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[clang] Annotating C++'s `operator new` with more attributes

Authored by lebedev.ri on Feb 25 2020, 2:37 PM.

Description

[clang] Annotating C++'s operator new with more attributes

Summary:
Right now we annotate C++'s operator new with noalias attribute,
which very much is healthy for optimizations.

However as per [basic.stc.dynamic.allocation],
there are more promises on global operator new, namely:

  • non-std::nothrow_t operator new *never* returns nullptr
  • If std::align_val_t align parameter is taken, the pointer will also be align-aligned
  • global operator new-returned pointer is __STDCPP_DEFAULT_NEW_ALIGNMENT__-aligned It's more caveated than that.

Supplying this information may not cause immediate landslide effects
on any specific benchmarks, but it for sure will be healthy for optimizer
in the sense that the IR will better reflect the guarantees provided in the source code.

The caveat is -fno-assume-sane-operator-new, which currently prevents emitting noalias
attribute, and is automatically passed by Sanitizers (PR16386) - should it also cover these attributes?
The problem is that the flag is back-end-specific, as seen in test/Modules/explicit-build-flags.cpp.
But while it is okay to add noalias metadata in backend, we really should be adding at least
the alignment metadata to the AST, since that allows us to perform sema checks on it.

Reviewers: erichkeane, rjmccall, jdoerfert, eugenis, rsmith

Reviewed By: rsmith

Subscribers: xbolva00, jrtc27, atanasyan, nlopes, cfe-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D73380

Details

Committed
lebedev.riFeb 25 2020, 2:37 PM
Reviewer
rsmith
Differential Revision
D73380: [clang] Annotating C++'s `operator new` with more attributes
Parents
rGb8fdafe68ce2: [Sema] Perform call checking when building CXXNewExpr
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