1228579 - BacktrackingAllocator: computeSpillWeight does a lot of L1 cache misses.

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, defect)

Description

[Nicolas B. Pierron [:nbp]]

I profiled the BacktrackingAllocator within callgrind with almost everything enabled.  The result highlighted that BacktrackingAllocator::computeSpillWeight does a lot of L1 cache misses (37% of GenerateLIR cache misses), including one line which represents 72% of the cache misses of this function.

Which means that this one line is accountable for ~27% of the L1 cache misses of GenerateLIR.  More over, we approximately use 14 bytes (because multiples structures might be adjacent) per cache line.

We should figure a different way of storing/computing data such that we improve our cache usage in BacktrackingAllocator::computeSpillWeight.

Comment 1

[Nicolas B. Pierron [:nbp]]

Attachment: BacktrackingAllocator: Pack the LUse::Policy next to the LUse pointer of UsePosition.

This patch removes 25.5% of the L1 cache misses under GenerateLIR. This
patch avoid loading new cache lines by making a copy of the LUse::Policy on
the UsePosition structure.  Thus we no longer have to access the content of
the LUse pointer, only for reading the policy bits out of it.

This patch removed a bit more than half of the cache misses of the
computeSpillWeight function, the remaining cache misses are caused by random
accesses of the UsePosition structures.  This is another issue which should
probably be solved by improving the data locality of the UsePosition list elements.

Comment 2

[Nicolas B. Pierron [:nbp]]

Comment on attachment 8693775 [details] [diff] [review]
BacktrackingAllocator: Pack the LUse::Policy next to the LUse pointer of UsePosition.

(works better with a reviewer)

Comment 3

[Brian Hackett [Laid off!]]

Comment on attachment 8693775 [details] [diff] [review]
BacktrackingAllocator: Pack the LUse::Policy next to the LUse pointer of UsePosition.

Review of attachment 8693775 [details] [diff] [review]:
-----------------------------------------------------------------

Cool!

::: js/src/jit/BacktrackingAllocator.h
@@ +148,4 @@
>      CodePosition pos;
>  
> +    LUse* use() const {
> +        // printf("UsePosition::use: %p %p %x\n", this, use_, use_->policy());

rm

Comment 4

[Pulsebot]

https://hg.mozilla.org/integration/mozilla-inbound/rev/66921f340b4c

Comment 5

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/66921f340b4c