Re: [gimple] assignments to volatile

[Nathan Sidwell <nathan@codesourcery.com>]

On 07/01/10 02:02, Mike Stump wrote:
> On Jun 27, 2010, at 11:28 PM, Nathan Sidwell wrote:
>> On 06/25/10 17:41, Mike Stump wrote:
>>> On Jun 24, 2010, at 11:35 PM, Nathan Sidwell wrote:
>>>> On 06/24/10 16:42, Mike Stump wrote:
>>>>
>>>>> Sorry for not expounding.  I thought it would be obvious I was talking about well formed code...  First case would be something like:
>>>>>
>>>>> volatile int i, j, k;
>>>>> volatile int vi;
>>>>> void foo(volatile int&    j) {
>>>>>    foo(i = k);
>>>>> }
>>>>>
>>>>> here, we need to bind j to the address of i.  The second case is something like:
>>>>
>>>> Correct.  The patch does not change this behaviour -- the C++ front end already has to explicitly break out the lvalue from the assignment to conform to the semantics of TREEs.  The current G++ behaviour is not to re-read the assigned-to value.
>>>
>>> g++ seems not to share your position:
>>>
>>> 	movl	_i, %eax
>>> 	movl	%eax, (%esp)
>>>
>>> volatile int i, j, k;
>>> volatile int vi;
>>> void foo(int j) {
>>>    foo(i = k);
>>> }
>>>
>>> GNU C++ (GCC) version 4.6.0 20100620 (experimental) [trunk revision 161045] (x86_64-apple-darwin10)
>>> 	compiled by GNU C version 4.2.1 (Apple Inc. build 5659), GMP version 5.0.1
>>>
>>> ?  Certainly I must misunderstand something, can you spot it?
>>
>> You have changed the testcase to be passing by value from passing by reference.  Why do you think those would be equivalent?
>
> The read happens when the the object referred to by the reference is accessed:

You're now changing the example again.  This time with a reference to volatile 
object.

>
> volatile int i, j;
> int x, data, expr, k;
>
> static inline void foo(volatile int&r) {
>    int fetch = r;
> }
>
> int main() {
>    foo(i=k);
> }
>
> 	movl	_k, %eax
> 	movl	%eax, _i
> 	movl	_i, %eax
>
> here, i is re-read.  Here, g++ in fact does re-read the assigned to value.

Yes I would expect this -- there is a sequence point between the assignment to i 
in the argument list, and the read of r in the inlined function.  I do not 
understand the point you are trying to make.

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery