Both the ARM processor and the IA64 architecture have predicated instruction execution. In what way is the IA64’s predication mechanism superior to the ARM processor’s?

What will be an ideal response?

The ARM processors have a very basic predicated execution mode. The current instruction execution can be
predicated on the condition code register; for example



CMP r1,r2 ;compare r1 and r2

ADDEQ r1,r1,#1 ;if equal r1 = r1 + 1

ADDLT r1,r1,#2 ;if less than r1 = r1 + 2

ADDGT r1,r1,#3 ;if greater than r1 = r1 + 3

This code uses a test and then three predicated instructions to implement a simple three?way if construct
without branch operations. Furthermore, we can carry out several operations for each test; for example

CMP r1,r2 ;compare r1 and r2
ADDEQ r1,r1,#1 ;if equal r1 = r1 + 1
MULEQ r4,r1,r2 ;if equal r4 = r1 × r2 (two operations predicated on equality)
ADDLT r1,r1,#2 ;if less than r1 = r1 + 2

The Itanium IA64 architecture is a VLIW processor and the concept of a condition code register becomes
meaningless when instructions are carried out in parallel. The Itanium has 64 predication registers p0 to p63.
Operations can be carried out using several predicate registers; for example,

(p1) add r1 = r1,1 //
(p2) add r2 = r3,r4 //
(p3) add r6 = r7,r4 //

Here we can perform three predicated operations using three different predicate registers that have been
generated by previous operations. Of course, these instructions can be executed in parallel.
The following example from the text demonstrates how Itanium’s predication can be used in a more
sophisticated way to implement complicated logical constructs. Consider the following where three
comparisons are made in the same cycle using the same predicate register with the OR completer. If any one of
them is true, the predicate is true.



if ((a == 0)||(b == 1)||(c =! 2)) {x = 5;}

cmp.ne p1 = r0,r0;; //trick code to set p1 = 0

cmp.eq.or p1 = 0,r1 //do a == 0 (set p1 if true)

cmp.eq.or p1 = 1,r2 //do b == 1 (set p1 if true)

cmp.ne.or p1 = 2,r3;; //do c =! 2 (set p1 if true)

(p1) mov r4 = 5 //if P1 true then r4 = 5