Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!snorkelwacker!apple!mips!fred
From: fred@mips.COM (Fred Chow)
Newsgroups: comp.sys.mips
Subject: Re: Can some one explain this compiler output?
Message-ID: <31346@batman.mips.COM>
Date: 14 Nov 89 00:56:25 GMT
References: <HAM.89Nov11161238@Neon.Stanford.EDU>
Reply-To: fred@mips.COM (Fred Chow)
Distribution: comp
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 27

In article <HAM.89Nov11161238@Neon.Stanford.EDU> ham@Neon.Stanford.EDU (Peter R. Ham) writes:
>Question: Why does cc allocate 24 bytes of stack for a non-leaf procedure
>with no local variables when calling another procedure that accepts no
>arguments?
>
>Given this C code:
>
>extern void bar();
>
>void
>foo()
>{
>  bar();
>}

	For a non-leaf, 4 words are always allocated as argument build area
for callees.  This is done even if the callee has 0 parameter because we
always pass the first four parameters in registers 4 thru 7.  And "bar" can
be a function with variable number of parameters (vararg).  Such functions 
will always home registers 4 thru 7 to the incoming argument build area and 
then access the parameters thru a pointer.  So the space for 4 parameters
is always allocated.  As you can see in the code, another word is allocated
to store the return address in r31.  Altogether 6 words are allocated
because the stack pointer always points to a double word boundary.

---------
Fred Chow (fred@mips.com)