Re: GCC turns &~ into | due to undefined bit-shift without warning

[Andrew Haley <aph at redhat dot com>]

On 3/20/19 2:08 PM, Moritz Strübe wrote:
> 
> Ok, I played around a bit. Interestingly, if I set
> -fsanitize=udefined and -fsanitize-undefined-trap-on-error the
> compiler detects that it will always trap, and optimizes the code
> accordingly (the code after the trap is removed).* Which kind of
> brings me to David's argument: Shouldn't the compiler warn if there
> is undefined behavior it certainly knows of?

Maybe an example would help.

Consider this code:

for (int i = start; i < limit; i++) {
  foo(i * 5);
}

Should GCC be entitled to turn it into

int limit_tmp = i * 5;
for (int i = start * 5; i < limit_tmp; i += 5) {
  foo(i);
}

If you answered "Yes, GCC should be allowed to do this", would you
want a warning? And how many such warnings might there be in a typical
program?

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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