Shankar Easwaran
2014-Apr-02 04:19 UTC
[LLVMdev] [lld] adding demangler for symbol resolution
Hi Nick, Bigcheese,
When lld is used to link C++ code, it would be required to demangle
symbol names by default/user driven option.
The Gnu linker has the following options :-
--demangle=[style]
--no-demangle
I found that clang/llvm-symbolizer use __cxx_demangle function.
I would think that lld also need to call the same function, and I think
the way we want to demangle is to have the function in LinkingContext as
various flavors may choose to use different API's to demangle symbol names.
The API's that would be in LinkingContext would be :-
* virtual bool canDemangle() = 0; // Does the flavor provide a
way to demangle symbol names ?
* virtual std::string demangle(StringRef symbolName) = 0; //
demangle the symbol name
Thoughts / Suggestions ?
Thanks
Shankar Easwaran
--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation
I agree that LinkingContext should be the right place to have a function for demangling. Such feature is needed. It'd be useful in many situations such as printing an error message or logging. But I'd want to keep the number of API's each LinkingContext needs to support minimum, at least for now, as we can add more as we need it. On Tue, Apr 1, 2014 at 9:19 PM, Shankar Easwaran <shankare at codeaurora.org>wrote:> Hi Nick, Bigcheese, > > When lld is used to link C++ code, it would be required to demangle symbol > names by default/user driven option. > > The Gnu linker has the following options :- > > --demangle=[style] > --no-demangle > > I found that clang/llvm-symbolizer use __cxx_demangle function. > > I would think that lld also need to call the same function, and I think > the way we want to demangle is to have the function in LinkingContext as > various flavors may choose to use different API's to demangle symbol names. > > The API's that would be in LinkingContext would be :- > > * virtual bool canDemangle() = 0; // Does the flavor provide a > way to demangle symbol names ? > * virtual std::string demangle(StringRef symbolName) = 0; // > demangle the symbol name >I wouldn't add canDemangle, but instead add "virtual std::string demangle(StringRef symbolName)" with the default implementation that returns a given string as is. Override the function if you want to support demangling. Do nothing if you don't. With this approach you can simply call demangle() unconditionally. If you are lucky you'll get a demangled symbol name, and if not, you'll get a fallback string. Thoughts / Suggestions ?> > Thanks > > Shankar Easwaran > > -- > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted > by the Linux Foundation > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140401/7e633f6c/attachment.html>
On Apr 1, 2014, at 9:19 PM, Shankar Easwaran wrote:> Hi Nick, Bigcheese, > > When lld is used to link C++ code, it would be required to demangle symbol names by default/user driven option. > > The Gnu linker has the following options :- > > --demangle=[style] > --no-demangle > > I found that clang/llvm-symbolizer use __cxx_demangle function. > > I would think that lld also need to call the same function, and I think the way we want to demangle is to have the function in LinkingContext as various flavors may choose to use different API's to demangle symbol names. > > The API's that would be in LinkingContext would be :- > > * virtual bool canDemangle() = 0; // Does the flavor provide a way to demangle symbol names ? > * virtual std::string demangle(StringRef symbolName) = 0; // demangle the symbol name > > Thoughts / Suggestions ?Wouldn't it be simpler to have one demangle() method that does nothing (returns input string) if demangling is not available, the string is not a mangled symbol, or demangling was turned off (--no-demangle). Then, you just wrap a demangle() call around every use. The __cxa_demangle function has an odd interface that requires a malloc allocated block. Having demangle() return a std::string means yet another allocation. We might not care if this is just used in diagnostic outputs, but a more efficient way would be to pass the stream object to demangle and have it write directly to the stream instead of creating a std::string. Seems like a demangling utility might be something to add at the LLVM level. Either directly to raw_ostream or a wrapper like format(). -Nick
Shankar Easwaran
2014-Apr-02 18:02 UTC
[LLVMdev] [lld] adding demangler for symbol resolution
On 4/2/2014 12:23 PM, Nick Kledzik wrote:> On Apr 1, 2014, at 9:19 PM, Shankar Easwaran wrote: > >> Hi Nick, Bigcheese, >> >> When lld is used to link C++ code, it would be required to demangle symbol names by default/user driven option. >> >> The Gnu linker has the following options :- >> >> --demangle=[style] >> --no-demangle >> >> I found that clang/llvm-symbolizer use __cxx_demangle function. >> >> I would think that lld also need to call the same function, and I think the way we want to demangle is to have the function in LinkingContext as various flavors may choose to use different API's to demangle symbol names. >> >> The API's that would be in LinkingContext would be :- >> >> * virtual bool canDemangle() = 0; // Does the flavor provide a way to demangle symbol names ? >> * virtual std::string demangle(StringRef symbolName) = 0; // demangle the symbol name >> >> Thoughts / Suggestions ? > Wouldn't it be simpler to have one demangle() method that does nothing (returns input string) if demangling is not available, the string is not a mangled symbol, or demangling was turned off (--no-demangle). Then, you just wrap a demangle() call around every use.Are you mentioning that one demangle function in LinkingContext ? One demangle method wouldnt work as the ItaniumABI uses one method to demangle, ARMCXXABI uses a different method, and MSVC uses a different one. I am not sure about Mach-O here ?> The __cxa_demangle function has an odd interface that requires a malloc allocated block. Having demangle() return a std::string means yet another allocation. We might not care if this is just used in diagnostic outputs, but a more efficient way would be to pass the stream object to demangle and have it write directly to the stream instead of creating a std::string.I dont know if diagnostics in clang, already redirect things directly to a stream. May be for now, as an initial implementation, we can have a single demangle function that returns a std::string. As part of this, I was thinking to cleanup the way the errors are displayed to the user from the Resolver, we could have functions in SymbolTable with raiseError(SymbolErrorKind, filename, symbolname) raiseError(SymbolErrorKind, filename, symbolname, filename, symbolname) SymbolErrorKind : MultipleDefinition Undefined GroupError Note (for tracing) ...> Seems like a demangling utility might be something to add at the LLVM level. Either directly to raw_ostream or a wrapper like format().I have browsed discussions in llvm related to this to move the demangler function which is housed in libcxx, and I dont think there is a plan to move that. I think the format() specifier would be one thing that would be useful, but I am not sure on how different linking contexts in lld, could route calls with a central format specifier. Can you share more info on this ? Thanks Shankar Easwaran -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation
The simplest long-term solution would probably be to just add demangling in a portable way to libSupport. Then we don't need to conditionalize or anything. We just call llvm::demangle(). This is the only viable solution for proper cross-linking anyway. Given that there should be existing demanglers we can reuse, I'm not sure that this long term solution is significantly more difficult than the short term solution you are proposing. -- Sean Silva On Tue, Apr 1, 2014 at 9:19 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:> > Hi Nick, Bigcheese, > > When lld is used to link C++ code, it would be required to demangle symbol > names by default/user driven option. > > The Gnu linker has the following options :- > > --demangle=[style] > --no-demangle > > I found that clang/llvm-symbolizer use __cxx_demangle function. > > I would think that lld also need to call the same function, and I think > the way we want to demangle is to have the function in LinkingContext as > various flavors may choose to use different API's to demangle symbol names. > > The API's that would be in LinkingContext would be :- > > * virtual bool canDemangle() = 0; // Does the flavor provide a > way to demangle symbol names ? > * virtual std::string demangle(StringRef symbolName) = 0; // > demangle the symbol name > > Thoughts / Suggestions ? > > Thanks > > Shankar Easwaran > > -- > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted > by the Linux Foundation > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20141211/d33938e5/attachment.html>
Shankar Easwaran
2014-Dec-12 05:38 UTC
[LLVMdev] [lld] adding demangler for symbol resolution
One source of confusion is libstdc++ owns the API __cxa_demangle, when the demangle code is moved from libc++ to llvm, does libc++ link with libSupport ? libc++ needs to continue to own the demangle API too, IMO. Shankar Easwaran On 12/11/2014 6:24 PM, Sean Silva wrote:> The simplest long-term solution would probably be to just add demangling in > a portable way to libSupport. Then we don't need to conditionalize or > anything. We just call llvm::demangle(). This is the only viable solution > for proper cross-linking anyway. > > Given that there should be existing demanglers we can reuse, I'm not sure > that this long term solution is significantly more difficult than the short > term solution you are proposing. > > -- Sean Silva > > On Tue, Apr 1, 2014 at 9:19 PM, Shankar Easwaran <shankare at codeaurora.org> > wrote: >> Hi Nick, Bigcheese, >> >> When lld is used to link C++ code, it would be required to demangle symbol >> names by default/user driven option. >> >> The Gnu linker has the following options :- >> >> --demangle=[style] >> --no-demangle >> >> I found that clang/llvm-symbolizer use __cxx_demangle function. >> >> I would think that lld also need to call the same function, and I think >> the way we want to demangle is to have the function in LinkingContext as >> various flavors may choose to use different API's to demangle symbol names. >> >> The API's that would be in LinkingContext would be :- >> >> * virtual bool canDemangle() = 0; // Does the flavor provide a >> way to demangle symbol names ? >> * virtual std::string demangle(StringRef symbolName) = 0; // >> demangle the symbol name >> >> Thoughts / Suggestions ? >> >> Thanks >> >> Shankar Easwaran >> >> -- >> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted >> by the Linux Foundation >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >>-- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation