On 30/04/2016 12:23 AM, Windt, W.K.F. van der (Fred) wrote:
Sent from my new iPad
On 29 Apr 2016, at 18:10, Peter Relson <[email protected]> wrote:
Since the origin for the starting post apparently lay in branching around
the eyecatcher (which really is not necessarily at all the same as a
branch in a 2 instruction loop), I was surprised that none of the posts
that I glanced at mentioned Instruction-cache misses.
Just because you think something is high frequency does not mean that the
operating system or the machine agrees with you. If the module's first
instruction is not in I-cache, then whether that first instruction is a
branch or anything else, it will show up as a lot hotter than a somewhat
similar instruction that is in the I-cache.
That's an interesting point: does this mean that every instruction at the start
of an 256 byte boundary will probably appear hotter because it sits at the
start of a cache line and profilers will attribute the time it takes to load
the line into cache to this instruction? The other instructions in the cached
line quietly benefit from this behavior.
That's a good question. Is there a doubt that profilers are unreliable?
Jim Mulder gave a good explanation where that is certainly the case in a
previous post.
You could probably validate this by inserting NOPS in the code. The hotspots
should shift by the length of the NOP.
And the entry point of a module might be more likely to sit on a 256 byte
boundary if it is also at the very start of the code of that module...
Very interesting.
Fred!
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