Bug 151741 – [JSC] Add a function attribute for Pure functions in B3

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Benjamin Poulain]

[JSC] Add a function attribute for Pure functions in B3

[Benjamin Poulain]

Created attachment 266435 [details]
Patch

[Geoffrey Garen]

Comment on attachment 266435 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=266435&action=review

r=me

> Source/JavaScriptCore/b3/B3CCallValue.h:38
> +    enum Pure { PureCall };

Usually we call this "enum PureCallTag { PureCall }".

[Benjamin Poulain]

Committed r192967: <http://trac.webkit.org/changeset/192967>

[Filip Pizlo]

Comment on attachment 266435 [details]
Patch

This doesn't belong here at all.  A pure function can be labeled by setting its Effects appropriately.  Also, I really hate the word "pure" since it has no precise meaning.  Do you mean that a call to the function with the same incoming values will yield the same result?  Do you also mean that the function has no hidden effects that cause it to require execution even if the result is unused?  Do you also mean that it will never read any memory, or can it still read some memory?

I created B3::Effects with the intent that it can answer all of these questions precisely:
- You can say that a function reads no memory.
- You can say that a function writes no memory.
- You can say that a function is not control-dependent.
- You can say that a function will not side-exit (for C functions a side-exit would be if it triggered unwinding - so this can always be cleared for all of the C functions we call).

The strongest purity guarantee is "any call to this function with the same arguments will yield the same result".  I believe that this guarantee can be expressed by saying that it does not read or write memory, does not have SSA effects, doesn't do any of the other things (control-dependent, side-exit, terminal).  My goal with CSE was to have pure CSE rules on calls and patchpoints that do this.

So, can we please roll this out?