks can be resolved like any other branch relocation.

BTW, one change I would like to try once all thunk logic is implemented
is to change the relocation structure to have an SectionBase instead of
a symbol.

This would avoid an indirection and remove the need for R_PLT_PC for
example, as it would be just R_PC with the plt section.

But I really want to wait for all the thunk logic is implemented. It
seems much easier to change the relocation logic once all the uses are
known than to change it and potentially discard the changes if it turns
out they are insufficient.

Cheers,
Rafael