On 06/26/2014 06:59 AM, David Rajchenbach-Teller wrote:
I would be interested in adding boundary checks and invariant checks
that could be eliminated in opt builds. Is this in the scope of your
project?
No, I don't think so. S2S could certainly do it, but you wouldn't want
to use S2S on all browser code in opt builds. Too expensive.
But: there's already a pattern you can use to enable assertions that
will be zero-cost if your code runs in IonMonkey! I didn't realize this
until you mentioned it. But suppose you start out with this:
function DEBUG() {
return false;
}
function assert(condition) {
if (!condition) {
throw new Error("assertion failed");
}
}
Then you can write:
if (DEBUG()) assert(flowers.areBeautiful());
Now of course `assert(flowers.areBeautiful())` will not execute whether
IonMonkey kicks in or not, because DEBUG() returns false.
But when IonMonkey compiles this, it can do even better. First, DEBUG()
will be inlined. Then, since the return value is always `false`, the
whole if-statement will be optimized away. (Currently, we don't optimize
away if-statements that depend on global vars or even consts. Only
functions.)
Admittedly this pattern is not quite as pretty as
`assert(flowers.areBeautiful())`. Here's why that wouldn't work. JS
doesn't have macros. :( Arguments to a function are evaluated before the
function is called. This means that if we wrote assertions this way,
`flowers.areBeautiful()` would have to be called even in non-debug
builds. The language spec requires it. We need the if-statement to get
the right debug-only behavior.
-j
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