http://d.puremagic.com/issues/show_bug.cgi?id=10854

           Summary: debug should also bypass safety and nothrow
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: monarchdodra@gmail.com


--- Comment #0 from monarchdodra@gmail.com 2013-08-19 08:42:51 PDT ---
debug instructions/blocks were allowed to nicely bypass function purity. This allows inserting impure calls for testing reasons inside pure functions.

This should be expanded for @safe and nothrow. Its really a natural evolution IMO.

There is nothing more annoying than inserting a test "writeln()" in a function, only to be refused because said function is @safe or nothrow.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #1 from bearophile_hugs@eml.cc 2013-08-19 09:21:47 PDT ---
(In reply to comment #0)

> debug instructions/blocks were allowed to nicely bypass function purity. This allows inserting impure calls for testing reasons inside pure functions.
> 
> This should be expanded for @safe and nothrow. Its really a natural evolution IMO.

If in module A you call a nothrow function F from module B, and you pre-compile module A with aggressive optimizations that rely on F never throwing, and then you run the module B in debug mode and now the function F throws, what does it happens?

If F is also tagged with 'pure' and when you debug B you call something not pure, like printf(), inside the debug{}, the optimizations of the compilation of the module A could cause some of those calls to printf() to never happen. But if the code inside debug{} is well behaved this doesn't cause significant problems. Peraphs with nothrow the situation is worse.


> There is nothing more annoying than inserting a test "writeln()" in a function, only to be refused because said function is @safe or nothrow.

For that I try to use printf(), that is nothrow.

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--- Comment #2 from monarchdodra@gmail.com 2013-08-19 10:40:40 PDT ---
(In reply to comment #1)
> (In reply to comment #0)
> 
> > debug instructions/blocks were allowed to nicely bypass function purity. This allows inserting impure calls for testing reasons inside pure functions.
> > 
> > This should be expanded for @safe and nothrow. Its really a natural evolution IMO.
> 
> If in module A you call a nothrow function F from module B, and you pre-compile module A with aggressive optimizations that rely on F never throwing, and then you run the module B in debug mode and now the function F throws, what does it happens?

If the *context* of the debug instruction is nothrow, then the debug could be automatically expanded to try {BODY} catch (Exception e){assert(0, format"Exception %s was thrown from a debug clause in a nothrow context");}

Or something with "scope(failure)", to avoid scoping the debug instruction/

> If F is also tagged with 'pure' and when you debug B you call something not pure, like printf(), inside the debug{}, the optimizations of the compilation of the module A could cause some of those calls to printf() to never happen.

But that is what we already have. Also, I don't think the new pure definitions mean a pure function can be outright optimized out. But I'm unsure.

> But if the code inside debug{} is well behaved this doesn't cause significant problems. Peraphs with nothrow the situation is worse.
> 
> 
> > There is nothing more annoying than inserting a test "writeln()" in a function, only to be refused because said function is @safe or nothrow.
> 
> For that I try to use printf(), that is nothrow.

That is indeed a workaround, but much less powerful than a writef. It is also a workaround to something (IMO) you shouldn't have to. But thanks for the tip.

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--- Comment #3 from bearophile_hugs@eml.cc 2013-08-19 13:05:40 PDT ---
(In reply to comment #2)

> Also, I don't think the new pure definitions
> mean a pure function can be outright optimized out. But I'm unsure.

The DMD compiler completely optimizes out strongly pure functions in some situations. And I expect better optimizers to remove some more calls. Observe:


import std.stdio;

int sqr(in int x) pure nothrow {
    debug printf("sqr\n");
    return x ^^ 2;
}

int twoSqr(in int x) pure nothrow {
    return sqr(x) + sqr(x);
}

void main() {
    writeln(twoSqr(3));
}



...>dmd -debug -run test.d
sqr
sqr
18

...>dmd -O -release -debug -run test.d
sqr
18

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monarchdodra@gmail.com changed:

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


--- Comment #4 from monarchdodra@gmail.com 2013-08-20 08:48:31 PDT ---
*** This issue has been marked as a duplicate of issue 8464 ***

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