Page 139 - A Practical Guide from Design Planning to Manufacturing

112 Chapter Four

TABLE 4-14 Control Flow Relative Frequency
Instruction Integer programs Floating-point programs
Branch 75% 82%
Jump 6% 10%
Call & return 19% 8%

Unconditional jumps always direct execution to a new point in the pro-
gram. Conditional jumps, also called branches, redirect or not based on
defined conditions. The same subroutines may be needed by many dif-
ferent parts of a program. To make it easy to transfer control and then
later resume execution at the same point, most architectures define call
and return instructions. A call instruction saves temporary values and
the instruction pointer (IP), which points to next instruction address,
before transferring control. The return instruction uses this informa-
tion to continue execution at the instruction after the call, with the same
architectural state. When requesting services of the operating system,
the program needs to transfer control to a subroutine that is part of the
operating system. An interrupt instruction allows this without requiring
the program to be aware of the location of the needed subroutine.
The distribution of control flow instructions measured on the SpecInt
2000 and SpecFP2000 benchmarks for the DEC Alpha architecture is
6
shown in Table 4-14. Branches are by far the most common control flow
instruction and therefore the most important for performance.
The performance of a branch is affected by how it determines whether
it will be taken or not. Branches must have a way of explicitly or implic-
itly specifying what value is to be tested in order to decide the outcome
of the branch. The most common methods of evaluating branch condi-
tions are shown in Table 4-15.
Many architectures provide an implicit condition code register that con-
tains flags specifying important information about the most recently
calculated result. Typical flags would show whether the results were
positive or negative, zero, an overflow, or other conditions. By having all
computation instructions set the condition codes based on their result,
the comparison needed for a branch is often performed automatically. If
needed, an explicit compare instruction is used to set the condition codes
based on the comparison. The disadvantage of condition codes is they
make reordering of instructions for better performance more difficult
because every branch now depends upon the value of the condition codes.
Allowing branches to explicitly specify a condition register makes
reordering easier since different branches test different registers.

6
Hennessy and Patterson, p. 113.