Re: [PATCH] C undefined behavior fix

dewar
2002-01-09T20:32:34Z

<<Ah... so (paraphrasing) -- if you have two byte size volatile objects,
and they happen to end up adjacent in memory, the compiler is
explicitly forbidden from turning an access to one of them into a
wider access -- because that would be an access to both of them, which
is a *different* side effect. (Certainly that exactly matches the
hardware-centric view of why "volatile" exists.) And the compiler
isn't allowed to change side effects, including causing them when the
source code didn't ask you to cause them.
>>

Right, and as you see that is covered by the language on external effects
in the Ada standard (remember the intent in Ada was to exactly match the
C rules :-)

But one thing in the Ada world that we consider left open is whether a
compiler is free to combine two volatile loads into a single load. Probably
the answer should be no, but the language at least in the Ada standard does
not seem strong enough to say this.