Code Optimization Witawas Srisa-an CSCE 496: Embedded Systems Design and Implementation

Agenda Talk about possible exam ideas Code optimization techniques –Not everyone has reconfigurable processors! Credits –Most of slides in this lecture are based on slides created by Profs. Raj Rajkumar and Professor Priya Narasimhan from ECE Dept at Carnegie Mellon

Exam Ideas

Code Optimization Programmers can improve program performance by writing better code –Improve data structure and/or algorithms Merge vs. bubble sorts –Reorganize code or provide flags to help compilers –Last option is to write in assembly

Better Algorithms Merge vs. bubble sorts –Which one runs faster? –Which one causes more cache misses?

Common Optimization Techniques Sub-expression elimination Dead code elimination Induction variables Strength reduction Loop unrolling In-lining

Common Techniques (cont.) Sub-expression elimination myfunction: index1 = 8 * i x = a [index1] temp = 8 * i index2 = 4 * j t = a[index2] a[temp] = t temp2 = 4 * j a[temp2] = x goto myfunction

Common Techniques (cont.) Dead code elimination int i = 0; i = i + 1; if (i == 0) j = j * 8; else j = j * 10; use ASSERT and #ifdef to advice the compiler about deadcode

Common Techniques (cont.) Induction variables and strength reduction i = 0 j = 0 label j = j + 1 i = 4 * j a[i * 2] = b [i] if (i < 1000) goto label

Optimization Techniques (cont.) In-lining main: addi $s0, $t1, 0 addi $s1, $t2, 0 jal mult add $t3, $v0, 0 mult: addi $sp, $sp -12 sw $s1, 4($sp) sw $s0, 8($sp) sw $ra, 12($sp) sll $v0, $s0, $s1 lw $s1, 4($sp) lw $s0, 8($sp) lw $ra, 12($sp) addi $sp, $sp, 12 jr $ra What’s wrong with this picture?

Optimization Techniques (cont.) Loop unrolling –Eliminate branches (why?)

Architecture Dependent Optimizations X = Y * 64 Convert 8-bit RGB to 8-bit YCC Y = 0.299R G B Cb = R G B Cr = 0.500R G – 0.082B + 128

Architecture Dependent Optimizations (cont.) Address Register Register Bank Mem Addr RegisterWrite Data RegisterRead Data/Instr Reg RAM Addr Incrementer Barrel Shifter 32-bit ALU Dout[31:0] Data[31:0] ALU Bus B Bus A Bus Incrementer Bus Write Buffer (holds address and data)

Summary No magic bullet –optimizations sometimes don’t work –programmers need to help –various techniques that may require prior knowledge of the hardware