Dominique Pellé wrote:
> "Bram Moolenaar wrote:
>
> >> getchar.c: In function 'check_map':
> >> getchar.c:5221:7: warning: assuming signed overflow does not occur when
> >> assuming that (X - c) <= X is always true [-Wstrict-overflow]
> >> if (len > mp->m_keylen - 3)
> >> ^
> >
> > That looks like a compiler problem.
>
>
> Line getchar.cpp:5221 is as follows:
>
> 5221 if (len > mp->m_keylen - 3)
>
> I don't think that it is spurious warnings, What gcc is saying
> here is that when it compiled the code, it made an optimization
> which assumed that an addition or subtraction did not overflow
> on 32 bits. This is a fair assumption, since overflow with sign
> integer is undefined behavior. It is only well defined with unsigned
> integers. However, lots of code wrongly assume that signed
> integer overflow wrap, which is not correct, and can cause real
> bugs when cranking up optimizations. Hence the warning.
Unfortunately this is caused by recent C standard where people writing
compiler optimizers forced this "undefined behavior" so that they could
make programs 0.1% faster. Before this signed integers always
predictably overflowed, since practically all CPUs work that way.
I never like "undefined behavior".
> In this case, if mp->m_keylen was INT_MIN, then
> "mp->keylen - 3" would overflow which would be undefined
> behavior (bug!). This is probably impossible in this case,
> because m_key_len is always a length so positive, so
> removing 3 will never overflow.
>
> That warning does not happen with -O2 but happens with -O3
> on my machine.
>
> Attached patch fixes the warning in getchar.cpp by casting
> to unsigned.
That makes it:
if ((unsigned)len > (unsigned)mp->m_keylen - 3U)
That fixes the warning, but isn't that code wrong? if mp->m_keylen is
zero the result must be TRUE.
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