Re: [PATCH v2] compiler.h: give up __compiletime_assert_fallback()

[Daniel Santos]

Hello Nick,

On 08/27/2018 03:09 PM, Nick Desaulniers wrote:
>> Now we're back to the question of "what do you mean by 'constant'"?  If
>> you mean a C constant expression (as defined in the C standard) than
>> almost none of this code fits that criteria.  For these compile-time
>> assertions to work, we are concerned with the data flow analysis and
>> constant propagation performed by the compiler during optimization.  You
>> will notice in include/linux/compiler.h that __compiletime_assert is a
>> no-op when __OPTIMIZE__ is not defined.
> Depending on optimizations for static assertions sounds problematic.

(with my best Palpatine voice) It is unavoidable.

Actually it's theoretically possible, but the compiler would have to do
something akin to copying it's control flow graph et. al, run -O2-ish
optimizations, perform the static assertions and then throw away the
optimized control flow graph and emit code based upon the original.


>>> As the comment in <linux/build_bug.h>
>>> says, GCC (and Clang as well) only emits the error for obvious cases.
>>>
>>> When '__cond' is a variable,
>>>
>>>     ((void)sizeof(char[1 - 2 * __cond]))
>>>
>>> ... is not obvious for the compiler to know the array size is negative.
>>>
>>> Reverting that commit would break BUILD_BUG() because negative-size-array
>>> is evaluated before the code is optimized out.
>>>
>>> Let's give up __compiletime_assert_fallback().  This commit does not
>>> change the current behavior since it just rips off the useless code.
>> Clang is not the only target audience of
>> __compiletime_assert_fallback().  Instead of ripping out something that
>> may benefit builds with gcc 4.2 and earlier, why not override its
> Note that with commit cafa0010cd51 ("Raise the minimum required gcc
> version to 4.6") that gcc < 4.6 is irrelevant.

Ah, I guess I'm not keeping up, that's wonderful news! Considering that
I guess I would be OK with its removal, but I still think it would be
better if a similar mechanism to break the Clang build could be found.

>> definition in compiler-clang.h with something that will break the build
>> for Clang?  It would need an #ifndef __compiletime_error_fallback here
>> though.
>>
>>> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>>> ---
>>>
>>> Changes in v2:
>>>   - Rebase
>>>
>>>  include/linux/compiler.h | 17 +----------------
>>>  1 file changed, 1 insertion(+), 16 deletions(-)
>>>
>>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>>> index 681d866..87c776c 100644
>>> --- a/include/linux/compiler.h
>>> +++ b/include/linux/compiler.h
>>> @@ -314,29 +314,14 @@ static inline void *offset_to_ptr(const int *off)
>>>  #endif
>>>  #ifndef __compiletime_error
>>>  # define __compiletime_error(message)
>>> -/*
>>> - * Sparse complains of variable sized arrays due to the temporary variable in
>>> - * __compiletime_assert. Unfortunately we can't just expand it out to make
>>> - * sparse see a constant array size without breaking compiletime_assert on old
>>> - * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
>>> - */
>>> -# ifndef __CHECKER__
>>> -#  define __compiletime_error_fallback(condition) \
>>> -     do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
>>> -# endif
>>> -#endif
>>> -#ifndef __compiletime_error_fallback
>>> -# define __compiletime_error_fallback(condition) do { } while (0)
>>>  #endif
>>>
>>>  #ifdef __OPTIMIZE__
>>>  # define __compiletime_assert(condition, msg, prefix, suffix)                \
>>>       do {                                                            \
>>> -             int __cond = !(condition);                              \
>>>               extern void prefix ## suffix(void) __compiletime_error(msg); \
>>> -             if (__cond)                                             \
>>> +             if (!(condition))                                       \
>>>                       prefix ## suffix();                             \
>>> -             __compiletime_error_fallback(__cond);                   \
>>>       } while (0)
>>>  #else
>>>  # define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
>> To give any more meaningful feedback I think I will need to experiment
>> with Clang, older GCC versions and icc.  It occurred to me that I should
>> probably clean up and publish my __builtin_constant_p test program and
>> also generate results for more recent compilers.  I can extend it to
>> test various negative sized array constructs and it could help inform
>> this decision.
>>
>> IMO, the most ideal solution would be a set of C2x (or future)
>> extensions providing something similar to C++'s constexpr, GCC's
>> __builtin_constant_p and our BUILD_BUG_ON.  This would cross deeply into
> Note that __builtin_constant_p is a wild beast with lots of edge
> cases, and dependencies on compiler and optimization level.  I'm
> trying to sort out some of these differences right now with llvm
> developers.

Yes it is. IIRC, I even found cases where __builtin_constant_p returned
false, but the emitted code replaced the value in question with a
constant. So at least at one point, constant propagation and
__builtin_constant_p weren't always entirely coherent.

I was working on a GCC extension that would solve this problem earlier
this year but bills and paying work interrupted me. :)Â The solution
involved adding a "const" attribute for function parameters and
variables that would simply create a warning or error if the value
wasn't compiled away at the time middle-end optimizations completed and
RTL emitted. It isn't finished -- maybe for gcc10.

Daniel

>
>> territory traditionally considered to belong to the implementation, so
>> it's no small request.  A lot would have to be resolved for it to work
>> in the standard.
>>
>> Daniel