Re: [gimple] assignments to volatile

[Mike Stump <mikestump@comcast.net>]

On Jun 23, 2010, at 10:54 PM, Mark Mitchell wrote:
> Mike Stump wrote:
>> If someone wanted to fix gcc, the proper way would be to get the C
>> and C++ committees on the same page, nail it down, or make it a hard
>> error, and remove any latitude and confusion and make them agree.
> 
> Are you saying that C and C++ require different behavior for:
> 
>  x = y = z;
> 
> where "y" is volatile?

In my book no, they do not.  I think a reasonable person can also say that they do, because the C standard can be read in a way that is difference from the C standard.  C++ requires a re-read of y.  For consistency, the C standard should be read in a way that requires the re-read of y.

> Or are you saying that C++ requires that "y" be read while C does not specify it one way or the other?

This is the essence of the second point above.

> Please state your position more clearly.

C++ requires a re-read of y, the patch was going to remove the re-read, I objected because the patch then makes the compiler not conform to the C++ standard.  The patch was going to make C differ from C++, should the semantic for C++ be put back to match the standard. I really don't like to see the behavior of C++ and C differ when they don't need to.

>> An assignment expression has the value of the left operand after the 
>> assignment
> 
> I don't think this is nearly as clear as you do.

Ok.  Let's talk about it.  I objected because people seems to be going in the wrong direction.  Maybe I just misunderstood the effect of the patch.  The patch removes the re-read of y, right?

> Furthermore, I think existing practice from other embedded compilers is pretty important;

But to the exclusion of all of gcc users and the c++ standard?

> However, I cannot see any good justification for:
> 
>  int x;
>  volatile int y;
>  int z;
>  x ? y = z : 0
> 
> generating a read from "y" while:
> 
>  x ? y = 0 : 0
> 
> does not.

I wasn't arguing this case.  I believe both standards have them behaving the same.