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digitalmars.D.bugs - [Issue 5787] New: Invisible multiple function calls in static foreach

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5787

           Summary: Invisible multiple function calls in static foreach
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2011-03-27 08:10:09 PDT ---
Through profiling I have found a performance problem in my code. I have later
reduced the case and created this demo code:


import std.typecons: tuple;
import std.c.stdio: printf;
auto foo() {
    printf("foo\n");
    return tuple(1, 2);
}
void main() {
    foreach (x; foo().tupleof)
        printf("%d\n", x);
}


Its output, DMD 2.052:

foo
1
foo
2

In my code foo() was an expensive computation.
In my opinion in this situation foo() needs to be computed only once (or I'd
like some other solution to avoid this invisible multiple calls, like some kind
of error, etc).

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next sibling parent d-bugmail puremagic.com writes:
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--- Comment #1 from bearophile_hugs eml.cc 2011-03-28 09:49:05 PDT ---
Kai Meyer has shown this related program:


import std.typecons: tuple;
import std.c.stdio: printf;
auto foo() {
    printf("foo\n");
    return tuple(1, 2);
}
void main() {
    auto f2 = foo().tupleof;
}


Its output:

foo
foo

So maybe the problem seems isn't caused by the static foreach.

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--- Comment #2 from kennytm gmail.com 2011-03-28 12:02:32 PDT ---
The problem is in .tupleof, where (expr).tupleof is rewritten to
TypeTuple!((expr).field0, (expr).field1, (expr).field2, ...) even if expr have
side effect, e.g.

-----------

import std.c.stdio : printf;

struct S {
    int x;
    int y;
}

void main() {
    cast(void)
    (printf("Hi\n"), S(2,3)).tupleof;
}

// print "Hi" twice.

-----------

This should be rewritten as (tmp=expr, TypeTuple!(tmp.field0, ...)).

(The ',' here is a comma expression.)

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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SomeDude <lovelydear mailmetrash.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lovelydear mailmetrash.com


--- Comment #3 from SomeDude <lovelydear mailmetrash.com> 2012-04-23 03:26:46
PDT ---
With 2.059 Win32

First example gives:

PS E:\DigitalMars\dmd2\samples> rdmd bug
foo
1
2

Second example:
PS E:\DigitalMars\dmd2\samples> rdmd bug
foo

Third example:
PS E:\DigitalMars\dmd2\samples> rdmd bug
Hi

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prev sibling parent d-bugmail puremagic.com writes:
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Kenji Hara <k.hara.pg gmail.com> changed:

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


--- Comment #4 from Kenji Hara <k.hara.pg gmail.com> 2012-04-23 10:40:02 PDT ---
This issue has been already fixed as part of bug 4940.

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