On Friday, 13 April 2012 at 13:54:39 UTC, Manu wrote:
No other processors have branch prediction units anywhere near
the sophistication of modern x86. Any call through a function
pointer
stalls the pipeline, pipelines are getting longer all the time,
and PPC has
even more associated costs/hazards.
Most processors can only perform trivial binary branch
prediction around an
'if'.
It also places burden on the icache (unable to prefetch), and
of course the
dcache, both of which are much less sophisticated than x86
aswell.
Allocation of small aggregated objects usually involves
allocation of several equally small objects of different types in
a row, so they sit one after another in heap and gc will visit
them in a row every time calling function different from the
previous time, so to x86 processor it would result in constant
misprediction: AFAIK x86 processor caches only one target address
per branch (ARM caches a flag?). And icache should not suffer in
both cases: once you prefetched the function, it will remain in
the icache and be reused from there the next time.
