Re: [PATCH] C undefined behavior fix

From: Alexandre Oliva (
Date: Thu Jan 03 2002 - 13:44:14 EST

On Jan 2, 2002, David Woodhouse <> wrote:

> said:
>> (and for CONSTANT < 5 it of course generated correct code to fill dst
>> with string contents; and yes, I know that code will sigsegv on run
>> because of dst is not initialized - but it should die at runtime, not
>> at compile time).

> An ICE, while it's not quite what was expected and it'll probably get fixed,
> is nonetheless a perfectly valid implementation of 'undefined behaviour'.

Not really. Think of code whose execution is guarded by a test that
guarantees it won't invoke undefined behavior, but whose test is in a
separate translation unit. Or even of code that is never executed.
The compiler has no business telling whether the code may potentially
invoke undefined behavior, or whether it's going to be executed at
all, it has to compile it to something that does whatever it wishes
*if* it's executed. So, it should indeed die at runtime, not at
compile time.

Alexandre Oliva   Enjoy Guarana', see
Red Hat GCC Developer                  aoliva@{,}
CS PhD student at IC-Unicamp        oliva@{,}
Free Software Evangelist    *Please* write to mailing lists, not to me
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