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

[Jonathan Roelofs via llvm-dev]

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:

   #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


Jon

>
> 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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-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded