rprichard added inline comments.
================
Comment at: libunwind/src/assembly.h:82
.globl SYMBOL_NAME(aliasname) SEPARATOR
\
- WEAK_SYMBOL(aliasname) SEPARATOR
\
+ EXPORT_SYMBOL(SYMBOL_NAME(aliasname)) SEPARATOR
\
SYMBOL_NAME(aliasname) = SYMBOL_NAME(name)
----------------
steven_wu wrote:
> rprichard wrote:
> > compnerd wrote:
> > > Does this not change the behaviour on MachO? This symbol is now
> > > `private_extern` rather than a `weak_reference`. A weak reference will
> > > be set to 0 by the loader if it is not found, and a `private_extern` is a
> > > strong internal reference.
> > Is `.weak_reference` the right directive to use here, instead of
> > `.weak_definition`? We're defining a symbol (`aliasname`) and setting its
> > value to that of another symbol (`name`).
> >
> > I think marking `unw_*` weak is intended to let some other strong
> > definition override it. Its value won't ever be set to 0.
> >
> > Currently on Mach-O, the hide-symbols flag hides almost everything
> > (including `_Unwind_*`) but leaves all of the `unw_*` alias symbols as
> > extern (and not private-extern) and not weak. With my change, they're still
> > not weak, but they're private-extern.
> >
> > libunwind's current assembly.h behavior for a weak alias:
> >
> > .globl aliasname
> > .weak_reference aliasname
> > aliasname = name
> >
> > The LLVM Mach-O assembler ignores the `.weak_reference` directive. If I
> > change it to `.weak_definition`, it is still ignored. AFAICT, the LLVM
> > assembler uses the WeakDef/WeakRef attributes from the `name` symbol.
> >
> > e.g.
> >
> > ```
> > $ cat test.S
> > .text
> > .space 0x42
> >
> > // Define foo.
> > .globl foo
> > foo:
> > ret
> >
> > // Define a weak alias, bar.
> > .globl bar
> > .weak_reference bar
> > bar = foo
> >
> > $ ~/clang11/bin/clang test.S -c && ~/clang11/bin/llvm-readobj --syms test.o
> >
> > File: test.o
> > Format: Mach-O 64-bit x86-64
> > Arch: x86_64
> > AddressSize: 64bit
> > Symbols [
> > Symbol {
> > Name: bar (1)
> > Extern
> > Type: Section (0xE)
> > Section: __text (0x1)
> > RefType: UndefinedNonLazy (0x0)
> > Flags [ (0x0)
> > ]
> > Value: 0x42
> > }
> > Symbol {
> > Name: foo (5)
> > Extern
> > Type: Section (0xE)
> > Section: __text (0x1)
> > RefType: UndefinedNonLazy (0x0)
> > Flags [ (0x0)
> > ]
> > Value: 0x42
> > }
> > ]
> > ```
> >
> > The Flags are empty.
> >
> > OTOH, if I flip things around:
> >
> > ```
> > .text
> > .space 0x42
> >
> > // Define a weak function, foo.
> > .globl foo
> > .weak_reference foo
> > foo:
> > ret
> >
> > // Define an alias, bar.
> > .globl bar
> > bar = foo
> > ```
> >
> > Now both symbols are WeakRef:
> >
> > ```
> > File: test.o
> > Format: Mach-O 64-bit x86-64
> > Arch: x86_64
> > AddressSize: 64bit
> > Symbols [
> > Symbol {
> > Name: bar (1)
> > Extern
> > Type: Section (0xE)
> > Section: __text (0x1)
> > RefType: UndefinedNonLazy (0x0)
> > Flags [ (0x40)
> > WeakRef (0x40)
> > ]
> > Value: 0x42
> > }
> > Symbol {
> > Name: foo (5)
> > Extern
> > Type: Section (0xE)
> > Section: __text (0x1)
> > RefType: UndefinedNonLazy (0x0)
> > Flags [ (0x40)
> > WeakRef (0x40)
> > ]
> > Value: 0x42
> > }
> > ]
> > ```
> >
> > I'm wondering if this LLVM behavior is actually correct, but I'm also
> > unfamiliar with Mach-O. (i.e. We want to copy the symbol's address, but
> > should we be copying the WeakRef/WeakDef flags?) It looks like the behavior
> > is controlled in `MachObjectWriter::writeNlist`. `writeNlist` finds the
> > aliased symbol and uses its flags:
> > ```
> > // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
> > // value.
> > bool EncodeAsAltEntry =
> > IsAlias && cast<MCSymbolMachO>(OrigSymbol).isAltEntry();
> >
> > W.write<uint16_t>(cast<MCSymbolMachO>(Symbol)->getEncodedFlags(EncodeAsAltEntry));
> > ```
> >
> > The PrivateExtern attribute, on the other hand, isn't part of these encoded
> > flags:
> > ```
> > if (Data.isPrivateExtern())
> > Type |= MachO::N_PEXT;
> > ```
> > `Data` continues to refer to the alias symbol rather than the aliased
> > symbol. However, apparently the author isn't completely sure about things?
> > ```
> > // FIXME: Should this update Data as well?
> > ```
> >
> > In libunwind, there is one usage of assembly.h's WEAK_ALIAS.
> > UnwindRegistersSave.S defines a hidden non-weak __unw_getcontext function,
> > and also a weak alias unw_getcontext. My patch's goal is to make
> > unw_getcontext hidden or not-hidden depending on a CMake config variable.
> >
> > Currently, on Mach-O and on Windows, `WEAK_ALIAS` doesn't actually make the
> > alias weak. I'm just making it a bit more explicit on Mach-O.
> >
> > Alternatively:
> > - Instead of a weak alias, we could output a weak thunk -- a weak function
> > with a branch to the internal hidden symbol. That's more of a functional
> > change, though.
> > - Or, on Mach-O, both the `unw_*` and `__unw_*` functions could be WeakDef.
> > - Maybe the hide-symbols flag should only affect ELF?
> >
> I guess the symbol is never really `weak` for Darwin. The `weak_import`
> attribute will turn all the reference to the symbol to `weak_reference` but
> since the alias is declared in `.cpp` file and never referenced, it will not
> create any weak linkage to the symbol. I am also not sure if a weak alias is
> possible on Darwin :) I think making everything `.private_extern` for Darwin
> should be fine?
>
> @ldionne The change was added: https://reviews.llvm.org/D59921. I am not sure
> why the alias need to be weak?
> I guess the symbol is never really `weak` for Darwin. The `weak_import`
> attribute will turn all the reference to the symbol to `weak_reference` but
> since the alias is declared in `.cpp` file and never referenced, it will not
> create any weak linkage to the symbol. I am also not sure if a weak alias is
> possible on Darwin :) I think making everything `.private_extern` for Darwin
> should be fine?
>
> @ldionne The change was added: https://reviews.llvm.org/D59921. I am not sure
> why the alias need to be weak?
I'm guessing the `unw_*` symbols are marked weak because they're not "reserved
for the implementation" the way the underscore-uppercase symbols are
(`_Unwind_*`), so if a program wants to define its own `unw_*` symbols, it
should be able to without breaking unwinding. This situation could cause a
multiply-defined symbol error at link-time, at least when libunwind is linked
statically.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D93003/new/
https://reviews.llvm.org/D93003
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