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On 10-Apr-06, at 7:09 AM, Ronald G Minnich wrote:
There's lots of other bad stuff that happened too. Linker-sets, for
example. Or, oh, the stuff like:
if (likely(a != b)) {eatme();}
Although I'm willing to claim that branch prediction hints do belong
somewhere, although probably in only in assembly.
One processor I'm dealing with has something like a 19 cycle branch
miss penalty, on a machine where it's memory is only 7 cycles away,
and with 4-way SIMD-in-a-word stuff. You can put your entire loop
into those 19 cycles, and the compiler had better get the (explicit,
coded, not predicted) branch hint right. I have a piece of TLB-like
code that likewise needs the common case to be detected for "free"
for efficiency (12 cycle lookup for a hit, >2000 for a miss), and the
compiler does get it wrong, there being no loop involved.
Yuck.
What's the right high-level way to deal with branch prediction? I'm
guessing there isn't one...
Worst quote from Sony: "Think in assembler, but code in C++"
Paul
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