[PATCH] D129541: [llvm][IPO] Add IR function attribute fine_grained_bitfields

[John McIver via Phabricator via llvm-commits]

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The IR function attribute fine_grained_bitfields is used to prevent the IPO
inlining of functions with different bit-field addressing schemes. Use of fine
grained and non fine grained bit-fields can result in data corruption. See the
following example:

  // File A: compiled with -ffine-grained-bitfield-accesses
  struct X {
    int a : 8;
    int b : 24;
  };
  
  void callee(struct X*);
  
  int caller() {
    struct X x;
    x.a = 10;   // Variable a is directly stored to.
    callee(&x);
    return x.a;
  }

  // File B: compiled with -fno-fine-grained-bitfield-accesses
  struct X {
    int a : 8;
    int b : 24;
  };
  
  void callee(struct X* x) {
    x->b = 10;  // Load occurs on struct object, followed by freeze,
                        // clear, set, and store sequence to assign b.
  }

Because the caller uses `fine-grained-bitfield-accesses`, only the byte
associated with `a` is assigned and the value of `b` remains `poison`. The
callee does not have individual member variable addressing and thus loads the
full 32-bits (8-bits of value and 24-bits `poison`) resulting in a load of
`poison`. The proceeding `freeze` in the freeze, clear, set, and store sequence
will corrupt the already assigned value of `a`.

The IPO inlining issue was identified in D128501 <https://reviews.llvm.org/D128501>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129541

Files:
  llvm/docs/BitCodeFormat.rst
  llvm/docs/LangRef.rst
  llvm/include/llvm/Bitcode/LLVMBitCodes.h
  llvm/include/llvm/IR/Attributes.td
  llvm/lib/Bitcode/Reader/BitcodeReader.cpp
  llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
  llvm/lib/Transforms/Utils/InlineFunction.cpp
  llvm/test/Bitcode/compatibility.ll
  llvm/test/Transforms/Inline/fine_grained_bitfields.ll
  llvm/test/Transforms/Inline/module-fine_grained_bitfield-both.ll
  llvm/test/Transforms/Inline/module-fine_grained_bitfield-callee.ll
  llvm/test/Transforms/Inline/module-fine_grained_bitfield-caller.ll
  llvm/utils/emacs/llvm-mode.el
  llvm/utils/vim/syntax/llvm.vim
  llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml

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