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

[Reid Kleckner via llvm-dev]

I totally support solving this problem, but please don't use dynamic
initialization to do it. :) That's everyone's least favorite feature of
libstdc++ iostream.

At least on Windows, we can solve this with #pragma detect_mismatch.
https://msdn.microsoft.com/en-us/library/ee956429.aspx

On platforms without such a feature, I'd rather mangle NDEBUG into one of
the core LLVM initialization routines than suffer the runtime and code size
penalty of initializers.

On Wed, Nov 16, 2016 at 10:48 AM, Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> 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*:STRINGUsed 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.
>
> 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);
> +#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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>
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