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Re: [Public WebGL] some samples and a question about looping



Ok, it seems to me that on platforms where infinite or otherwise too
long loops in the shader can crash the system,  unrolling should be
forced, or at least the driver should be made check that no such loops
are possible. Hopefully that is already the case in current WebGL
implementations.

Anyway, as you pointed out, the restriction in WebGL (Appendix A.4 in
GLSL ES 1.0 spec) doesn't exclude infinite or too long loops (and also
while-loops can be unrolled by drivers if they have constant number of
iterations). Just like WebGL doesn't enforce other minimum
requirements of ES (for example VTF is allowed), it should not enforce
this one. In fact, this restriction doesn't prevent any unportable
programs, because the loop bound can be selected high enough so that
it won't work on platforms that unroll it. Actually the restriction
can make programs even less portable (for example my Xmas star doesn't
seem to work on ATI GL drivers for Windows 7, it just allocates
gigabytes of memory). I think that the only way to make loops portable
is testing on different platforms, this restriction isn't helping at
all...

-- Sami

On Sat, Dec 25, 2010 at 3:26 PM, Steve Baker <steve@sjbaker.org> wrote:
> The WebGL spec requires that GLSL implementations limit themselves to
> the MINIMUM OpenGLES2.0 requirements - so we don't allow loops with a
> non-constant number of iterations (and we specifically disallow those
> where the value of a uniform is used to determine the termination
> condition).
>
> In other words, only loops that could (in principle) be unrolled at
> compile time are allowed.
>
> I believe that there are two reasons for this - one is that a lot of
> hardware doesn't support loops at all - so the compiler is forced to
> unroll them.  The other is as a kind of security measure to make sure
> the bad guy can't lock up your computer by putting a shader into an
> infinite loop.
>
> Now - I agree that there is an issue here...while it's reasonable to
> require loops that could in principle be unrolled - should WebGL force
> them to actually be unrolled in practice?   Arguably, it shouldn't
> because that makes the shader code much longer than it needs to be on
> machines that CAN support looping at the hardware level.
>
> I say "arguably" because the current restrictions don't seem to prevent
> you from saying something like:
>
>     for ( float i = 100000000 ; i < 100000001 ; i += 0.1 )
>       dosomething ;
>
> ...which is legal (per the specification) - and on paper, only iterates
> 10 times.   But in practice, it would probably loop forever because math
> precision isn't good enough and 100000000+0.1 evaluates to 100000000, so
> the loop never reaches its target.  If loops are not literally unrolled
> by the compiler, this kind of infinite looping would be hard to
> definitively exclude.
>
> You might imagine that the shader verifier could try unrolling the loop
> to make sure it's possible - and if that works, pass on the original
> rolled-up loop to the underlying GLSL compiler - but that's awfully
> risky if you don't know the precise inner workings of the GPU's floating
> point math unit.  The bad guy might easily find a 'chink' in that armor
> with carefully-chosen data.
>
> Your work-around of looping a very large number of times and then
> breaking out in the middle is legal in principle - but if the compiler
> does unroll loops, you'll run out of shader instructions and it won't
> compile.
>
>  -- Steve
>
>
> On 12/24/2010 10:51 AM, Sami Mäkelä wrote:
>> Hello!
>>
>> I've updated my samples at http://programming-progress.com/webgl.html
>> so that they now work (or do not work) as expected on the latest
>> browsers. At least check out the Xmas star.
>>
>> One question: It seems like the ATI GL drivers on at least Windows 7
>> 64bit seem to always unfold loops that have constant number of
>> iterations. My workaround was to force the looping to be unknown (by
>> using uniforms). This doesn't work anymore because ANGLE validator
>> doesn't accept it. Is there some example on any platform where
>> converting the shader code to the form accepted by ANGLE would
>> actually convert a non-working shader to a shader that works with the
>> driver?
>> Anyway even if there is some driver that cannot handle full looping,
>> it should not matter, because any program can be converted into the
>> form accepted by ANGLE for example using
>> while (cond) body; ===> for (int i = 0; i < INFINITY-1; i++) { if
>> (!(cond)) break; else body; }
>> As summary, this check might cause some shaders to become impossible
>> on some environments, and it does not add to compatibility, because
>> the shaders could be easily converted into form that should be
>> accepted, if the original code is rejected by the driver. Wouldn't it
>> just be better to drop this check and say that the loops are "too
>> long" (and hence undefined) if they are not supported by the driver?
>>
>> -- Sami
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>
>

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