[lldb-dev] [llvm-dev] [RFC] Runtime checks for ABI breaking build of LLVM

[Mehdi Amini via lldb-dev]

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.

— 
Mehdi



> On Nov 16, 2016, at 12:12 PM, Mehdi Amini via llvm-dev <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
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> 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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