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

[Haoran Xu via llvm-dev]

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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