[cfe-dev] Clarification on undefined behaviour and/or how to get warned for these

[David Blaikie via cfe-dev]

UBSan catches this

$ clang++-tot undef.cpp -fsanitize=undefined
$ ./a.out
undef.cpp:14:8: runtime error: value inf is outside the range of
representable values of type 'long'

On Tue, Oct 6, 2015 at 5:07 AM, Eloy Durán <cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> I’m fairly new to the world of C compiler edge cases and their undefined
> behaviour or what the C standards say about this particular one, but as
> it’s behaviour changed ‘recently’ I’d like to ask for some clarification
> nonetheless.
>
> Since moving the build process of our iOS app to Xcode 7, an animation
> that was expected to run for a long time suddenly ran for a very short
> period. The issue turned out to be a change in how `HUGE_VAL` as an
> argument was converted to a `long`, which is the parameter type the
> function expected. The full case in question can be found here
> https://github.com/artsy/eigen/issues/838#issuecomment-145600551, but I
> have reduced it to the following example for ease of understanding.
>
> ——
>
> ~/tmp » cat huge-vail.c
> // This could use the HUGE_VAL macro instead of __builtin_huge_val, but I’m
> // using the built-in one here, because that’s what the macro would use
> under
> // the hood.
>
> #include <math.h>
> #include <assert.h>
>
> void succeed(double value) { assert(value > 43); }
> void fail(long value)      { assert(value > 43); }
>
> int main(void)
> {
>   asm volatile ("movq $42, %%rdi" :);
>
>   // When passing to a function with the correct parameter type, this
> works as expected.
>   succeed(__builtin_huge_val());
>   // When passing to a function that changes the type, the value in the
> $rdi register is passed as-is.
>   fail(__builtin_huge_val());
>
>   return 0;
> }
>
> ——
>
> ~/tmp » clang -v
> Apple LLVM version 7.0.0 (clang-700.0.72)
> Target: x86_64-apple-darwin14.5.0
> Thread model: posix
> ~/tmp » clang huge-vail.c -o huge-vail
> ~/tmp » ./huge-vail
> Assertion failed: (value > 43), function fail, file huge-vail.c, line 9.
> fish: Job 1, './huge-vail ' terminated by signal SIGABRT (Abort)
>
> When looking at the LLVM IR generated for this file, there’s a clear
> difference in what gets passed to the function.
> Specifically the undef part, which is considered a ‘undefined variable’:
>
> define i32 @main() #0 {
>   %1 = alloca i32, align 4
>   store i32 0, i32* %1
>   call void asm sideeffect "movq $$42, %rdi",
> "~{dirflag},~{fpsr},~{flags}"() #3, !srcloc !1
>   call void @succeed(double 0x7FF0000000000000)
>   call void @fail(i64 undef)
>   ret i32 0
> }
>
> ——
>
> With an older clang this works as I would (naively) expect and how it has
> been working for the past 3 years:
>
> ~/tmp »
> /Applications/Xcode-6.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
> -v
> Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
> Target: x86_64-apple-darwin14.5.0
> Thread model: posix
> ~/tmp »
> /Applications/Xcode-6.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
> huge-vail.c -o huge-vail
> ~/tmp » ./huge-vail
> ~/tmp » echo $status
> 0
>
> The LLVM IR for that is:
>
> define i32 @main() #0 {
>   %1 = alloca i32, align 4
>   store i32 0, i32* %1
>   call void asm sideeffect "movq $$42, %rdi",
> "~{dirflag},~{fpsr},~{flags}"() #4, !srcloc !1
>   call void @succeed(double 0x7FF0000000000000)
>   call void @fail(i64 9223372036854775807)
>   ret i32 0
> }
>
> ——
>
> I understand that `HUGE_VAL` is actually a `double` and so the `double` to
> `long` conversion is probably where this behaviour comes from, but I wonder:
> * What the reason is to mark it as undefined now vs the large value used
> previously?
> * If there is a compiler flag that would have turned on checks for this
> undefined behaviour? I’ve tried the flags listed here in the manual, but
> none of those triggered a warning for me:
> http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation
>
> Simply pointing me to a doc that explains this case is perfectly fine too!
>
> Kind regards,
> Eloy Durán
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20151006/18f3fb23/attachment.html>