[Toybox] [CLEANUP] paste

[Rob Landley]

Description of the paste cleanup that accidentally got checked in  
during commit 944 because hg import -f has side effects.

   http://landley.net/hg/toybox/rev/944

(Yes, that commit description is actively ironic. Scroll down to  
patch.c.)

Minor tweaks to the first loop: if -d isn't set then TT.delim will be  
NULL, and as long as that's the value we're playing with anyway the  
optimizer tends to do better if it can load that into a register and  
perform several operations on it, rather than load/mask/test an  
otherwise unrelated integer. So yank the test and instead start the  
loop with p = TT.delim ? TT.delim : "\t"; (In theory gcc offers an "x ?  
: y;" syntax where the blank between ? : substitutes in the result of  
the test again, so I _could_ say p = TT.delim ? : "\t";. In practice,  
c99 never picked that one up so I feel uncomfortable using it.)

I was on the fence about moving the assignment from the else case into  
the if, but did it anyway, so:

-    if (*p == '\\') {
-    } else *buf = *p;

becomes:

+  if ((*buf = *p) == '\\') {
+  }

This means we can load *p into a register once and then perform both  
the assignment and the test from that register. The initial write only  
goes into L1 cache and if it gets overwritten a couple lines later we  
only have one write to main memory. In general modern processors have a  
better time eliding writes than predicting branches. A lot of stuff  
like:

   if (x) y = 1;
   else y = 2;

Is going to get rewritten by the optimizer into:

   y = 2;
   if (x) y = 1;

So the "if (x)" can become a conditional assignment instruction instead  
of a branch instruction. (This avoids bubbles in the instruction  
pipeline due to branch mispredicts, and/or wasted work from speculative  
execution on mispredicted branches eating your battery power and then  
being discarded.)

I was on the fence because this is phrasing easier for the computer to  
do, and less repetitive (only one mention of *p instead of 2), but  
slightly less easy to read.

Turned the p++ on its own line into a ++p in the stridx.

Changed the error message to say "bad -d" instead of "bad delimiter" so  
it's one less word to translate to other languages.

Moved the "// Sequential" comment from on the same line to the line  
before. I know it uses more vertical screen space and I ordinarily try  
to conserve that, but mixing comment and code on the same line is  
something I only really do describing data values. (Describing  
structure members, table entries, or magic constants when a symbol name  
is not conveniently available in a header.) This comment is more "what  
does this block of code do", and that I prefer to put on its own line.

This loop is what loopfiles() is for. That handles the aliasing of "-"  
with stdin and everything.

(No, I'm probably not done cleaning up paste, this was just my initial  
pass that got accidentally checked in.)

Rob
 1374218153.0