[all-commits] [llvm/llvm-project] b40a2a: [clang] Add support for optional flag -fnew-infall...

[modiking via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b40a2a533a9dfb8dd5afb1f3b7d277da1e19f235
      https://github.com/llvm/llvm-project/commit/b40a2a533a9dfb8dd5afb1f3b7d277da1e19f235
  Author: modimo <modimo at fb.com>
  Date:   2021-08-02 (Mon, 02 Aug 2021)

  Changed paths:
    M clang/docs/ClangCommandLineReference.rst
    M clang/include/clang/Basic/LangOptions.def
    M clang/include/clang/Driver/Options.td
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/lib/Sema/SemaExprCXX.cpp
    A clang/test/CodeGenCXX/new-infallible.cpp

  Log Message:
  -----------
  [clang] Add support for optional flag -fnew-infallible to restrict exception propagation

The declaration for the global new function in C++ is generated in the compiler front-end. When examining exception propagation, we found that this is the largest root throw site propagator requiring unwind code to be generated for callers up the stack. Allowing this to be handled immediately with termination stops upward propagation and leads to significantly less landing pads generated. This in turns leads to a performance and .text size win.

With `-fnew-infallible` this annotates the declaration with `throw()` and `__attribute__((returns_nonnull))`.  `throw()` allows the compiler to assume exceptions do not propagate out of new and eliminate it as a root throw site. Note that the definition of global new is user-replaceable so users should ensure that the one used follows these semantics.

Measuring internally, we're seeing at 0.5% CPU win in one of our large internal FB workload. Measuring on clang self-build (cd0a1226b50081e86eb75a89d01e8782423971a0) we get:

thinlto/

        "dwarfehprepare.NumCleanupLandingPadsRemaining": 153494,
        "dwarfehprepare.NumNoUnwind": 26309,
thinlto_newinfallible/

        "dwarfehprepare.NumCleanupLandingPadsRemaining": 143660,
        "dwarfehprepare.NumNoUnwind": 28744,

a 1-143660/153494 = 6.4% reduction in landing pads and a 28744/26309 = 9.3% increase in the number of nounwind functions.

Testing:
ninja check-all
new test case to make sure these attributes are added correctly to global new.

Reviewed By: urnathan

Differential Revision: https://reviews.llvm.org/D105225