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From: fy@lucid.com (Frank Yellin)
Newsgroups: comp.sys.mips
Subject: Re: Desperately Seeking Bltzal and Bgezal.
Summary: Lisp's use of bgezal.  Avoiding standing on your head.
Keywords: bltzal bgezal
Message-ID: <2281@heavens-gate.lucid.com>
Date: 7 Sep 90 21:54:37 GMT
References: <1179@cluster.cs.su.oz> <2509@rossignol.Princeton.EDU>
Organization: Lucid, Inc. Menlo Park, CA
Lines: 44

In article <2509@rossignol.Princeton.EDU>, appel@cs.Princeton.EDU (Andrew Appel) writes:
> The "Standard ML of New Jersey" compiler generates position-independent 
> code (useful, because code is moved around by the garbage collector).
> To do this, it is sometimes necessary to get the PC into a register.
> A failing bltzal (BLTZAL r0,anywhere) is a convenient way to do this.

Lucid Common Lisp has a similar problem.  To handle catch frames and 
unwind-protect code, we need to store the address of code on 
the stack.   The easiest way to do this is

         bgezal %0, label       ; always branch and link
         move %31, <stack location>
         <code to deal with unwind-protect or catch frame>
      label
         <continuation of normal code>

It seems, though, that no one has yet come up with a case in which bgezal
or bltzal is ever used with any register other than %0, so that the branch
is either always taken or never taken, respectively.

> On the SPARC, which has no such instruction, we must stand on our heads
> to do it.

Sure it does:

     my-address
        call label      ;; go to label.  Put address of my-address into %o7
        <some delay slot instruction>
        ......
     label      

Any place in the code can now be reached by jumping to an offset from %o7

There doesn't have to be any code between the delay slot instruction and
the label.  And unlike the MIPS, the delay slot instruction can even be a
branch, since the preceding instruction is an unconditional branch.

-- Frank Yellin
   fy@lucid.com
   ...!decwrl!edsel!fy
   ...!sun!edsel!fy
-- 

-- Frank