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digitalmars.D.bugs - [Issue 2423] New: Erroneous unreachable statement warning

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423

           Summary: Erroneous unreachable statement warning
           Product: D
           Version: 1.035
          Platform: Other
        OS/Version: All
            Status: NEW
          Keywords: diagnostic
          Severity: regression
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: larsivar igesund.net


Consider the following function:

void foo() {

    do {
        if (false)
            return 1;
    } while (true);
}

Compiling with -w, results in

warning - whiletrue.d(6): Error: statement is not reachable

Minimized from a module in Tango, meaning Tango does not compile with warnings
on.

This regression was introduced in DMD 1.032.


--
Oct 20 2008
next sibling parent reply Don <nospam nospam.com.au> writes:
d-bugmail puremagic.com wrote:

 http://d.puremagic.com/issues/show_bug.cgi?id=2423
 
            Summary: Erroneous unreachable statement warning
            Product: D
            Version: 1.035
           Platform: Other
         OS/Version: All
             Status: NEW
           Keywords: diagnostic
           Severity: regression
           Priority: P2
          Component: DMD
         AssignedTo: bugzilla digitalmars.com
         ReportedBy: larsivar igesund.net
 
 
 Consider the following function:
 
 void foo() {
 
     do {
         if (false)
             return 1;
     } while (true);
 }
 
 Compiling with -w, results in
 
 warning - whiletrue.d(6): Error: statement is not reachable
 
 Minimized from a module in Tango, meaning Tango does not compile with warnings
 on.
 
 This regression was introduced in DMD 1.032.
 
 


Why is that wrong? "return 1" looks unreachable to me.
Oct 20 2008
parent reply Lars Ivar Igesund <larsivar igesund.net> writes:
Don wrote:


 d-bugmail puremagic.com wrote:

 http://d.puremagic.com/issues/show_bug.cgi?id=2423
 
            Summary: Erroneous unreachable statement warning
            Product: D
            Version: 1.035
           Platform: Other
         OS/Version: All
             Status: NEW
           Keywords: diagnostic
           Severity: regression
           Priority: P2
          Component: DMD
         AssignedTo: bugzilla digitalmars.com
         ReportedBy: larsivar igesund.net
 
 
 Consider the following function:
 
 void foo() {
 
     do {
         if (false)
             return 1;
     } while (true);
 }
 
 Compiling with -w, results in
 
 warning - whiletrue.d(6): Error: statement is not reachable
 
 Minimized from a module in Tango, meaning Tango does not compile with
 warnings on.
 
 This regression was introduced in DMD 1.032.
 
 


 Why is that wrong? "return 1" looks unreachable to me.


It is 

while(true)

that is line 6 (you can move the while a few lines down to see that it isn't
just a line number error). The return makes the statement on that line
("true" I assume) unreachable, which is correct without the conditional. So
it is the presence of the if (false) that makes this an error.

-- 
Lars Ivar Igesund
blog at http://larsivi.net
DSource, #d.tango & #D: larsivi
Dancing the Tango
Oct 20 2008
parent Lars Ivar Igesund <larsivar igesund.net> writes:
Lars Ivar Igesund wrote:


 Don wrote:
 

 d-bugmail puremagic.com wrote:

 http://d.puremagic.com/issues/show_bug.cgi?id=2423
 
            Summary: Erroneous unreachable statement warning
            Product: D
            Version: 1.035
           Platform: Other
         OS/Version: All
             Status: NEW
           Keywords: diagnostic
           Severity: regression
           Priority: P2
          Component: DMD
         AssignedTo: bugzilla digitalmars.com
         ReportedBy: larsivar igesund.net
 
 
 Consider the following function:
 
 void foo() {
 
     do {
         if (false)
             return 1;
     } while (true);
 }
 
 Compiling with -w, results in
 
 warning - whiletrue.d(6): Error: statement is not reachable
 
 Minimized from a module in Tango, meaning Tango does not compile with
 warnings on.
 
 This regression was introduced in DMD 1.032.
 
 


 Why is that wrong? "return 1" looks unreachable to me.


 
 It is
 
 while(true)
 
 that is line 6 (you can move the while a few lines down to see that it
 isn't just a line number error). The return makes the statement on that
 line ("true" I assume) unreachable, which is correct without the
 conditional. So it is the presence of the if (false) that makes this an
 error.
 


Of course, any conditional should remove this warning - in the module
(tango/io/vfs/ZipFolder.d) where I found this problem, the return is in the
else block.

-- 
Lars Ivar Igesund
blog at http://larsivi.net
DSource, #d.tango & #D: larsivi
Dancing the Tango
Oct 20 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423






There is also a related issue that will show in the same function if that has a
return value:

int foo() {
    do {
        return 1;
    } while  (true);
}

warning - whiletrue.d(4): Error: statement is not reachable
warning - whiletrue.d(1): function whiletrue.foo no return at end of function

I understand that the "no return" is a semantic challenge, but the rule is
fairly simple: if there is a while(true), then all code after it is dead code
unless there is also a break. This is also a regression.

Note that whereas the first is an obvious bug and impossible to workaround,
this one is possible to workaround, but still a question about quality of
implementation. The "no return" bug does also affect/break Tango when using
warnings.


--
Oct 26 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423








 There is also a related issue that will show in the same function if that has a
 return value:
 
 int foo() {
     do {
         return 1;
     } while  (true);
 }
 
 warning - whiletrue.d(4): Error: statement is not reachable
 warning - whiletrue.d(1): function whiletrue.foo no return at end of function
 
 I understand that the "no return" is a semantic challenge, but the rule is
 fairly simple: if there is a while(true), then all code after it is dead code
 unless there is also a break. This is also a regression.
 
 Note that whereas the first is an obvious bug and impossible to workaround,
 this one is possible to workaround, but still a question about quality of
 implementation. The "no return" bug does also affect/break Tango when using
 warnings.


In this case there's no doubt a simple flow analysis will take care of things.
The challenge is only when conditions are complex; in this case that doesn't
matter. The code could as well be:

do {
    return 1;
} while (P == NP);

My explanation for the bug is that Walter's front-end rewrites loops with
terminal test as loops with initial test with a jump:

do stmt while (cond); 

==>

goto __label; while (cond) __label: stmt

The rewritten form makes it a tad more difficult to figure out what's going on.

Andrei


--
Oct 26 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423








 Consider the following function:
 
 void foo() {
 
     do {
         if (false)
             return 1;
     } while (true);
 }
 
 Compiling with -w, results in
 
 warning - whiletrue.d(6): Error: statement is not reachable
 
 Minimized from a module in Tango, meaning Tango does not compile with warnings
 on.
 
 This regression was introduced in DMD 1.032.
 


Erm, actually, I wonder if this is even valid.  do-while loops in D do not
require a semicolon at the end.  The "unreachable statement" is simply the
empty statement that follows the "while(true)".  The following code:

int foo() {
   do {
       return 1;
   } while  (true)
}

gives no warnings.


--
Oct 26 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423










 Consider the following function:
 
 void foo() {
 
     do {
         if (false)
             return 1;
     } while (true);
 }
 
 Compiling with -w, results in
 
 warning - whiletrue.d(6): Error: statement is not reachable
 
 Minimized from a module in Tango, meaning Tango does not compile with warnings
 on.
 
 This regression was introduced in DMD 1.032.
 


 
 Erm, actually, I wonder if this is even valid.  do-while loops in D do not
 require a semicolon at the end.  The "unreachable statement" is simply the
 empty statement that follows the "while(true)".  The following code:
 
 int foo() {
    do {
        return 1;
    } while  (true)
 }
 
 gives no warnings.
 


You are right. I won't decide whether there is still a bug in the compiler, but
it is no longer a problem for Tango.


--
Oct 26 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423


smjg iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com







 Erm, actually, I wonder if this is even valid.  do-while loops in D do not
 require a semicolon at the end.  The "unreachable statement" is simply the
 empty statement that follows the "while(true)".  The following code:


It's valid, it just throws a warning.  ISTM not having the semicolon at the end
of DoStatement was a bad design decision - if you stumble upon

    }
    while (whatever)
    {

in the middle of some code, you have to look through possibly screenfuls of
code to determine whether the while applies to the preceding block (and the
following one just opens a new scope for whatever reason) or the following
block.

Meanwhile, the conditions under which "statement is not reachable" is thrown
ought to be changed to exclude empty statements.


--
Nov 19 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423


Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                 CC|                            |clugdbug yahoo.com.au





To remove the warning from empty statements:

PATCH:
statement.c, line 564 (DMD2)

-        if (!(result & BEfallthru) && !s->comeFrom())
+        if (!(result & BEfallthru) && !s->comeFrom() && !s->isEmpty())
        {
        s->warning("statement is not reachable");
        }

And then add this line to ExpStatement, in statement.h line 140:

    virtual int isEmpty() { return exp==NULL; }


Side effect: This will make {;;;;;} an empty statement; at the moment, it
isn't. The patch below makes the code below compile (into return 2;). Currently
it won't compile, but works if try{;} is changed into try{}.

nothrow int main() {
 int x= 2;
 try { ; } catch(Exception e) { x=4; throw new Exception("xxx"); } 
 return x;
}

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Aug 07 2009
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2423


Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED



Fixed DMD1.050 and DMD2.035.

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