[llvm-dev] clang10 mis-compiles simple C program transpiled from brainfxxk

[Haoran Xu via llvm-dev]

I did a simple bisect on clang version, and it seems like clang 8.0.0 works
correctly, but clang 9.0.0 failed to compile the code correctly.
https://godbolt.org/z/676Grr  <- if you change the clang version to 8.0.0,
you will see the expected output in 'output' section.
I don't have the ability to bisect on clang git history. I would greatly
appreciate it if any one is willing to do that.

Thanks!

Haoran Xu <haoranxu510 at gmail.com> 于2020年10月21日周三 下午8:47写道：

> Hello,
>
> I'm really amazed to find out that under -O3, a simple piece of C code
> generated from a brainfxxk-to-C transpiler is miscompiled.
> As one probably know, the C code transpiled from brainfxxk only contains 3
> kind of statements:
>
>> (1) ++(*ptr) / --(*ptr)
>> (2) ++ptr / --ptr
>> (3) while (*ptr) { ... }
>>
> where ptr is a uint8_t*.
> So it seems very clear to me that the code contains no undefined behavior
> (the pointer is uint8_t* and unsigned integer overflow is not UD).
>
> After further investigation, it seems like clang compiled this loop:
>
>> while (*ptr) {
>>  --(*ptr);
>>  ++ptr;
>>  ++(*ptr);
>>  --ptr;
>> }
>>
>  to an unconditional infinite loop under -O3, resulting in the bug. The
> code snippet above seems completely benign to me.
>
> I attached the offending program. With
>
>> clang a.c -O0
>>
> it worked fine (it should print out an ASCII-art picture of mandelbrot
> fracture). However, with -O1 or -O3, it goes into a dead loop (in the code
> snippet above) after printing out a few characters.
>
> I also tried UndefinedBehaviorSanitizer. Strangely, when compiling using
>
>> clang a.c -O3  -fsanitize=undefined
>>
> the code worked again, with no infinite loop, and no undefined behavior
> reported.
>
> So it seems to me a LLVM optimizer bug. I would greatly appreciate if any
> one is willing to investigate.
>
> Best,
> Haoran
>
>
>
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