[PATCH] D106030: [Clang] add support for error+warning fn attrs

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers created this revision.
nickdesaulniers added reviewers: rsmith, aaron.ballman, craig.topper, efriedma, lebedev.ri, jdoerfert, arsenm.
Herald added subscribers: dexonsmith, hiraditya.
nickdesaulniers requested review of this revision.
Herald added subscribers: llvm-commits, cfe-commits, wdng.
Herald added projects: clang, LLVM.

Add support for the GNU C style __attribute__((error(""))) and
__attribute__((warning(""))). These attributes are meant to be put on
definitions of functions whom should not be called.

They are frequently used to provide compile time diagnostics similar to
_Static_assert, but which may rely on non-ICE conditions (ie. relying on
compiler optimizations). While users may instead simply call undefined
functions in such cases to get a linkage failure from the linker, these
provide a much more ergonomic and actionable diagnostic to users and do
so at compile time rather than at link time.

These are used throughout the Linux kernel in its implementation of
BUILD_BUG and BUILD_BUG_ON macros. These macros generally cannot be
converted to use _Static_assert because many of the parameters are not
ICEs. The Linux kernel still needs to be modified to make use of these
when building with Clang; I have a patch that does so I will send once
this feature is landed.

To do so, we create a new IR level Function attribute,
"user-diagnostic"="" which contains the string parameter from source
language function attributes (both error and warning boil down to one IR
Fn Attr). Then, similar to calls to inline asm, we attach a !srcloc
Metadata node to call sites of such attributed callees.

The backend diagnoses these before instruction selection, while we still
know that a call is a call (vs say a JMP that's a tail call) in an arch
agnostic manner.

The frontend then reconstructs the SourceLocation from that Metadata,
and determines whether to emit an error or warning based on the callee's
attribute.

Some things to iron out TODO:

- Is user-diagnostic the best identifier for the new fn attr? Maybe "diagnose-if-called"? I also use `UserDiagnostic` throughout much of this patch for C++ class names; surely there's something better?
- When is the best time for the backend to produce such diagnostics? If we wait until post-ISEL, then we need to diagnose instructions which may be JMPs, not just CALLs.

Link: https://bugs.llvm.org/show_bug.cgi?id=16428
Link: https://github.com/ClangBuiltLinux/linux/issues/1173


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D106030

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/include/clang/Basic/DiagnosticFrontendKinds.td
  clang/include/clang/Basic/DiagnosticGroups.td
  clang/lib/CodeGen/CGCall.cpp
  clang/lib/CodeGen/CodeGenAction.cpp
  clang/lib/CodeGen/CodeGenModule.cpp
  clang/lib/Sema/SemaDeclAttr.cpp
  clang/test/CodeGen/attr-error.c
  clang/test/CodeGen/attr-warning.c
  clang/test/Frontend/backend-attribute-error-warning-optimize.c
  clang/test/Frontend/backend-attribute-error-warning.c
  clang/test/Frontend/backend-attribute-error-warning.cpp
  clang/test/Misc/pragma-attribute-supported-attributes-list.test
  clang/test/Sema/attr-error.c
  clang/test/Sema/attr-warning.c
  llvm/docs/LangRef.rst
  llvm/include/llvm/IR/DiagnosticInfo.h
  llvm/lib/CodeGen/PreISelIntrinsicLowering.cpp
  llvm/lib/IR/DiagnosticInfo.cpp
  llvm/test/CodeGen/Generic/attr-user-diagnostic.ll

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