[Vm-dev] [commit] r2463 - CogVM source as per VMMaker.oscog-eem.105. Fix signed32BitValueOf for most neg-

[Eliot Miranda]

On Thu, Jul 21, 2011 at 3:17 PM, Stefan Marr <squeak at stefan-marr.de> wrote:

>
> Hi Eliot:
>
> On 21/07/11 19:35, Eliot Miranda wrote:
>
>
>> On Thu, Jul 21, 2011 at 4:30 AM, David T. Lewis <lewis at mail.msen.com<mailto:
>> lewis at mail.msen.com>> wrote:
>>
>>
>>
>>    Hi Eliot,
>>
>>    Can you say what the issue was with signed32BitValueOf? I can
>>    see the changes in InterpreterPrimitives>>**signed32BitValueOf:
>>    but I'm not clear on whether this is something that affects
>>    Alien, or if it is something that has been causing problems
>>    more generally but went unnoticed. Also, I'd like to document
>>    this with a unit test, so if you can suggest a code snippet
>>    that would be great.
>>
>>
>> The unit test is in the lien tests and is the attempt to assign max neg
>> int (-2^31) through signedLongAt:put:.  The problem is due to a pervasive C
>> compiler bug with optimization.
>>
> Maybe I misread this blog post: http://blog.llvm.org/2011/05/**
> what-every-c-programmer-**should-know.html<http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html>(*Signed integer overflow:)*
> but from that I would conclude that it is not a compiler bug, but undefined
> behavior in C instead. Thus, GCC and ICC are just doing there job in terms
> of what they are allowed to do by the spec.
>

Thanks, Stefan.  I hadn't known this (or had forgotten):

*Signed integer overflow:* If arithmetic on an 'int' type (for example)
overflows, the result is undefined. One example is that "INT_MAX+1" is not
guaranteed to be INT_MIN. This behavior enables certain classes of
optimizations that are important for some code. For example, knowing that
INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". Knowing the
multiplication "cannot" overflow (because doing so would be undefined)
allows optimizing "X*2/2" to "X". While these may seem trivial, these sorts
of things are commonly exposed by inlining and macro expansion. A more
important optimization that this allows is for "<=" loops like this:

for (i = 0; i <= N; ++i) { ... }


In this loop, the compiler can assume that the loop will iterate exactly N+1
times if "i" is undefined on overflow, which allows a broad range of loop
optimizations to kick in. On the other hand, if the variable is defined to
wrap around on overflow, then the compiler must assume that the loop is
possibly infinite (which happens if N is INT_MAX) - which then disables
these important loop optimizations. This particularly affects 64-bit
platforms since so much code uses "int" as induction variables.

Makes evil sense :)


>
> Best regards
> Stefan
>
>


-- 
best,
Eliot
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