[Webkit-unassigned] [Bug 94779] To reduce cost of converting type during modulo for integers

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Tue Sep 18 00:34:13 PDT 2012


https://bugs.webkit.org/show_bug.cgi?id=94779





--- Comment #20 from Filip Pizlo <fpizlo at apple.com>  2012-09-18 00:34:43 PST ---
(In reply to comment #19)
> (In reply to comment #18)
> > (In reply to comment #17)
> > > (In reply to comment #16)
> > > I prefer a browser that doesn't crash over a browser that computes prime numbers quickly.
> > 
> > I totally agree with your thought. I'm appreciated for your comment.
> 
> I figured out why the crash didn't occur on my device. That's because of the compile optimization level.

I'm curious about this.  Can you share more details?

For me, here's what I see:

If I make it obvious to my compiler (clang+llvm) what the constants are in the modulo operation, like so:

int a = -2147483647 - 1;
int b = -1;
return a % b; // a and b are known constants

Then I only crash when optimizations are disabled.  With optimizations, I do get a garbage result, however.

But if I make it non-obvious, such as in my test program that I attached, by virtue of the fact that the integers a and b are parsed from the command-line, then I do crash on (-2^31)%-1, regardless of optimization level.

Is there something different that is happening for you?

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