[LLVMdev] C as used/implemented in practice: analysis of responses

Tue Jul 7 12:09:26 PDT 2015

On 7 July 2015 at 18:26, Chris Lattner <clattner at apple.com> wrote:
>
> On Jul 1, 2015, at 3:20 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>
>
> On Wed, Jul 1, 2015 at 12:22 PM, Russell Wallace <russell.wallace at gmail.com>
> wrote:
>>
>> I am arguing in favor of a point, and I understand you disagree with it,
>> but I don't think I'm dismissing any use cases except a very small
>> performance increment.
>
>
> I'm sure Google has numbers about how much electricity/server cost they save
> for X% performance improvement.
> I'm sure Apple has numbers about how much money they make with X% improved
> battery life.
> I'm not convinced that the cost of some of these bugs is actually larger
> than the benefit of faster programs. Nor am I convinced about the inverse.
> I'm just pointing out that pointing to a "bad bug" caused by a certain
> optimization without comparing the cost of the bug to the benefit of the
> optimization is basically meaningless. You'll need to quantify "very small
> performance improvement" and put it in context of the bugs you're talking
> about.
>
>
> As with many things, it is more complicated than that.  The performance
> effects of optimizations are often non-linear, and you can take a look at
> many of the worst forms of UB in C and easily show cases where they allow 2x
> speedups, not just 2%.
>
> For example, consider undefined behavior for integer overflow:
>
>   for (int i = 0; i <= N; ++i) {
>      …
>   }
>
> When compiling for a 64-bit machine, you really want to promote the
> induction variable to 64-bits.  Further, knowing the trip count of a loop is
> extremely important for many loop optimizations.  Unfortunately, without
> being able to assume undefined integer wraparound, you get neither of these
> from C.
>
> -fstrict-aliasing is another great example.  In many cases, it makes no
> difference whatsoever.  OTOH, on code like:
>
> void doLoopThing(float *array, int *N) {
>     for (int i = 0; i < *N; ++i) {
>        array[i] = array[i] + 1;
>   }
>
> You can easily get a 2x or more speedup due to auto-vectorization if you can
> assume -fstrict-aliasing.  Of course usually you wouldn’t write this code,
> you’d get this because doLoopThing is a template, and N is passed in as a
> reference.
>
>
> Anyway, I could go on and on here, and I’ve spent a lot of time over the
> years thinking about how to improve the situation: can we make clang detect
> more of these, can we make the optimizer more conservative in certain cases
> etc?  This is why (for example) our TBAA uses simple structural points-to
> analysis before using TBAA.  With GCC’s implementation (circa GCC 4.0, I
> have no idea what they are doing now), GCC would “miscompile” code like:
>
> float bitcast(int x) {
>   return *(float*)&x;
> }
>
> This code is a TBAA violation, but is also “obvious” what the programmer
> meant.  LLVM being “nicer” in this case is a feature.  It is irritating that
> the union version of this is also technically UB or implementation defined
> behavior, so that isn’t portable either (a C programmer needs to magically
> know that memcpy is the safe way to do this).
>
> However, as I’ve continued to dig into this, my feeling is that there really
> is no satisfactory solution to these issues.  The problem here are pervasive
> structural problems in the C language: In the first example above, it is
> that “int” is the default type people generally reach for, not “long”, and
> that array indexing is expressed with integers instead of iterators.  This
> isn’t something that we’re going to “fix" in the C language, the C
> community, or the body of existing C code.  Likewise, while C++ has made
> definite progress here by replacing some of these idioms (e.g. with
> iterators), it adds its own layers of UB on, and doesn’t actually *subtract*
> the worst things in C.
>
> My conclusion is that C, and derivatives like C++, is a very dangerous
> language the write safety/correctness critical software in,

This is certainly true, but (as you say below) the shift to better
languages for systems code isn't going to happen any time soon, so,
given the level of disagreement and confusion about what can be relied
on that we saw in that survey, I think we have to do something to
improve matters for C more-or-less as it is.

Taking those survey questions as an (incomplete) starting point:

  http://www.cl.cam.ac.uk/~pes20/cerberus/notes51-2015-06-21-survey-short.html

- for some, it seems that clang and gcc already support a
stronger-than-ISO behaviour on "normal" platforms (e.g. perhaps the
null-pointer cases).  Can we identify and document those?

- for all the others where there is substantial code relying on them,
it seems that the minimal thing is at least to identify whatever
current optimisation-limiting compiler option(s) are enough to
guarantee it's supported, where that exists. That'll have a cost, of
course, but in many contexts the clarity may be worth it, just as many
of our respondents routinely use -fno-strict-aliasing.

- other cases might need new such options

- sometimes the non-ISO behaviour may be so prevalent that there
really isn't any reasonable sense in which compilers should be
exploiting it (I'm suspicious about temporarily out-of-bounds pointer
construction, for example)

- others might really need new attributes or other extensions, e.g. to
say that some pointers should support inter-object arithmetic, if
there are relatively few places where that occurs in practice.

- and, of course, some UBs are really essential to normal C
implementation; we can't hope to give sensible semantics to wild
writes, and also some of the arithmetic issues, in any way other than
declaring them UB.

I'd hope that identifying which of the survey questions is which can
be reasonably straightforward for the compiler development community,
and, given that, we should be in a position to precisely define what
*that* version of C is, at least.

Peter

----
Peter Sewell
Computer Laboratory, University of Cambridge
http://www.cl.cam.ac.uk/users/pes20

>  and my personal
> opinion is that it is almost impossible to write *security* critical
> software in it.  This isn’t really C’s fault if you consider where it was
> born and evolved from (some joke that it started as a *very* nice high level
> assembler for the PDP11
> https://en.wikipedia.org/wiki/C_(programming_language)#Early_developments
> :-).
>
> There are many more modern and much safer languages that either eliminate
> the UB entirely through language design (e.g. using a garbage collector to
> eliminate an entire class of memory safety issues, completely disallowing
> pointer casts to enable TBAA safely, etc), or by intentionally spending a
> bit of performance to provide a safe and correct programming model (e.g. by
> guaranteeing that integers will trap if they overflow).  My hope is that the
> industry will eventually move to better systems programming languages, but
> that will take a very very long time...
>
> -Chris
>
>
>
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