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A better test to see if IE9 is cheating is to remove/rearrange code and rename variables. I'd avoid changing operators. Adding a 'true;' or 'return;' may seem harmless, but if their analysis is fragile they may just throw as "may have side-effects" on those statements or (in the case of the 'return;') it may not do liveness analysis on the other side of the block.

This code (taken from this thread) seems like a good test:

function numNumNum() { var I; var num = 10; for (I = 0; I < 10; I++) { num = num * num * num * num * num % num; } }

Except it uses two new operators: '*' and '%'. Test the same code using '+' and '-'.

This will give a much better idea of it the analysis is just fragile or if this code was being targeted.




At what point does "fragile" become "targeted"? Seriously, if it's that narrow...


Well there's really three words of interest here: fragile, targeted, and cheating.

Cheating is really doing something like looking specifically for sunspider and then doing DCE based on knowing the function.

Fragile is distinct from cheating in that there is actually a real analysis framework in place, but the analysis can be invalidated easily. For example, it's not uncommon to see analysis assume function calls may write to all globals and modify all byref arguments. Looking at the code you can say, "with interprocedural analysis its obvious that this function has no side effects", but the analysis may not be that smart. That's an example of fragility.

Now with this example, given that the browser is in Beta/CTP I wouldn't be at all surprised if their framework was simply incomplete. The 'return;' statement causing a problem, but renaming and reordering variables doesn't is the clearest indication IMO. It seems to indicate that they aren't doing any liveness analysis on the backside, but they aren't doing simple pattern matching on the text, nor the IR.

Targeting is really about how one brings up the framework. I actually wouldn't be surprised to hear that they did target sunspider, and that sunspider is probably part of their regression suite. With that said, this is EXTREMELY common in the compiler industry.

Now the question you're arguing is does targeting == cheating? In most cases, no. In fact my suspicioun is that what we're seeing here is the result of either an incomplete implementation where they did target sunspider, or a more complete implementation that broke, but no one noticed because its main DCE test was sunspider.

If IE9 can turn this around with a fix in their next CTP, it was probably not cheating and just a case of targeting. The reason being that doing a static analysis framework that is capable of being robust in these situations is non-trivial, and not something you just add in post-beta.

And if someone could run the test I posted above with '+' '-' rather than '*' '%' we'd have a first step in our answer. I would do it, but I neither know the sunspider harness, and don't have IE9 installed (and getting a new VM on this particular machine is a hassle).




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