[all-commits] [llvm/llvm-project] b27430: Treat `std::move`, `forward`, etc. as builtins.

Richard Smith via All-commits all-commits at lists.llvm.org
Sun Apr 17 13:35:55 PDT 2022

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b27430f9f46b88bcd54d992debc8d72e131e1bd0
  Author: Richard Smith <richard at metafoo.co.uk>
  Date:   2022-04-17 (Sun, 17 Apr 2022)

  Changed paths:
    M clang/docs/CommandGuide/clang.rst
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/Basic/Builtins.def
    M clang/include/clang/Basic/Builtins.h
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/lib/AST/ExprConstant.cpp
    M clang/lib/Analysis/BodyFarm.cpp
    M clang/lib/Basic/Builtins.cpp
    M clang/lib/CodeGen/CGBuiltin.cpp
    M clang/lib/CodeGen/CGCall.cpp
    M clang/lib/Sema/SemaChecking.cpp
    M clang/lib/Sema/SemaDecl.cpp
    M clang/lib/Sema/SemaExpr.cpp
    M clang/lib/Sema/SemaExprCXX.cpp
    M clang/lib/Sema/SemaInit.cpp
    M clang/lib/Sema/SemaOverload.cpp
    M clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    M clang/test/Analysis/inner-pointer.cpp
    M clang/test/Analysis/use-after-move.cpp
    A clang/test/CodeGenCXX/builtin-std-move.cpp
    M clang/test/CodeGenCXX/builtins.cpp
    M clang/test/CodeGenCXX/microsoft-abi-throw.cpp
    A clang/test/SemaCXX/builtin-std-move-nobuiltin.cpp
    A clang/test/SemaCXX/builtin-std-move.cpp
    M clang/test/SemaCXX/unqualified-std-call-fixits.cpp
    M clang/test/SemaCXX/unqualified-std-call.cpp
    M clang/test/SemaCXX/warn-consumed-analysis.cpp
    M clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp

  Log Message:
  Treat `std::move`, `forward`, etc. as builtins.

This is extended to all `std::` functions that take a reference to a
value and return a reference (or pointer) to that same value: `move`,
`forward`, `move_if_noexcept`, `as_const`, `addressof`, and the
libstdc++-specific function `__addressof`.

We still require these functions to be declared before they can be used,
but don't instantiate their definitions unless their addresses are
taken. Instead, code generation, constant evaluation, and static
analysis are given direct knowledge of their effect.

This change aims to reduce various costs associated with these functions
-- per-instantiation memory costs, compile time and memory costs due to
creating out-of-line copies and inlining them, code size at -O0, and so
on -- so that they are not substantially more expensive than a cast.
Most of these improvements are very small, but I measured a 3% decrease
in -O0 object file size for a simple C++ source file using the standard
library after this change.

We now automatically infer the `const` and `nothrow` attributes on these
now-builtin functions, in particular meaning that we get a warning for
an unused call to one of these functions.

In C++20 onwards, we disallow taking the addresses of these functions,
per the C++20 "addressable function" rule. In earlier language modes, a
compatibility warning is produced but the address can still be taken.

The same infrastructure is extended to the existing MSVC builtin
`__GetExceptionInfo`, which is now only recognized in namespace `std`
like it always should have been.

This is a re-commit of
  a571f82a50416b767fd3cce0fb5027bb5dfec58c, and
which were reverted in
due to a crasher bug where CodeGen would emit a builtin glvalue as an
rvalue if it constant-folds.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D123345

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