[PATCH] D152986: [clang] Allow 'nomerge' attribute for function pointers

[Eduard Zingerman via Phabricator via cfe-commits]

eddyz87 created this revision.
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Allow specifying 'nomerge' attribute for function pointers,
e.g. like in the following C code:

  extern void (*foo)(void) __attribute__((nomerge));
  void bar(long i) {
    if (i)
      foo();
    else
      foo();
  }

With the goal to attach 'nomerge' to both calls done through 'foo':

  @foo = external local_unnamed_addr global ptr, align 8
  define dso_local void @bar(i64 noundef %i) local_unnamed_addr #0 {
    ; ...
    %0 = load ptr, ptr @foo, align 8, !tbaa !5
    ; ...
  if.then:
    tail call void %0() #1
    br label %if.end
  if.else:
    tail call void %0() #1
    br label %if.end
  if.end:
    ret void
  }
  ; ...
  attributes #1 = { nomerge ... }

Report a warning in case if 'nomerge' is specified for a variable that
is not a function pointer, e.g.:

  t.c:2:22: warning: 'nomerge' attribute is ignored because 'j' is not a function pointer [-Wignored-attributes]
      2 | int j __attribute__((nomerge));
        |                      ^

The intended use-case is for BPF backend.

BPF provides a sort of "standard library" functions that are called
helpers. BPF also verifies usage of these helpers before program
execution. Because of limitations of verification / runtime model it
is important to keep calls to some of such helpers from merging.

An example could be found by the link [1], there input C code:

  if (data_end - data > 1024) {
      bpf_for_each_map_elem(&map1, cb, &cb_data, 0);
  } else {
      bpf_for_each_map_elem(&map2, cb, &cb_data, 0);
  }

Is converted to bytecode equivalent to:

  if (data_end - data > 1024)
    tmp = &map1;
  else
    tmp = &map2;
  bpf_for_each_map_elem(tmp, cb, &cb_data, 0);

However, BPF verification/runtime requires to use the same map address
for each particular `bpf_for_each_map_elem()` call.

The 'nomerge' attribute is a perfect match for this situation, but
unfortunately BPF helpers are declared as pointers to functions:

  static long (*bpf_for_each_map_elem)(void *map, ...) = (void *) 164;

Hence, this commit, allowing to use 'nomerge' for function pointers.

[1] https://lore.kernel.org/bpf/03bdf90f-f374-1e67-69d6-76dd9c8318a4@meta.com/


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152986

Files:
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/CodeGen/CGCall.cpp
  clang/lib/Sema/SemaDeclAttr.cpp
  clang/test/CodeGen/attr-nomerge.cpp
  clang/test/Misc/pragma-attribute-supported-attributes-list.test
  clang/test/Parser/stmt-attributes.c
  clang/test/Parser/stmt-attributes.cpp
  clang/test/Parser/stmt-attributes.m
  clang/test/Sema/attr-nomerge-ast.cpp
  clang/test/Sema/attr-nomerge.cpp

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