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There are no widely deployed standard libraries providing sized
deallocation functions, so we have to punt and ask the user if they want
us to use sized deallocation. In the future, when such libraries are
deployed, we can teach the driver to detect them and enable this
feature.
N3536 claimed that a weak thunk from sized to unsized deallocation could
be emitted to avoid breaking backwards compatibility with standard
libraries not providing sized deallocation. However, this approach and
other variations don't work in practice.
With the weak function approach, the thunk has to have default
visibility in order to ensure that it is overridden by other DSOs
providing sized deallocation. Weak, default visibility symbols are
particularly expensive on MachO, so John McCall was considering
disabling this feature by default on Darwin. It also changes behavior
ELF linking behavior, causing certain otherwise unreferenced object
files from an archive to be pulled into the link.
Our second approach was to use an extern_weak function declaration and
do an inline conditional branch at the deletion call site. This doesn't
work because extern_weak only works on MachO if you have some archive
providing the default value of the extern_weak symbol. Arranging to
provide such an archive has the same challenges as providing the symbol
in the standard library. Not to mention that extern_weak doesn't really
work on COFF.
Reviewers: rsmith, rjmccall
Differential Revision: http://reviews.llvm.org/D8467
llvm-svn: 232788
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This reverts commit r230580.
extern_weak functions don't appear to work on Darwin (PR22951), so we'll
need to come up with a new approach.
llvm-svn: 232731
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Do not declare sized deallocation functions dependently on whether it is found in global scope. Instead, enforce the branching in emitted code by (1) declaring the functions extern_weak and (2) emitting sized delete expressions as a branching between both forms delete.
llvm-svn: 230580
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if sized delete is not provided in global scope, and -fdefine-sized-deallocation option is disabled.
llvm-svn: 230160
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documentation in r229818.
llvm-svn: 229950
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llvm-svn: 229915
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Fixes PR22635.
llvm-svn: 229913
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-fdefine-sized-deallocation, and from DefaultSizedDelete to DefineSizedDeallocation.
llvm-svn: 229597
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generate weak definitions of the sized operator delete (in terms of unsized operator delete). Instead, provide the funcitonality via a new compiler flag, -fdef-sized-delete.
The current implementation causes link-time ODR violations when the delete symbols are exported into the dynamic table.
llvm-svn: 229241
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llvm-svn: 228110
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llvm-svn: 228108
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visibility.
llvm-svn: 228107
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