MP A,B
BZ STMT
....
________________________________
From: Programming <[email protected]> on behalf of Henry
Rich <[email protected]>
Sent: Monday, July 29, 2019 1:51:34 AM
To: [email protected] <[email protected]>
Subject: [Jprogramming] A bit-twisting puzzle
Many of the members of this Forum will remember the days of assembler
language
...and who could less than merry be
when writing out BXLE?
AND, OR, XOR were our meat. Those days are returning.
You have two integer variables a and b and you want to do something if
one or both of them are 0. In C, you might write
if(a==0 || b==0)stmt;
but that will generate the code
cmp a,0
bz stmt
cmp b,0
bnz notstmt
stmt:
...
notstmt:
Here's the problem: your machine is very slow at branch instructions.
Each branch takes 30 times as long as a regular instruction. (I am
describing a state-of-the-art Intel CPU when it cannot predict the
branches effectively, perhaps because the data is... unpredictable).
Obviously, you want to use only one branch instruction. You may use as
many arithmetic and logic instructions as you like, but only one
branch. The usual condition codes, ZNCV, are available. How tight can
you make the code?
Example: suppose the problem were to execute stmt if one or both of a
and b is NOT zero. Then you would simply write
or a,b
bnz notstmt
...
Checking for zero seems to be harder.
No hurry. I have pondered this problem for over a year, and just today
I found a solution I consider acceptable.
Henry Rich
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