Re: Compile error when not using -ftree-ter

[Andrew Haley <aph at redhat dot com>]

On 08/19/2010 01:15 PM, Job Noorman wrote:
> On Thursday 19 August 2010 13:24:49 you wrote:
>>
>> Finally, does the "g" constraint really not work for you with a recent
>> gcc?
> 
> It does work. I just forgot to reply on that in my last mail :-)
> 
>> Hmm, this seems to generalize "i" to symbolic constants, but not to
>> expressions that yield addresses.
> 
>> ... a translator is not required to do anything in particular with a program 
>> that has undefined behaviour.
> 
> So basically you are saying that passing an expression that yields
> an address to an "i" constraint results in undefined behavior?

We know that passing anything other than a constant (which for this
purpose seems to include address constants) to an "i" constraint is
not defined by gcc's documentation.  However, when dealing with things
that are as implementation-specific as this it's hard to make a strict
boundary between undefined and implementation-defined.

There are parts of gcc that are rather underspecified: this is one of
them.  Traditionally people use the gcc implementation as the
specification, and this results in howls of anguish when things later
break.

> One thing, however: My interpretation of the sentence
> 
> "This includes symbolic constants whose values will be known only at
> assembly time or later."
> 
> is that it is allowed to pass "address constant expressions" (5.19.4
> of the C++ standard) because they will be resolved to constants "at
> assembly time or later" (at link time).

I think so, but wouldn't be prepared to make a categorical statement
without taking a vote of the C++ front-end maintainers!

However, you cannot guarantee that every CPU allows a full address as
an immediate operand.  Some only allow short immediates of (say) 20
bits.  And on such machines your example would fail.

Andrew.