lld/mac: Implement support for linker optimization hints (.loh)

[nico]

Bugzilla Link	51055
Version	unspecified
OS	All
CC	@gkmhub,@int3,@smeenai

Extended Description

LLVM supports .loh directives (llvm/trunk/test/MC/AArch64/arm64-directive_loh.s). Per https://reviews.llvm.org/D49929:

"""
The hints are there so that the linker can turn (non-adjacent) ADRP/ADD sequences, and similar, into ADR/NOP if the referenced symbol ends up close enough; this saves a cycle or so on Apple CPUs. To do that the compiler marks instruction-pairs used to generate a single address and saves that data off in a special MachO load command. I suppose it is sort of a souped-up relocation which specifies two addresses to be relocated instead of just one.
"""

I think we don't support that yet in lld.

[BertalanD]

I'm working on this.

[nico]

current ld64.lld loh status:

• LOH_ARM64_ADRP_ADRP
• LOH_ARM64_ADRP_LDR
• LOH_ARM64_ADRP_ADD_LDR
• LOH_ARM64_ADRP_LDR_GOT_LDR
• LOH_ARM64_ADRP_ADD_STR
• LOH_ARM64_ADRP_LDR_GOT_STR
• LOH_ARM64_ADRP_ADD
• LOH_ARM64_ADRP_LDR_GOT

If I remember correctly, @BertalanD said that the implemented ones hit the majority of instances in Chromium Framework (@BertalanD, do you have a histogram at hand?).

[BertalanD]

❯ ld @response.txt -verbose_optimization_hints 2>&1 | grep 'transformed to' | cut -d ' ' -f1 | sort | uniq -c | sort -n
  24 adrp-add-str
 319 adrp-add-ldr
 341 adrp-ldr-got-str
4056 adrp-ldr-got
115745 adrp-add
121578 adrp-ldr-got-ldr

[BertalanD]

LOH_ARM64_ADRP_ADD_LDR will be added in https://reviews.llvm.org/D130505.

[int3]

@BertalanD is there anything left to do here?

[BertalanD]

We are still missing LOH_ARM64_ADRP_ADD_STR and LOH_ARM64_ADRP_LDR_GOT_STR.