www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - Insight into the DMD back-end

reply %u <e ee.com> writes:
Yay for more comments like this one.

Don 2010-12-06 11:53:27 PST
--
Bearophile -- That's an interesting [automatic fuzzy testing]link. Currently,
DMD back-end bugs are
being found at the rate of about 3 per year. So yes, fuzzy testing of DMC could
probably flush out some backend bugs a bit faster.
-------------------

Here's what's happening. First, in this code:

    for (int i = 0; i < 10; i++) {
        foo(i * 5 - 6);
    }
it sees that i and 10 are always >=0, so the signed comparison "i < 10" is
replaced with an unsigned one. (This happens in the backend in constprop() ).
Then, while dealing with loop invariants, it rewrites the loop into:

for (int _i2 = -6; _i2 < 10*5 - 6; _i2 += 5)
{
  foo(_i2);
}

Fine. Except that it had changed the comparison into an unsigned one!
Particularly interesting is the case where the call is foo(i*5-50);
Then, the loop becomes:
for (int _i2 = -50; _i2 < 0; _i2 += 5)

Since an unsigned value is NEVER less than zero, it just drops the loop
completely!

Nasty.
--
http://d.puremagic.com/issues/show_bug.cgi?id=5294
Dec 06 2010
parent reply Stephan <spam extrawurst.org> writes:
On 07.12.2010 06:57, %u wrote:
 Yay for more comments like this one.

 Don 2010-12-06 11:53:27 PST
 --
 Bearophile -- That's an interesting [automatic fuzzy testing]link. Currently,
 DMD back-end bugs are
 being found at the rate of about 3 per year. So yes, fuzzy testing of DMC could
 probably flush out some backend bugs a bit faster.
 -------------------

 Here's what's happening. First, in this code:

      for (int i = 0; i<  10; i++) {
          foo(i * 5 - 6);
      }
 it sees that i and 10 are always>=0, so the signed comparison "i<  10" is
 replaced with an unsigned one. (This happens in the backend in constprop() ).
 Then, while dealing with loop invariants, it rewrites the loop into:

 for (int _i2 = -6; _i2<  10*5 - 6; _i2 += 5)
 {
    foo(_i2);
 }

 Fine. Except that it had changed the comparison into an unsigned one!
 Particularly interesting is the case where the call is foo(i*5-50);
 Then, the loop becomes:
 for (int _i2 = -50; _i2<  0; _i2 += 5)

 Since an unsigned value is NEVER less than zero, it just drops the loop
 completely!

 Nasty.
 --
 http://d.puremagic.com/issues/show_bug.cgi?id=5294

What i think is more disturbing is Walters response:
 I'm not sure how to fix that one yet, but it has been there for 25 years
 now, so I am not sure it is urgent!

I often ran into this strange behaviour when using -O optimization without knowing where it came from and it is so disturbing when i think of people newly getting interested in D making the experience when trying to compare it with C/C++ and then finding out the optimization makes strange things. I think out of a image perspective such bugs must be high priority, ESPECIALLY if it lies there for 25years already. Regards, Stephan
Dec 07 2010
next sibling parent reply Walter Bright <newshound2 digitalmars.com> writes:
Stephan wrote:
 I often ran into this strange behaviour when using -O optimization 
 without knowing where it came from and it is so disturbing when i think 
 of people newly getting interested in D making the experience when 
 trying to compare it with C/C++ and then finding out the optimization 
 makes strange things. I think out of a image perspective such bugs must 
 be high priority, ESPECIALLY if it lies there for 25years already.

I've had probably 10,000 bug reports over the years and this has never appeared as one before now. Thank you for reporting it.
Dec 07 2010
parent reply Stephan <spam extrawurst.org> writes:
On 07.12.2010 11:52, Walter Bright wrote:
 Stephan wrote:
 I often ran into this strange behaviour when using -O optimization
 without knowing where it came from and it is so disturbing when i
 think of people newly getting interested in D making the experience
 when trying to compare it with C/C++ and then finding out the
 optimization makes strange things. I think out of a image perspective
 such bugs must be high priority, ESPECIALLY if it lies there for
 25years already.

I've had probably 10,000 bug reports over the years and this has never appeared as one before now. Thank you for reporting it.

Possibly because it was so hard to nail down and reproduce in a simple testcase. Slightly changing the code made it disappear every now and then again.
Dec 07 2010
parent reply Walter Bright <newshound2 digitalmars.com> writes:
Stephan wrote:
 On 07.12.2010 11:52, Walter Bright wrote:
 Stephan wrote:
 I often ran into this strange behaviour when using -O optimization
 without knowing where it came from and it is so disturbing when i
 think of people newly getting interested in D making the experience
 when trying to compare it with C/C++ and then finding out the
 optimization makes strange things. I think out of a image perspective
 such bugs must be high priority, ESPECIALLY if it lies there for
 25years already.

I've had probably 10,000 bug reports over the years and this has never appeared as one before now. Thank you for reporting it.

Possibly because it was so hard to nail down and reproduce in a simple testcase. Slightly changing the code made it disappear every now and then again.

I do appreciate the small test case, too.
Dec 07 2010
parent Stephan <spam extrawurst.org> writes:
On 07.12.2010 12:49, Walter Bright wrote:
 Stephan wrote:
 On 07.12.2010 11:52, Walter Bright wrote:
 Stephan wrote:
 I often ran into this strange behaviour when using -O optimization
 without knowing where it came from and it is so disturbing when i
 think of people newly getting interested in D making the experience
 when trying to compare it with C/C++ and then finding out the
 optimization makes strange things. I think out of a image perspective
 such bugs must be high priority, ESPECIALLY if it lies there for
 25years already.

I've had probably 10,000 bug reports over the years and this has never appeared as one before now. Thank you for reporting it.

Possibly because it was so hard to nail down and reproduce in a simple testcase. Slightly changing the code made it disappear every now and then again.

I do appreciate the small test case, too.

Its fixed! Thanks a lot! That will hopefully make D perform better in a benchmark vs. C that i make.
Dec 08 2010
prev sibling parent Don <nospam nospam.com> writes:
Stephan wrote:
 On 07.12.2010 06:57, %u wrote:
 Yay for more comments like this one.

 Don 2010-12-06 11:53:27 PST
 -- 
 Bearophile -- That's an interesting [automatic fuzzy testing]link. 
 Currently,
 DMD back-end bugs are
 being found at the rate of about 3 per year. So yes, fuzzy testing of 
 DMC could
 probably flush out some backend bugs a bit faster.
 -------------------

 Here's what's happening. First, in this code:

      for (int i = 0; i<  10; i++) {
          foo(i * 5 - 6);
      }
 it sees that i and 10 are always>=0, so the signed comparison "i<  10" is
 replaced with an unsigned one. (This happens in the backend in 
 constprop() ).
 Then, while dealing with loop invariants, it rewrites the loop into:

 for (int _i2 = -6; _i2<  10*5 - 6; _i2 += 5)
 {
    foo(_i2);
 }

 Fine. Except that it had changed the comparison into an unsigned one!
 Particularly interesting is the case where the call is foo(i*5-50);
 Then, the loop becomes:
 for (int _i2 = -50; _i2<  0; _i2 += 5)

 Since an unsigned value is NEVER less than zero, it just drops the loop
 completely!

 Nasty.
 -- 
 http://d.puremagic.com/issues/show_bug.cgi?id=5294

What i think is more disturbing is Walters response:
 I'm not sure how to fix that one yet, but it has been there for 25 years
 now, so I am not sure it is urgent!

I often ran into this strange behaviour when using -O optimization without knowing where it came from and it is so disturbing when i think of people newly getting interested in D making the experience when trying to compare it with C/C++ and then finding out the optimization makes strange things. I think out of a image perspective such bugs must be high priority, ESPECIALLY if it lies there for 25years already.

Yes, such bugs are given maximum priority. The only question is, should we delay the next release until this one is fixed? BTW, I said there are about 3 of these bugs per year. Here's the ones from the past two years, together with the time elapsed between reporting and fix: 2697 [6 months] 3521 [3 weeks] 3558 [10 weeks] 3633 [1 week] 3736 [2 weeks] 4443 [3 weeks] Other wrong-code bugs were in the front end, or the glue layer. True back-end bugs are very rare, and are always very serious (largely because when you encounter them, they are almost impossible to recognize).
Dec 07 2010