digitalmars.D - Possible bug in std.algorithm.map

Magnus Lie Hetland <mlh idi.ntnu.no> writes:
```Hi!

Just read Andrei Alexandrescu's new book, and I'm starting to
experiment with using D in my algorithms research. Loved the book, and
I'm loving the language so far :D

I just hit a snag, though ... I was doing something simple, for which
my prototype code (in Python) was

d, u = max((D(u,v), v) for v in V)

I first started writing it explicitly with loops, but it got a bit too
verbose for my taste. Thought I'd use map and reduce, perhaps
(although I'm still not sure if that's practical, as I'm reducing with
max, but I'd like the argmax as well...).

Anyway -- while using attempting to use map, I suddenly got a
segfault. As I hadn't really done any crazy stuff with pointers, or
circumvented the bounds checks or the like, I was a bit surprised. I
have now boiled things down to the following little program:

import std.algorithm;
void f() {
auto x = 0;
double g(int z) { // Alt. 1: return int
auto y = x;   // Alt. 2: remove this
return 0;
}
auto seq = [1, 2, 3];
auto res = map!(g)(seq);
}
void main() {
f();
}

When I compile and run this (dmd 2.051, OS X 10.5.8), I get a
segmentation fault.

Oddly enough, if I *either* change the return type to int *or* remove
the "y = x" line, things work just fine.

Am I correct in assuming this is a bug?

--
Magnus Lie Hetland
http://hetland.org
```
Jan 29 2011
Don <nospam nospam.com> writes:
```Magnus Lie Hetland wrote:
Hi!

Just read Andrei Alexandrescu's new book, and I'm starting to experiment
with using D in my algorithms research. Loved the book, and I'm loving
the language so far :D

I just hit a snag, though ... I was doing something simple, for which my
prototype code (in Python) was

d, u = max((D(u,v), v) for v in V)

I first started writing it explicitly with loops, but it got a bit too
verbose for my taste. Thought I'd use map and reduce, perhaps (although
I'm still not sure if that's practical, as I'm reducing with max, but
I'd like the argmax as well...).

Anyway -- while using attempting to use map, I suddenly got a segfault.
As I hadn't really done any crazy stuff with pointers, or circumvented
the bounds checks or the like, I was a bit surprised. I have now boiled
things down to the following little program:

import std.algorithm;
void f() {
auto x = 0;
double g(int z) { // Alt. 1: return int
auto y = x;   // Alt. 2: remove this
return 0;
}
auto seq = [1, 2, 3];
auto res = map!(g)(seq);
}
void main() {
f();
}

When I compile and run this (dmd 2.051, OS X 10.5.8), I get a
segmentation fault.

Oddly enough, if I *either* change the return type to int *or* remove
the "y = x" line, things work just fine.

Am I correct in assuming this is a bug?

Yes. Sounds like bug 5064.
http://d.puremagic.com/issues/show_bug.cgi?id=5064
It's currently #8 on my list of most important bugs to fix in DMD.
```
Jan 29 2011
"Robert Jacques" <sandford jhu.edu> writes:
```On Sat, 29 Jan 2011 11:12:28 -0500, Magnus Lie Hetland <mlh idi.ntnu.no>
wrote:

Hi!

Just read Andrei Alexandrescu's new book, and I'm starting to experiment
with using D in my algorithms research. Loved the book, and I'm loving
the language so far :D

I just hit a snag, though ... I was doing something simple, for which my
prototype code (in Python) was

d, u = max((D(u,v), v) for v in V)

I first started writing it explicitly with loops, but it got a bit too
verbose for my taste. Thought I'd use map and reduce, perhaps (although
I'm still not sure if that's practical, as I'm reducing with max, but
I'd like the argmax as well...).

Anyway -- while using attempting to use map, I suddenly got a segfault.
As I hadn't really done any crazy stuff with pointers, or circumvented
the bounds checks or the like, I was a bit surprised. I have now boiled
things down to the following little program:

import std.algorithm;
void f() {
auto x = 0;
double g(int z) { // Alt. 1: return int
auto y = x;   // Alt. 2: remove this
return 0;
}
auto seq = [1, 2, 3];
auto res = map!(g)(seq);
}
void main() {
f();
}

When I compile and run this (dmd 2.051, OS X 10.5.8), I get a
segmentation fault.

Oddly enough, if I *either* change the return type to int *or* remove
the "y = x" line, things work just fine.

Am I correct in assuming this is a bug?

Yes, it's Issue 5073.
test case using the listed patch + DMD 2.051 and it works. The issue used
to be a bad compile time error in earlier compiler versions but in DMD
2.051 it turned into a runtime error. The underlying error in DMD has to
do with an alias of a delegate having a bad hidden pointer. The reason
commenting out 'auto y = x;' works is that the incorrect hidden pointer is
never called, thus never causing a segfault (IIRC). Issue 5073's patch
works by passing delegates by value instead of by alias. Looking over
Issue 5064, Don is probably right in it being the root cause in DMD, but
if you just want map to work correctly, you might want to try the patch
from 5073.
```
Jan 29 2011
Magnus Lie Hetland <mlh idi.ntnu.no> writes:
```On Jan 30, 2011, at 08:19 , Robert Jacques wrote:

Yes, it's Issue 5073. (http://d.puremagic.com/issues/show_bug.cgi?id=5073
).

OK.

I've tested your test case using the listed patch + DMD 2.051 and it
works.

Great.

Looking over Issue 5064, Don is probably right in it being the root
cause in DMD, but if you just want map to work correctly, you might
want to try the patch from 5073.

Thanks. I also see (from 5064) that using -inline works around the
problem (worked for me). Slightly less invasive solution for right
now, at least.

Thanks for the help.

--
Magnus Lie Hetland
http://hetland.org
```
Jan 30 2011