[PATCH] D153096: [MC] Fold A-B when A's fragment precedes B's fragment

[Fangrui Song via Phabricator via llvm-commits]

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When the MCAssembler is non-null and the MCAsmLayout is null, we can fold A-B
when

- A and B are in the same fragment, or
- A's fragment suceeds B's fragment, and they are not separated by non-data fragments (D69411 <https://reviews.llvm.org/D69411>)

This patch allows folding when A's fragment precedes B's fragment so
that `9997b - . == 0` below can be evaluated as true:

  nop
  .arch_extension sec
  9997:nop
  // old behavior: error: expected absolute expression
  .if 9997b - . == 0
  .endif

Some Darwin tests check that this folding does not work. Adjust them.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D153096

Files:
  llvm/lib/MC/MCExpr.cpp
  llvm/test/MC/AArch64/arm64-small-data-fixups.s
  llvm/test/MC/ARM/directive-if-subtraction.s
  llvm/test/MC/MachO/absolutize.s
  llvm/test/MC/MachO/darwin-x86_64-reloc.s
  llvm/test/MC/MachO/reloc-diff.s

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