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From: flint@gistdev.UUCP
Newsgroups: comp.lang.c
Subject: Re: Short code to determine compiler's
Message-ID: <7800016@gistdev>
Date: 22 Jul 89 23:03:04 GMT
References: <396@uop.uop.EDU>
Lines: 19
Nf-ID: #R:uop.uop.EDU:396:gistdev:7800016:000:1109
Nf-From: gistdev.UUCP!flint    Jul 21 17:32:00 1989


I think you're gonna come back to "you have to look at the assembler
output." For one thing, you really can't know that the compiler hasn't
decided (possibly stupidly) to reserve a register for something else, such
as a stack pointer, which would make your count too low.  Some other things
nobody has taken into account here is that there are some architectures
with multiple levels of register performance: such as machines that have X
registers, and then a second (or more) bank(s) of X alternate registers,
with a fast instruction to exchange the contents of the registers with the
contents of the alternate registers.  A timing loop is likely to see 4
levels of speed: when using only the first bank of registers, when you have
to switch to the other bank and use a register, when you have to switch to
the other bank, use a register, and then switch back, and when you have to
use memory. 

Flint Pellett, Global Information Systems Technology, Inc.
1800 Woodfield Drive, Savoy, IL  61874     (217) 352-1165
INTERNET: flint%gistdev@uxc.cso.uiuc.edu
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