[PATCH] D73070: Add function attribute "patchable-function-prefix" to support -fpatchable-function-entry=N,M where M>0

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: craig.topper, nickdesaulniers, nsz, ostannard, peter.smith.
Herald added subscribers: llvm-commits, hiraditya, kristof.beyls.
Herald added a project: LLVM.
MaskRay added a child revision: D73071: [X86] Support -fpatchable-function-entry=N,M where M>0.

Similar to the function attribute `prefix` (prefix data),
"patchable-function-prefix" inserts data (M NOPs) before the function
entry label.

-fpatchable-function-entry=2,1 (1 NOP before entry, 1 NOP after entry)
will look like:

    .type	foo, at function
  .Ltmp0:               # @foo
    nop
  foo:
  .Lfunc_begin0:
    # optional `bti c` (AArch64 Branch Target Identification) or
    # `endbr64` (Intel Indirect Branch Tracking)
    nop
  
    .section  __patchable_function_entries,"awo", at progbits,get,unique,0
    .p2align  3
    .quad .Ltmp0

-fpatchable-function-entry=N,0 + -mbranch-protection=bti/-fcf-protection=branch has two reasonable
placements (https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01185.html):

  (a)         (b)
  
  func:       func:
  .Ltmp0:     bti c
    bti c     .Ltmp0:
    nop       nop

(a) needs no additional code. If the consensus is to go for (b), we will
need more code in AArch64BranchTargets.cpp / X86IndirectBranchTracking.cpp .


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D73070

Files:
  llvm/include/llvm/CodeGen/AsmPrinter.h
  llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
  llvm/lib/IR/Verifier.cpp
  llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
  llvm/test/CodeGen/AArch64/patchable-function-entry-bti.ll
  llvm/test/CodeGen/AArch64/patchable-function-entry.ll
  llvm/test/Verifier/invalid-patchable-function-entry.ll

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