Date: Sun, 4 Jan 1998 14:28:57 -0800 (PST)
Message-Id: <199801042228.OAA26965@adit.ap.net>
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To: GAMMELJL AT SLU DOT EDU, djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: code which won't compile with -O2 on
Precedence: bulk

At 07:23  1/3/1998 -0600, GAMMELJL AT SLU DOT EDU wrote:
>     I do not understand what you mean by ri.  I assume that you mean
>r1,r2,etc.  So I write the input line:
>       : "r0" (z), "r1" (x), "r2" (y)   \
>and leave the output line blank ( : \ ).  
>     The compilation and running goes ok except the compiler issues
>warnings "operand 0 has constraint 0", "operand 1 has constraints 1", etc.
Are you sure? "ri" works for me. It tells the compiler to either place the
value in a register ("r"), or to generate an immediate "i"nteger constant.
Do not use the numbers.
>     While the compiler does not recognize ri, it does recognize rm which
>seems to be very similar to g.  If I use all g's (not followed by a numeral)
>the compiler issues "inconsistent constraints".
"rm" is exactly the same as "g". It places the value in either a "r"egister
or in "m"emory.
>     It is quite interesting that you mentioned %%ebx(,%%ecx,4) because
>that is exactly what I would like to do, namely, get a register for a 
>pointer.  Brennan mentions that immed32(basepointer,indexpointer,indexscale)
>and the formula
>            address=immediate32+basepointer+indexpointer+indexscale 
>go together.
The indexscale is *multiplied* by indexpointer, not added. Was that a typo?
>     Now, I suppose ebp can contain a basepointer--can it?  Suppose
>immediate32=_array--is it? I also suppose that the (, in the form
>_array(,%%ecx,4) implies that basepointer=0.  Here comes the question:
>If there were a null pointer so that immediate32=0, and if one could set
>basepointer=_array, then maybe (just maybe), one could write
>            0(%%ebp,%%ecx,4)
>with "ebp" (array) on the input line.  So one would have the x in my
>example in a register (ebp). 
>     That might really speed up some codes, but is it obvious nonsense?
No, that's perfectly reasonable. The 0 is superfluous, in fact. (Should be
$0, anyway). Also, you can use any register you want for the base pointer,
not just %ebp. (It may be difficult to use, since GCC uses it as a frame
pointer.)

Nate Eldredge
eldredge AT ap DOT net