Re: issue: unexpected results in optimizations

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Mon, 11 Dec 2023, 17:08 Jingwen Wu via Gcc, <gcc@gcc.gnu.org> wrote:

> Hello, I'm sorry to bother you. And I have some gcc compiler optimization
> questions to ask you.
> First of all, I used csmith tools to generate c files randomly. Meanwhile,
> the final running result was the checksum for global variables in a c file.
> For the two c files in the attachment, I performed the equivalent
> transformation of loop from *initial.**c* to *transformed.c*. And the two
> files produced different results (i.e. different checksum values) when
> using *-O2* optimization level, while the results of both were the same
> when using other levels of optimization such as *-O0*, *-O1*, *-O3*, *-Os*,
> *-Ofast*.
> Please help me to explain why this is, thank you.
>

Sometimes csmith can generate invalid code that gets miscompiled. It looks
like you're compiling with no warnings, which is a terrible idea:


> command line: *gcc file.c -O2 -lm -I $CSMITH_HOME/include && ./a.out*
>

You should **at least** enable warnings and make sure gcc isn't pointing
out any problems in the code.

You should also try the options suggested at http://gcc.gnu.org/bugs/ which
help identify invalid code.


version: gcc 12.2.0
> os: ubuntu 22.04
>