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From: Bart <bc@freeuk.com>
Newsgroups: comp.compilers
Subject: Re: Bounds checking, Optimization techniques and undefined behavior
Date: Wed, 8 May 2019 01:14:29 +0100
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Xref: csiph.com comp.compilers:2292

On 07/05/2019 10:04, David Brown wrote:

> Just for a laugh, try this code:
> #include <stdio.h>
>
> int main(void) {
>          int i;
>          int j;
>          int *pi = &i;
>          int *pj = &j;
>          int sum = 0;
>
>          printf("pi = %p, pj[-1] = %p\n", (void*) pi, (void*) &pj[-1]);
>
>          for (i = 0; i < 11; i++) {
>                  if (i == 3) {
>                          pj[-1] = 5;
>                  }
>                  sum += i;
>          }
>          printf("Sum is %i\n", sum);
> }

This is the sort of code that /I/ would say has undefined behaviour. In
this case because pj points to a 1-element block, and you writing
outside that block.

With my C compiler, it shows '55', whatever order i and j are defined
in. That's because these int32 types are 64-bit aligned so have a 4-byte
gap between. I could get '48' if I changed the ints to 64 bits.

Translating to my language, and in the knowledge that the (64-bit) ints
there are assigned consecutively, and knowing that j follows i, then I
would expect a result of 48.

I expect it because that's what I coded, and my compilers tend to do
what I tell them in the language. If I wanted to do something underhand
like this for some genuine reason, and which would be well-defined
according to the provisos above, then I don't want to have to fight the
compiler to achieve it.

In that case, a compiler generating '55' would be undesirable.