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

[Eloy Durán via cfe-dev]

Ok, interesting, I had to use `-fsanitize=undefined-trap -fsanitize-undefined-trap-on-error`, so I guess this is just an issue with the clang being included not having the current UBSan yet.

Thanks!

> On 06 Oct 2015, at 20:20, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 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 <mailto: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 <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 <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
> 
> 
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