http://d.puremagic.com/issues/show_bug.cgi?id=4835



--- Comment #10 from Luís Marques <luismarques@gmail.com> 2013-03-26 20:04:13 PDT ---
(In reply to comment #9)
> (In reply to comment #8)
> > https://github.com/D-Programming-Language/dmd/pull/1803
> 
> An error, even :-) So the issue name should be updated.
> 
> Thank you Walter. Let's see how this patch goes.

I hope the feedback I provided with being bitten by this corner case helped nudge this issue to the limelight, that way I can feel slightly important ;-). Thanks for addressing it so quickly Walter.

(Also, isn't it strange that this was addressed quicker than in Java, which
should have a larger support community, where the issue remains? Go D! (At
least it remains with javac 1.6.0_43))

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--- Comment #11 from Walter Bright <bugzilla@digitalmars.com> 2013-03-26 22:36:40 PDT ---
While I disagree with bearophile about doing such checks at runtime, due to the performance cost, doing them at compile time is a whole 'nother story.

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--- Comment #12 from Luís Marques <luismarques@gmail.com> 2013-03-26 22:46:13 PDT ---
(In reply to comment #11)
> While I disagree with bearophile about doing such checks at runtime, due to the performance cost, doing them at compile time is a whole 'nother story.

Why don't you make the runtime over/underflow checks opt-in? (compiler option). There's a 0 performance cost that way, both for debug and release, by default, so no performance excuse. Then we can always discuss it later if it should be opt-out for the debug builds.

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--- Comment #13 from Walter Bright <bugzilla@digitalmars.com> 2013-03-27 02:38:18 PDT ---
Because it'll screw things up right and left, for example, address calculations.

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--- Comment #14 from bearophile_hugs@eml.cc 2013-03-27 04:00:40 PDT ---
(In reply to comment #11)
> While I disagree with bearophile about doing such checks at runtime, due to the performance cost,

A feature of LLVM 3.3, it seems those costs are not too much large for C language:

http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation

See:
-fsanitize=signed-integer-overflow
-fsanitize=integer

More info:
http://embed.cs.utah.edu/ioc/

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--- Comment #15 from Luís Marques <luismarques@gmail.com> 2013-03-27 09:00:14 PDT ---
(In reply to comment #13)
> Because it'll screw things up right and left, for example, address calculations.

Walter, what do you mean, screw? Is that a limitation of the dmd backend, or are you arguing that it is problematic in general? LLVM implements it, as bearophile points out...

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--- Comment #16 from Walter Bright <bugzilla@digitalmars.com> 2013-03-28 00:39:03 PDT ---
(In reply to comment #15)
> Walter, what do you mean, screw? Is that a limitation of the dmd backend, or are you arguing that it is problematic in general? LLVM implements it, as bearophile points out...

Consider all the addressing modes used - they are all adds, with no overflow checks. Secondly, they all rely on wraparound (overflow) arithmetic, after all, that is how subtraction is done.

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