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From: sundar@cwruecmp.UUCP
Newsgroups: comp.arch
Subject: Re: subroutine frequency
Message-ID: <1859@cwruecmp.UUCP>
Date: Sat, 31-Jan-87 23:19:21 EST
Article-I.D.: cwruecmp.1859
Posted: Sat Jan 31 23:19:21 1987
Date-Received: Sun, 1-Feb-87 07:41:59 EST
References: <1881@homxc.UUCP> <898@moscom.UUCP> <476@mntgfx.MENTOR.COM>
Reply-To: sundar@cwruecmp.UUCP (Sundar Iyengar)
Organization: CWRU Dept. of Computer Engineering, Cleveland, Ohio
Lines: 19
Keywords: register stack frame variable

do compilers take the time to keep track of which registers have 
been used and only save the 'dirty' ones or do most call and
return mechanisms save the entire register set on the stack?
  It depends on the architecture and the compiler, the three easy ways
  to do it are
  a) save all register
  b) have the caller save only the registers it is using
  c) have the callee save only the registers it will use
  pdp-11's use "a" since they only have 3 register variables anyway.  Most
  68k compilers use "c" since you get about 12 register variables.  I don't
  know of anyone who uses "b" but it should be about as efficient as "c".

    I am just curious.  Isn't it a security hole to use the method c)
    above?  If the caller is a system routine and the callee is my program
    and I am supposed to save and restore registers that I intend to use,
    I can have some fun by not saving the registers in the first place and
    in addition destroying them. 

sri