digitalmars.D.bugs - [Issue 7353] New: NRVO not properly working with inferred return type
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http://d.puremagic.com/issues/show_bug.cgi?id=7353
Summary: NRVO not properly working with inferred return type
Product: D
Version: D1 & D2
Platform: All
OS/Version: All
Status: NEW
Keywords: performance
Severity: normal
Priority: P2
Component: DMD
AssignedTo: nobody puremagic.com
ReportedBy: mrmocool gmx.de
--- Comment #0 from Trass3r <mrmocool gmx.de> 2012-01-23 16:27:36 CET ---
import std.stdio;
struct S
{
static uint ci = 0;
uint i;
this(int x)
{
i = ci++;
writeln("new: ", i);
}
this(this)
{
i = ci++;
writeln("copy ", i);
}
~this()
{
writeln("del ", i);
}
S save1() // produces 2 copies in total
{
S s = this;
return s;
}
auto save2() // produces 3 copies in total
{
S s = this;
return s;
pragma(msg, typeof(return));
}
S save3()
{
return this;
}
}
void main()
{
{
S s = S(1);
S t = S(1);
t = s.save1();
}
writeln("-");
S.ci = 0;
{
S s = S(1);
S t = S(1);
t = s.save2();
}
writeln("-");
S.ci = 0;
{
S s = S(1);
S t = S(1);
t = s.save3();
}
}
$ dmd -run test.d
//or dmd -release -run test.d
//or dmd -release -O -run test.d
S
new: 0
new: 1
copy 2
del 1
del 2
del 0
-
new: 0
new: 1
copy 2
copy 3
del 2
del 1
del 3
del 0
-
new: 0
new: 1
copy 2
del 1
del 2
del 0
$ dmd -release -O -inline -run test.d
S
new: 0
new: 1
copy 2
del 1
del 2
del 0
-
new: 0
new: 1
copy 2
copy 3
del 2
del 1
del 3
del 0
-
new: 0
new: 1
del 1
del 0
del 0
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Jan 23 2012
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Trass3r <mrmocool gmx.de> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |wrong-code
--- Comment #1 from Trass3r <mrmocool gmx.de> 2012-01-23 16:32:24 CET ---
Hmm that last one even looks like a wrong-code bug, 0 is deleted twice.
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Jan 23 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353
Lionello Lunesu <lio+bugzilla lunesu.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |lio+bugzilla lunesu.com
Severity|normal |critical
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Feb 12 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353
Kenji Hara <k.hara.pg gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords|wrong-code |pull
Version|D1 & D2 |D2
--- Comment #2 from Kenji Hara <k.hara.pg gmail.com> 2012-02-15 06:39:48 PST ---
Postblit is only in D2 spec, so this is not 'D1 & d2' issue.
I think save3() is inlining problem, then I've separated it as bug 7506.
Therefore I remove 'wrong-code' keyword.
----
Current compiler implementation disables nrvo with auto function.
I've post a pull to fix it.
https://github.com/D-Programming-Language/dmd/pull/722
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Feb 15 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353
--- Comment #3 from Trass3r <mrmocool gmx.de> 2012-02-15 17:48:44 CET ---
You're right. The third one is an inlining problem.
But there's a different thing I'm now confused about. Why is postblit called
after all?
It's assignment to t, not construction.
See the docs:
struct S { ... }
S s; // default construction of s
S t = s; // t is copy-constructed from s => this(this)
t = s; // t is assigned from s => opAssign
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--- Comment #4 from Kenji Hara <k.hara.pg gmail.com> 2012-02-15 09:16:26 PST ---
(In reply to comment #3)
You're right. The third one is an inlining problem.
But there's a different thing I'm now confused about. Why is postblit called
after all?
It's assignment to t, not construction.
See the docs:
struct S { ... }
S s; // default construction of s
S t = s; // t is copy-constructed from s => this(this)
t = s; // t is assigned from s => opAssign
The assignment to t does not call postblit. Because the built-in opAssign is
implemented as 'swap and destroy' due to never create any unnecessary copy on
assignment.
First of all, S has postblit, then dmd creates implicit built-in opAssing like
follows:
ref S opAssign(S rhs) { // rhs receives rvalue, not lvalue
swap(this, rhs);
return this;
// rhs is destroyed the end of opAssign function scope
}
Next, the code sequence with explanation:
{
S s = S(1); // prints new: 0
S t = S(1); // prints new: 1
t = s.save1();
// is translated to t.opAssign(s.save1())
// inside save1():
S s = this; // prints copy 2
return s; // s is moved out by nrvo
// t.opAssign receives moved value, so is never copied
// inside t.opAssign():
// this (this.i==1) is swapped with rhs, and destroyed
// prints del 1
// s.i == 0
// t.i == 2
// destroyed local variables in reverse order
// prints del 2
// prints el 1
}
Last, the postblit call runs only once.
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Feb 15 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353 --- Comment #5 from Trass3r <mrmocool gmx.de> 2012-02-15 20:39:08 CET --- Ok, so in essence the postblit is the one called by 'S s = this;' in save1. But what about save3? -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Feb 15 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353 --- Comment #6 from github-bugzilla puremagic.com 2012-02-17 18:21:35 PST --- Commit pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/02ecef1cea5dea4f78bafb7533c896d0d9b07816 Merge pull request #722 from 9rnsr/fix7353 Issue 7353 - NRVO not properly working with inferred return type -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Feb 17 2012
http://d.puremagic.com/issues/show_bug.cgi?id=7353
Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |bugzilla digitalmars.com
Resolution| |FIXED
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