Re: Nop insertion

Chris F Clark <cfc@shell01.TheWorld.com>
Wed, 28 Oct 2009 12:49:46 -0400

          From comp.compilers

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From: Chris F Clark <cfc@shell01.TheWorld.com>
Newsgroups: comp.compilers
Date: Wed, 28 Oct 2009 12:49:46 -0400
Organization: The World Public Access UNIX, Brookline, MA
References: 09-10-032
Keywords: architecture
Posted-Date: 30 Oct 2009 11:53:54 EDT

As our moderator says, it's normally done at assembly time, but that's
because the number of stall cycles is usually small (1-3 instructions)
and localized. Large stalls that are non-localized are not usually
handled by inserting fixed numbers of noops, especially not ones
crossing basic block (or whatever the coding unit is) boundary. I
have heard large precise timing models called "crystal clock"
architectures, refering to the fragiility of a clock made out of
glass. They are not robust, even with automated tool support.


At Intel, we have a crypto multiplier unit we put on certain chips and
it has a vary long latency, but we don't try to time it exactly even
though for any given multiplication there is a precise number of
cycles the multiplication will take. Doing so would only work for
very simple fixed architectures. The more out-of-order one allows,
the more variable the interactions can make the timing.


Thus, semaphores (and other locks) are good things. Use judiciously,
but use them.


Hope this helps,
-Chris


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Chris Clark email: christopher.f.clark@compiler-resources.com
Compiler Resources, Inc. Web Site: http://world.std.com/~compres
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Berlin, MA 01503 USA twitter: @intel_chris


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