Range-Checks and Recklessness

http://prog21.dadgum.com/192.html

by fexl • 12 years ago

22 19 12 years ago

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Roboprog2• 12 years ago

It's funny the author uses an example of an array indexed by 199x values, as the 90s were in some way "the lost decade" of C/++ development, and an utter lack of concern for this sort of checking.

The realities of internet exposure put an end to this recklessness for casual^H^H^H^H^H "enterprise" software development.

I love C A Hoare's comments, reference in this link (http://en.wikipedia.org/wiki/Bounds_checking) about "some languages" (cough cough - C)

Interesting how the Go language now includes array bounds checking. While 2 of the main designers are ex Bell labs, 1 of them is from U of Zurich. (I'm assuming he would have had some Pascal/Modula exposure there)

fexl4• 12 years ago

I love C but I use it in a disciplined way. Here's the email I sent to the author:

James:

I liked your article on range checking.

I'm very keen on assertions, for example here:

https://github.com/chkoreff/Fexl/blob/fresh/base/src/buf.c#L...

It's still fast as greased lightning. I buffer up 2.6 MB here:

https://github.com/chkoreff/Fexl/blob/fresh/test/src/run.c#L...

It's too fast to notice. I can bump it up to 260 MB and try that:

  $ ./build && time ../bin/run
  : Buffering 260000000 bytes
    length = 260000000

  real	0m1.751s
  user	0m1.608s
  sys	0m0.140s

What the heck, let's try an even 2 GB:

  $ ./build && time ../bin/run
  Compile run
  Link run
  : Buffering 2000000000 bytes
    length = 2000000000

  real	0m13.350s
  user	0m12.049s
  sys	0m1.288s

Ah, but what if I remove my range check?

  $ ./build && time ../bin/run
  Compile buf
  Link run
  : Buffering 2000000000 bytes
    length = 2000000000

  real	0m13.103s
  user	0m11.781s
  sys	0m1.312s

Repeated testing of both ways shows no significant difference.

I think I'll leave in the range check.

Range checks also support one of my favorite programming maxims:

  No Silent Failure!

deletes• 12 years ago

How will assert help if the failure happens in production code? Wouldn't it be better if the error was logged to a file.

bunderbunder2• 12 years ago

Modern CPU design adds an interesting twist to the bounds checking question that (imo) renders it moot. At least on Intel CPUs, the array bounds check gets compiled down to a compare-and-jump instruction pair, which then ends up getting run as a single microinstruction on the CPU. Unless you're in an extremely tight loop, the cost of that instruction just isn't significant. The only time it costs much at all is in the event of a branch misprediction, in which case the very next thing that's going to happen is an exception so it's still insignificant given the context. And if you are in an extremely tight loop, it's probably structured in a way that makes it easy for compiler optimize away the bounds check.

srean2• 12 years ago

I am willing to be convinced, but it will take a well executed BLAS (, in particular a sparse BLAS) benchmark with and without range checks. If the range check enabled version is within 5~8% of the current standard of say a ATLAS/MKL/Eigen (note these aren't quite the fastest) consider me sold.

In my experience range checks did affect the speed, but range check alone may not be to blame. It might well be that code pieces became a tad bit larger than what a compiler would automatically inline, and those snow-balled.

In case you have such a benchmark handy would appreciate a link 'cause fast is good, but fast and correct is way better.

bunderbunder2• 12 years ago

If you're really in a position where you need to worry about performance down to the level of counting individual instructions then you probably shouldn't be relying on someone else to do your benchmarks for you, anyway.

al2o3cr• 12 years ago

I wonder if the "bound" instruction on x86 has the same issue that the VAX(?) opcode did - that it's faster to write out the two checks than to call the instruction...

deathanatos• 12 years ago

Range checking is great; the CPU cycles consumed are well worth the bugs they catch.

Thing is, they're a list thing, and there's more than just "is this index valid?" that can go wrong. One of the things I appreciate about C++ is that iterator invalidation is very precisely specified. Sadly, that's where it stops: it's just specified, but the onus is still on you to catch the errors. It'd be great to have that same thing: immediate errors when you use an invalidated iterator. (in vectors, it might just skip an element (some deletes) or see the same element twice (some inserts); Python warns about this during some cases with dicts:

  RuntimeError: dictionary changed size during iteration

which is nice, but I think you can still slip by that.) I don't believe Java or Python specify what happens to iterators when the collection changes, which to me, is a bit sad.

Thing is, to implement this, the collections would probably need to know about all outstanding iterators, so as to figure out where they are and whether they should be invalidated at all. Most operators then become O(number of active iterators + whatever the normal cost of this op is); I'd argue this might still be close to O(1 + ...) since the number of active iterators on a collection is probably usually 0 or 1. But there's a memory cost, a CPU cost, and if you have something like:

  if(weird condition) {
    use_that_iterator_i_just_accidentally_invalidated()
  }

Then your bug only gets caught if `weird_condition` is true. Is it worth it?

bmm6o• 12 years ago

Java can do it without too much trouble, you just have a version number on the collection that gets incremented with every mutation, and the iterator just checks to see if that has changed. C++ is much more complicated, allowing patterns like map.erase(i++) being legal but similar usages not.

TheLoneWolfling• 12 years ago

Simple answer:

Leave in all range checks. If the compiler can determine that the range check will never be hit, or can partially optimize it (hoisting it out of a loop, etc) great. Otherwise? It gets checked at runtime, which is typically not that expensive an operation anyways.

Range-Checks and Recklessness

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