[llvm-dev] [RFC] Runtime checks for ABI breaking build of LLVM
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Fri Nov 18 16:23:04 PST 2016
> On Nov 18, 2016, at 3:45 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> Hi,
>
> I had to revert it, because it breaks building LLDB on MacOS.
>
> It turns out it would break any client that is including llvm-config.h but not linking to libLLVMSupport.
> So either:
>
> - we shouldn’t allow to include llvm-config.h without linking to LLVM, in which case I need to look a bit closer as of why lldb includes this header in a context where they don’t link LLVM.
> - we should allow to include llvm-config.h without linking to LLVM (at least libSupport). In which case we need a new solution for this feature: it can be to use another header dedicated to this flag, that would be included only from headers that contain the ABI break.
The second approach is implemented here: https://reviews.llvm.org/D26876
I extracted the LLVM_ENABLE_ABI_BREAKING_CHECKS to its own header to have a fine granularity on which client needs to include the check.
—
Mehdi
>
>
>> On Nov 16, 2016, at 12:12 PM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>>
>>> On Nov 16, 2016, at 12:05 PM, Jonathan Roelofs <jonathan at codesourcery.com <mailto:jonathan at codesourcery.com>> wrote:
>>>
>>>
>>>
>>> On 11/16/16 11:48 AM, Mehdi Amini via llvm-dev wrote:
>>>> Hi all,
>>>>
>>>> An issue that come up from time to time and has cost hours for debug for
>>>> many of us and users of LLVM is that an assert build isn’t ABI
>>>> compatible with a release build.
>>>>
>>>> The CMake flags that controls this behavior is LLVM_ABI_BREAKING_CHECKS (
>>>>
>>>> *LLVM_ABI_BREAKING_CHECKS*:STRING
>>>> Used to decide if LLVM should be built with ABI breaking checks or
>>>> not. Allowed values
>>>> are WITH_ASSERTS (default), FORCE_ON and FORCE_OFF. WITH_ASSERTS turns
>>>> on ABI breaking checks in an assertion enabled
>>>> build. FORCE_ON (FORCE_OFF) turns them on (off) irrespective of
>>>> whether normal (NDEBUG-based) assertions are enabled or not. A
>>>> version of LLVM built with ABI breaking checks is not ABI compatible
>>>> with a version built without it.
>>>>
>>>>
>>>> I propose to add a runtime check to detect when we have incorrectly
>>>> setup build.
>>>>
>>>> The idea is that every translation unit that includes such header will
>>>> get a weak definition of a symbol with an initializer calling the
>>>> runtime check. The symbol name would be different if the ABI break is
>>>> enabled or not.
>>>
>>> Can it be made into a link-time check instead? I'm imagining something like:
>>
>> I’d love to, but didn’t find a universal solution unfortunately :(
>>
>>> #if LLVM_ENABLE_ABI_BREAKING_CHECKS
>>> extern int EnableABIBreakingChecks;
>>> __attribute__((weak)) int *VerifyEnableABIBreakingChecks = &EnableABIBreakingChecks;
>>> #else
>>> extern int DisableABIBreakingChecks;
>>> __attribute__((weak)) int *VerifyDisableABIBreakingChecks = &VDisableABIBreakingChecks;
>>> #endif
>>>
>>> in llvm-config.h.cmake, and:
>>>
>>> #if LLVM_ENABLE_ABI_BREAKING_CHECKS
>>> int EnableABIBreakingChecks;
>>> #else
>>> int DisableABIBreakingChecks;
>>> #endif
>>>
>>> in Error.cpp.
>>>
>>> Then it'll only link if Error.cpp's TU's setting of LLVM_ENABLE_ABI_BREAKING_CHECKS matches that of the TU that includes llvm-config.h
>>
>> It seems that this work, I thought I tried exactly this but got lost on the whiteboard at some point!
>>
>> Maybe because one drawback that I tried to avoid is that the export-list of a LLVM dylib would depend on the value of LLVM_ENABLE_ABI_BREAKING_CHECKS with this.
>>
>> Thanks,
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>
>>>
>>>
>>>>
>>>> The runtime check maintains two boolean to track if it there is in the
>>>> image at least a translation unit for each value of this flag. If both
>>>> flags are set the process is aborted.
>>>>
>>>> The cost is *one* static initializer per DSO (or per image I believe).
>>>>
>>>> A straw-man patch (likely not windows compatible because of weak) is:
>>>>
>>>> diff --git a/llvm/include/llvm/Config/llvm-config.h.cmake
>>>> b/llvm/include/llvm/Config/llvm-config.h.cmake
>>>> index 4121e865ea00..4274c942d3b6 100644
>>>> --- a/llvm/include/llvm/Config/llvm-config.h.cmake
>>>> +++ b/llvm/include/llvm/Config/llvm-config.h.cmake
>>>> @@ -80,4 +80,18 @@
>>>> /* LLVM version string */
>>>> #define LLVM_VERSION_STRING "${PACKAGE_VERSION}"
>>>>
>>>> +
>>>> +#ifdef __cplusplus
>>>> +namespace llvm {
>>>> +bool setABIBreakingChecks(bool Enabled);
>>>> +__attribute__((weak))
>>>> +#if LLVM_ENABLE_ABI_BREAKING_CHECKS
>>>> +bool
>>>> +ABICheckEnabled = setABIBreakingChecks(true);
>>>> +#else
>>>> +bool ABICheckDisabled = setABIBreakingChecks(true);
>>>
>>> Do you mean `false` here ^ ?
>>>
>>>> +#endif
>>>> +}
>>>> +#endif
>>>> +
>>>> #endif
>>>> diff --git a/llvm/lib/Support/Error.cpp b/llvm/lib/Support/Error.cpp
>>>> index 7436a1fd38ee..151fcdcbfb27 100644
>>>> --- a/llvm/lib/Support/Error.cpp
>>>> +++ b/llvm/lib/Support/Error.cpp
>>>> @@ -112,3 +112,17 @@ void report_fatal_error(Error Err, bool GenCrashDiag) {
>>>> }
>>>>
>>>> }
>>>> +
>>>> +
>>>> +bool llvm::setABIBreakingChecks(bool Enabled) {
>>>> + static char abi_breaking_checks_enabled = 0;
>>>> + static char abi_breaking_checks_disabled = 0;
>>>> + if (Enabled)
>>>> + abi_breaking_checks_enabled = 1;
>>>> + else
>>>> + abi_breaking_checks_disabled = 1;
>>>> + if (abi_breaking_checks_enabled && abi_breaking_checks_disabled)
>>>> + report_fatal_error("Error initializing LLVM: mixing translation
>>>> units built"
>>>> + "with and without LLVM_ABI_BREAKING_CHECKS");
>>>> + return true;
>>>> +}
>>>>
>>>>
>>>>
>>>> —
>>>> Mehdi
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>>>
>>>
>>> --
>>> Jon Roelofs
>>> jonathan at codesourcery.com <mailto:jonathan at codesourcery.com>
>>> CodeSourcery / Mentor Embedded
>>
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