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digitalmars.D - Warning: volatile does NOT do what you think it does. WRT. DS or

reply downs <default_357-line yahoo.de> writes:
Yesterday, I tried porting parts of libnds (the open-source utility library for
the Nintendo DS) to D.

Worked relatively fine too, templates work, structs work, volatile DOESN'T WORK.

Let me repeat that. Volatile Doesn't Work.

This was my code:


 struct VolatilePointerProxy(T) {
   T* ptr;
   T access(T i) { volatile (*ptr) = i; return i; }
   T access() { T res; volatile res = *ptr; return res; }
   mixin(PropertyForward!("access"));
 }


This prevents the compiler from optimizing memory accesses past access().

It does *NOT* prevent the compiler from removing "redundant" calls to access.

Like, say, three subsequent 0 writes to MATRIX_MULT4x4. ¹

It took me four hours to find this problem. Foolishly, I trusted volatile to
behave at least a _little_ like it does in C.

Walter: I'm certain that you thought changing the behavior of volatile was a
good idea at the time, ill-defined though it was, but let me assure you, the
act left a gaping hole in the language.

In any case, eventually I settled on this workakludge:


 struct VolatilePointerProxy(T) {
   T* ptr;
   pragma(GNU_attribute, noinline) T access(T i) { (*ptr) = i; return i; }
   pragma(GNU_attribute, noinline) T access() { return *ptr; }
   mixin(PropertyForward!("access"));
 }


This implies redundant function calls, but at least it works.

(Note: If anybody can tell me how to copy a short from memory to memory in ARM
assembly, I'd be much obliged)

To have some good news for a change, here's the pay-off :
http://img55.imageshack.us/img55/6811/victory2wv7.jpg .. D on the DS *is
feasible*.

 --downs

¹ This is definitely the problem: I dropped three 0 calls in the main function
and saw them compile to a single 0 write.
Jan 12 2009
next sibling parent reply grauzone <none example.net> writes:
Maybe that's why Walter just decided to remove volatile in D2.0:

http://www.digitalmars.com/d/1.0/statement.html#VolatileStatement
http://www.digitalmars.com/d/2.0/statement.html#VolatileStatement
http://www.digitalmars.com/d/2.0/changelog.html#new2_013

I guess in D2.0, volatile accesses will be explicitly done with 
read_atomic()/write_atomic() style functions?

PS: downs, why do you still trust the compiler, after all the bugs you 
have found?
Jan 12 2009
parent downs <default_357-line yahoo.de> writes:
grauzone wrote:

 Maybe that's why Walter just decided to remove volatile in D2.0:
 
 http://www.digitalmars.com/d/1.0/statement.html#VolatileStatement
 http://www.digitalmars.com/d/2.0/statement.html#VolatileStatement
 http://www.digitalmars.com/d/2.0/changelog.html#new2_013
 
 I guess in D2.0, volatile accesses will be explicitly done with
 read_atomic()/write_atomic() style functions?
 
 PS: downs, why do you still trust the compiler, after all the bugs you
 have found?


I'm addicted to D. It's an abusive, violent relationship. :)
Jan 12 2009
prev sibling next sibling parent reply Walter Bright <newshound1 digitalmars.com> writes:
downs wrote:

 Walter: I'm certain that you thought changing the behavior of
 volatile was a good idea at the time, ill-defined though it was, but
 let me assure you, the act left a gaping hole in the language.


You are using the GDC back end, and I don't know what it does with the 
volatile information provided by the front end. Please add a bugzilla 
bug report for GDC on this.
Jan 12 2009
parent downs <default_357-line yahoo.de> writes:
Walter Bright wrote:

 downs wrote:

 Walter: I'm certain that you thought changing the behavior of
 volatile was a good idea at the time, ill-defined though it was, but
 let me assure you, the act left a gaping hole in the language.


 
 You are using the GDC back end, and I don't know what it does with the
 volatile information provided by the front end. Please add a bugzilla
 bug report for GDC on this.


Filed. Thanks for the advice.
Jan 13 2009
prev sibling parent Sean Kelly <sean invisibleduck.org> writes:
downs wrote:

 Yesterday, I tried porting parts of libnds (the open-source utility library
for the Nintendo DS) to D.
 
 Worked relatively fine too, templates work, structs work, volatile DOESN'T
WORK.
 
 Let me repeat that. Volatile Doesn't Work.
 
 This was my code:
 

 struct VolatilePointerProxy(T) {
   T* ptr;
   T access(T i) { volatile (*ptr) = i; return i; }
   T access() { T res; volatile res = *ptr; return res; }
   mixin(PropertyForward!("access"));
 }


 
 This prevents the compiler from optimizing memory accesses past access().
 
 It does *NOT* prevent the compiler from removing "redundant" calls to access.


That's not a legal optimization of 'volatile'.  I'd file a bug report.


Sean
Jan 12 2009