Xref: news2.mv.net comp.os.msdos.djgpp:2142
From: brennan AT mack DOT rt66 DOT com (Brennan "Mr. Wacko" Underwood)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Is this ASM ok?
Date: 24 Mar 1996 17:42:47 -0700
Organization: None, eh?
Lines: 44
Message-ID: <4j4q67$h4b@mack.rt66.com>
References: <3150B8B2 DOT 8B2 AT jeffnet DOT org> <4ishog$obf AT mack DOT rt66 DOT com> <4ius89$76n AT agate DOT berkeley DOT edu>
NNTP-Posting-Host: mack.rt66.com
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <4ius89$76n AT agate DOT berkeley DOT edu>,
Eric J. Korpela <korpela AT islay DOT ssl DOT berkeley DOT edu> wrote:
>In article <4ishog$obf AT mack DOT rt66 DOT com>,
>Brennan "Mr. Wacko" Underwood <brennan AT mack DOT rt66 DOT com> wrote:
>>asm(
>>  "loopf:\n\t"
>>  "movb %%dh, (%%edi)\n\t"	/* write integer portion of color - U1 */
>>  "addl %%ebx, %%edx\n\t"       /* Add colstep to col             - V1 */
>>  "incl %%edi\n\t"		/* increment edi                  - U2 */
>>  "decl %ecx\n\t"		/* decrement count                - V2 */
>>  "jnz loopf"     	        /* If ecx not zero goto loop      - U3 */
>>  : : "D" (tscreen), "d" (tcol1), "b" (tcolstep), "c" (tlength)/*loaded regs*/
>>  : "%edi", "%ebx", "%ecx", "%edx"); /* clobbered regs */
>
>I think there is a problem with your clobbered register list.  Because 
>you have already specified the loaded registers gcc already knows that
>these registers will be modified.  Ordinarily I wouldn't think that
>would be a problem, but the GCC info pages state that "The input
>operands are guaranteed not to use any of the clobbered registers."
>There's clearly a contradiction here, and I would hope GCC flags
>this as an error, or at least gives a warning.

Nope, I found out the hard way, that if you tell GCC that x is in eax,
it expects it to still be there after your asm completes, unless you
tell it it's been clobbered. This is a Good Thing, actually, because
it can save a register load if it needs it later (and you didn't
clobber it.) It's part of how it optimizes. (You can put (x+1) in there,
and afterwards, if you respecify (x+1), it'll remember that it's already
got it in a register.)

Oh, I see what you mean about the Info pages. That refers to this register
loading method: "q" (tscreen) then use %0 for the register name, in case
GCC was already using eax or whatever you would have picked. Or something
to that effect. I haven't been able to get that to be useful yet, I just
specify the registers.

I did notice after I posted this that I specified ebx as clobbered when
it isn't, but it wouldn't break anything.


--brennan
I love the word clobber.
-- 
brennan AT rt66 DOT com  |  Do what you'll wish you had done.