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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.lang.c
Subject: Re: asm statements & the stack
Message-ID: <8743@utzoo.UUCP>
Date: Sat, 10-Oct-87 19:47:26 EDT
Article-I.D.: utzoo.8743
Posted: Sat Oct 10 19:47:26 1987
Date-Received: Sat, 10-Oct-87 19:47:26 EDT
References: <110@teletron.UUCP>, <7426@e.ms.uky.edu>
Organization: U of Toronto Zoology
Lines: 21

> The method of keeping the stack pointer pointing at an unused location is
> quite common, as I have used many unix compilers which do this...

Yes, people trying to work in assembler around C compilers should be aware
that this is a standard tactic.  There are three reasons for it.  First,
moving a value to an already-vacant stack-top location is quicker than
pushing it onto the stack in many machines.  Second, it becomes unnecessary
to remove the argument from the stack after calling a one-argument function.
And third, on some machines it is easier to remove one or two words from
the stack than to remove a more general number, and these efficient removal
sequences then apply to a larger fraction of all functions.

The first and second reasons also apply to using the stacktop location as
an expression-evaluation temporary.

This tactic does require careful attention to situations like function calls
within function argument lists, but it is a cheap and effective optimization
if done right.
-- 
"Mir" means "peace", as in           |  Henry Spencer @ U of Toronto Zoology
"the war is over; we've won".        | {allegra,ihnp4,decvax,utai}!utzoo!henry