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digitalmars.D - Compiler does some flow analysis with -O..?

reply Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
Try this.

void main()
{
    int x = void;
    int y = 3;

    if(y < 10)
        x += 5;
    else
        x = 2;
}

Notice that x is uninitialized, so "x += 5;" shouldn't work.

If you compile this, even with -w, DMD happily accepts it.  But if you
throw -O, it gives the error:

dtest.d(187): Error: variable x used before set

This seems to be a relatively recent development, and doesn't seem to
be documented.  It's also very surprising that it only happens when -O
is thrown.

I like it a lot.  Could this functionality be formalized and extended
(i.e. to include accesses to variables declared like "int x;", like it
says in the spec)?
Apr 09 2009
next sibling parent reply "Denis Koroskin" <2korden gmail.com> writes:
On Thu, 09 Apr 2009 18:19:02 +0400, Jarrett Billingsley
<jarrett.billingsley gmail.com> wrote:


 Try this.

 void main()
 {
     int x = void;
     int y = 3;

     if(y < 10)
         x += 5;
     else
         x = 2;
 }

 Notice that x is uninitialized, so "x += 5;" shouldn't work.

 If you compile this, even with -w, DMD happily accepts it.  But if you
 throw -O, it gives the error:

 dtest.d(187): Error: variable x used before set

 This seems to be a relatively recent development, and doesn't seem to
 be documented.  It's also very surprising that it only happens when -O
 is thrown.

 I like it a lot.  Could this functionality be formalized and extended
 (i.e. to include accesses to variables declared like "int x;", like it
 says in the spec)?



I believe this is just an optimization and not a general-purpose flow analysis:

Step 0:
int x = void;
int y = 3;

if (y < 10)
    x += 5;
else
    x = 2;

Step 1:
int x = void;
int y = 3;
if (true)
    x += 5;
else
    x = 2;

Step2:
int x = void;
int y = 3;
x += 5;  // Error: variable x used before set
Apr 09 2009
parent reply "Nick Sabalausky" <a a.a> writes:
"Denis Koroskin" <2korden gmail.com> wrote in message 
news:op.ur4kb8ffo7cclz soldat.creatstudio.intranet...

 Now I am convinced:

 extern(C) int rand();

 void main()
 {
    int x = void;
    if (rand() == 0) {
        x += 5; // line 7
    }
 }

 test.d(7): Error: variable x used before set

 It only happens with DMD1.042+

 Yay!



I guess I'm going to have to start using -O in my debug builds...unless this 
behavior becomes independant of the -O switch...wink wink nudge nudge...
Apr 09 2009
parent Robert Fraser <fraserofthenight gmail.com> writes:
Denis Koroskin wrote:

 BTW, that's a breaking change! XD



Ssh!
Apr 10 2009
prev sibling next sibling parent Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
On Thu, Apr 9, 2009 at 10:28 AM, Denis Koroskin <2korden gmail.com> wrote:


 I believe this is just an optimization and not a general-purpose flow
 analysis:



It happens even with a non-trivial condition.
Apr 09 2009
prev sibling next sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Thu, 09 Apr 2009 18:53:07 +0400, Jarrett Billingsley
<jarrett.billingsley gmail.com> wrote:


 On Thu, Apr 9, 2009 at 10:28 AM, Denis Koroskin <2korden gmail.com>  
 wrote:


 I believe this is just an optimization and not a general-purpose flow
 analysis:



 It happens even with a non-trivial condition.



Now I am convinced:

extern(C) int rand();

void main()
{
    int x = void;
    if (rand() == 0) {
        x += 5; 		// line 7
    }
}

test.d(7): Error: variable x used before set

It only happens with DMD1.042+

Yay!
Apr 09 2009
prev sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Thu, 09 Apr 2009 20:36:43 +0400, Nick Sabalausky <a a.a> wrote:


 "Denis Koroskin" <2korden gmail.com> wrote in message
 news:op.ur4kb8ffo7cclz soldat.creatstudio.intranet...

 Now I am convinced:

 extern(C) int rand();

 void main()
 {
    int x = void;
    if (rand() == 0) {
        x += 5; // line 7
    }
 }

 test.d(7): Error: variable x used before set

 It only happens with DMD1.042+

 Yay!



 I guess I'm going to have to start using -O in my debug builds...unless  
 this
 behavior becomes independant of the -O switch...wink wink nudge nudge...



Yeah, me too!

BTW, that's a breaking change! XD
Apr 09 2009