ECE232: Hardware Organization and Design

Transcription

1 ECE232: Hardware Organization and Design Lecture 5: MIPs Decision-Making Instructions Adapted from Computer Organization and Design, Patterson & Hennessy, UCB

2 Overview Computers need to make decisions Microprocessors support conditional operations Operations typically called branch operations Connections with common Java and C language constructs If, else, switch, for loops, do loops Conditional instructions have a specific format ECE232: Decision Making Instructions 2

3 Example: Compiling C if-then-else Example C Code if (i==j) f = g + h; else f = g - h; Assembly bne $s3, $s4, Else add $s0, $s1, $s2 j Exit; # new: unconditional jump Else: sub $s0, $s1, $s2 Exit: New Instruction: Unconditional jump j LABEL # goto Label ECE232: Decision Making Instructions 3

4 Compiling Loop Statements C code: while (save[i] == k) i += 1; i in $s3, k in $s5, address of save in $s6 Compiled MIPS code: Loop: sll $t1, $s3, 2 add $t1, $t1, $s6 lw $t0, 0($t1) bne $t0, $s5, Exit addi $s3, $s3, 1 j Loop Exit: ECE232: Decision Making Instructions 4

5 while statement while ( condition ) { statements } while_start_label: # MIPS code for the condition expression #(if condition satisfied set $t0=1) beq $t0, $zero, while_end_label # MIPS code for the statements j while_start_label while_end_label: ECE232: Decision Making Instructions 5

6 do-while statement do { statements } while ( condition ); do_start_label: # MIPS code for the statements do_cond_label: # MIPS code for the condition expression #(if condition satisfied set $t0=1) beq $t0, $zero, do_end_label j do_start_label do_end_label: ECE232: Decision Making Instructions 6

7 for loop for ( init ; condition ; incr ) { } statements for_start_label: for_end_label: # MIPS code for the init expression # MIPS code for the condition expression #(if condition satisfied set $t0=1) beq $t0, $zero, for_end_label # MIPS code for the statements # MIPS code for the incr expression j for_start_label ECE232: Decision Making Instructions 7

8 Basic Blocks A basic block is a sequence of instructions with No embedded branches (except at end) No branch targets (except at beginning) A compiler identifies basic blocks for optimization An advanced processor can accelerate execution of basic blocks ECE232: Decision Making Instructions 8

9 More Conditional Operations Set result to 1 if a condition is true Otherwise, set to 0 slt rd, rs, rt if (rs < rt) rd = 1; else rd = 0; slti rt, rs, constant if (rs < constant) rt = 1; else rt = 0; Use in combination with beq, bne slt $t0, $s1, $s2 # if ($s1 < $s2) bne $t0, $zero, L # branch to L ECE232: Decision Making Instructions 9

10 Comparisons - What about <,, >,? bne, beq provide equality comparison slt (set on less than) provides magnitude comparison slt $t0,$s3,$s4 # if $s3<$s4 $t0=1; # else $t0=0; Combine with bne or beq to branch: slt $t0,$s3,$s4 # if (a<b) condition register bne $t0,$zero,less # goto Less; Why not include a blt instruction in hardware? Supporting in hardware would lower performance Assembler provides this function if desired (by generating the two instructions) ECE232: Decision Making Instructions 10

11 switch statement switch ( expr ) { case const1: statement1 case const2: statement2... case constn: statementn default: default-statement } ECE232: Decision Making Instructions 11

12 MIPS code for switch statement # MIPS code for $t0=expr beq $t0, const1, switch_label_1 beq $t0, const2, switch_label_2... beq $t0, constn, switch_label_n j switch_default switch_label_1: # MIPS code to compute statement1 switch_label_2: # MIPS code to compute statement2... switch_default: # MIPS code to compute default-statement switch_end_label: ECE232: Decision Making Instructions 12

13 Switch Example switch (i) { //Assume i is in $s1 and j is in $s2; case 0: j = 3; break; case 1: j = 5; break; case 2: ; case 3: j = 11; break; case 4: j = 13; break; default: j = 17; } main: add $t0, $zero, $zero # $t0 = 0, temp. variable beq $t0, $s1, case0 # go to case0 addi $t0, $t0, 1 # $t0 = 1 beq $t0, $s1, case1 # go to case1 addi $t0, $t0, 1 # $t0 = 2 beq $t0, $s1, case2 # go to case2 addi $t0, $t0, 1 # $t0 = 3 beq $t0, $s1, case3 # go to case3 addi $t0, $t0, 1 # $t0 = 4 beq $t0, $s1, case4 # go to case4 j default # go to default case case0: addi $s2, $zero, 3 # j = 3 j finish # exit switch block ECE232: Decision Making Instructions 13

14 Unconditional branches Unconditional branch: j L1 jr $s5 (useful for large case statements and big jumps) ECE232: Decision Making Instructions 14

15 Example 2 Convert to assembly: while (save[i] == k) i += 1; i and k are in $s3 and $s5 and base of array save[] is in $s6 Loop: sll $t1, $s3, 2 add $t1, $t1, $s6 lw $t0, 0($t1) bne $t0, $s5, Exit addi $s3, $s3, 1 j Loop Exit: ECE232: Decision Making Instructions 15

16 SPIM Example switch (i) { case 0: j = 3; break; case 1: j = 5; break; case 2: ; case 3: j = 11; break; case 4: j = 13; break; default: j = 17; } //Assume i is in $s1 and j is in $s2; main: add $t0, $zero, $zero # $t0 = 0, temp. variable used in switch beq $t0, $s1, case0 # go to case0 addi $t0, $t0, 1 # $t0 = 1 beq $t0, $s1, case1 # go to case1 addi $t0, $t0, 1 # $t0 = 2 beq $t0, $s1, case2 # go to case2 addi $t0, $t0, 1 # $t0 = 3 beq $t0, $s1, case3 # go to case3 addi $t0, $t0, 1 # $t0 = 4 beq $t0, $s1, case4 # go to case4 j deflt # go to default case case0: addi $s2, $zero, 3 # j = 3 j fin # exit switch block ECE232: Decision Making Instructions 16

17 Summary Conditional operations affect program flow based on data values Microprocessor makes decision based on results of arithmetic and logic operation Know the difference between branch and jump Useful for C/Java loops, if-else, and switch statements ECE232: Decision Making Instructions 17