On Tue, 12 Dec 2023, Jonathan Wakely via Gcc wrote:
> 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. Let me also link the "Testing Compilers Using Csmith" page, which is currently available via the Wayback Machine, but not its original URL: https://web.archive.org/web/20230316072811/http://embed.cs.utah.edu/csmith/using.html It was written by the developers of Csmith. Alexander
