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From:	"Alexei A. Frounze" <alexfru AT chat DOT ru>
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Subject:	Re: gcc bug?
Date:	Wed, 18 Apr 2007 00:26:47 -0700
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Rod Pemberton wrote:
> "Alexei A. Frounze" <alexfru AT chat DOT ru> wrote in message
> news:G-CdnfEDh5KKDLnbnZ2dnUVZ_jydnZ2d AT comcast DOT com...
>> Rod Pemberton wrote:
>>> "Alexei A. Frounze" <alexfru AT chat DOT ru> wrote in message
>>> news:LbadnVsa3_H-ornbnZ2dnUVZ_sudnZ2d AT comcast DOT com...
>
> Very sleepy now on a topic I'm not well versed on...  This, from n1124
> 6.7.3, as I read it, seems to apply. (e.g., both the data _and_
> pointer need a volatile...)
>
> "5 If an attempt is made to modify an object defined with a
> const-qualified type through use
> of an lvalue with non-const-qualified type, the behavior is
> undefined. If an attempt is
> made to refer to an object defined with a volatile-qualified type
> through use of an lvalue
> with non-volatile-qualified type, the behavior is undefined.113)"
>
>
> Anyway, let's ignore the spec. and see if I've got it the issues
> straight:
>
> Volatile on the pointer works (i.e., f2).  If the pointer is
> volatile, then the data is volatile implicitly (i.e., new pointer
> therefore new data). And, both optimized and unoptimized, we see the
> pointer being reloaded from the stack and the data being reloaded
> from the pointer.
>
> Volatile on the data doesn't work (i.e., f1) supposedly just for -O2.
> If the pointer isn't volatile, but the data is, then just the data
> should be reloaded.  Unoptimized, the compiler treats both as
> volatile (both are refreshed), i.e., incorrect.  Optimized, the
> compiler treats both as non-volatile (neither are reloaded), i.e,
> (appears) incorrect.  So, technically, both are incorrect.  But, for
> the unoptimized case, since the pointer isn't changing, it's
> functionally correct.  For the optimized case, it would appear that
> it's incorrect unless the compiler can somehow determine that the
> data isn't changing...
>
> Going to sleep...

Consider this case:

fxn()
{
#if 0
volatile int* px; // variant 1
#else
int* volatile px; // variant 2
#endif
...
px = ...;
...
switch (*px)
{
case 0:
break;

case 1:
break;

default:
break;
}
...
}

On a different from GCC compiler variant 2 yielded this sort of asm code for 
the switch:
cmp [eax], 0
je L0
cmp [eax], 1
je L1
...
It didn't treat the pointer as volatile (maybe because it was local a local 
var not passed to any functions as argument) but generated 2 accesses to the 
data. These 2 accesses are a bit unexpected. Well, they're OK for a 
non-volatile variable, but either volatile pointer doesn't necessarily imply 
volatiliness of the object it points to or something's wrong or indeed 
undefined.

On the other hand for the variant 1 it generated this sort of code for the 
switch:
mov ecx, [eax]
cmp ecx, 0
je L0
cmp ecx, 1
je L1

which is what I wanted but screwed up.

Even though the code's fixed, the incident now puzzles me.

Alex

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