Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!pyramid!prls!mips!earl@wright.mips.com
From: earl@wright.mips.com (Earl Killian)
Newsgroups: comp.arch
Subject: Re: Procedure Call Protocol
Message-ID: <8522@wright.mips.COM>
Date: 19 Nov 88 20:19:20 GMT
References: <3300037@m.cs.uiuc.edu> <5938@killer.DALLAS.TX.US> <7580@aw.sei.cmu.edu> <3926@omepd>
Sender: earl@mips.COM
Organization: MIPS Computer Systems, Sunnyvale CA
Lines: 33
In-reply-to: mcg@omepd (Steven McGeady)

In article <3926@omepd>, mcg@omepd (Steven McGeady) writes:
>Mr. Firth wisely neglects to count in his 12 cycle cost the cost of saving
>procedure-local registers to the stack.  Typical C programs use between 4 and
>12 procedure-local variables (variables local to the procedure which are not
>referenced by enclosed scopes and thus can be allocated permanantly to
>registers).  This adds between 4 and 12 additional memory references at
>call and return in either the calling site or the called site (depending on
>whether you are using caller or callee save or some combination thereof).
>If you (charitably) consider the average to be 6, and (charitably) assume
>that Mr. Firth is running from zero-wait-state cache and that he gets no cache
>misses, and (charitably) assume that he can fill the delay slots on three
>of these stores, you are left with an additional 18 clocks, making Mr. Firth's
>"12 cycle" (unoptimized) call/return into a 30 cycle call/return.

Rather than "(charitably) considering the average to be 6", it would
be better to measure it.  I took 15 programs, sorted them by number of
registers dynamically saved/restored, and picked the median.  Here is
its register save/restore statistics:

Callee registers saved/restored at entry/exit:
   N      entry entry%   cum%  regs%   cum% instr%   cum%
   0     229298  11.8%  11.8%   0.0%   0.0%   0.0%   0.0%
   1    1698070  87.0%  98.8%  95.3%  95.3%   3.3%   3.3%
   2       6636   0.3%  99.1%   0.7%  96.1%   0.0%   3.3%
   3      10637   0.5%  99.7%   1.8%  97.9%   0.1%   3.4%
   4       2648   0.1%  99.8%   0.6%  98.5%   0.0%   3.4%
   6         91   0.0%  99.8%   0.0%  98.5%   0.0%   3.4%
   7       3854   0.2% 100.0%   1.5% 100.0%   0.1%   3.5%
  10          1   0.0% 100.0%   0.0% 100.0%   0.0%   3.5%
Total procedure entries: 1951235
Total register save/restore instructions: 1951235 (3.5%)
Average registers saved per entry: 0.9
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