DR. SIMING LIU SPRING 2016 COMPUTER SCIENCE AND ENGINEERING UNIVERSITY OF NEVADA, RENO Session 11 Conditional Operations

Chapter 2 — Instructions: Language of the Computer — 2 Conditional Operations Branch to a labeled instruction if a condition is true  Otherwise, continue sequentially beq rs, rt, L1  if (rs == rt) branch to instruction labeled L1; bne rs, rt, L1  if (rs != rt) branch to instruction labeled L1; j L1  unconditional jump to instruction labeled L1 §2.7 Instructions for Making Decisions

Chapter 2 — Instructions: Language of the Computer — 3 Compiling If Statements C code: if (i==j) f = g+h; else f = g-h;  f, g, h, i, j in $s0, $s1, $s2, $s3, $s4 Compiled MIPS code: bne $s3, $s4, Else add $s0, $s1, $s2 j Exit Else: sub $s0, $s1, $s2 Exit: … Assembler calculates addresses

Chapter 2 — Instructions: Language of the Computer — 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, $s3, 1 j Loop Exit: … #$t1=4*i #$t1=&save[i] #$t0=save[i] #check save[i]==k #i++

Chapter 2 — Instructions: Language of the Computer — 5 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

MIPS Control Flow Instructions MIPS conditional branch instructions  Ex:  Instruction Format (I format): bne $s0, $s1, L1 #go to L1 if $s0≠$s1 beq $s0, $s1, L1 #go to L1 if $s0=$s1 If (i==j) h=i+j bne $s0, $s1, L1 add $s3, $s0, $s1 L1: … 0x bit offset

Specifying Branch Destinations Use a register (like in lw and sw) added to the 16-bit offset  Which register? Instruction Address Register (the PC)  Its use is automatically implied by instruction  PC gets updated (PC+4) during the fetch cycle so that it holds the address of the next instruction  Limits the branch distance to to (word) instructions from the (instruction after the) branch instruction. But most branches are local anyway  to (word) => to (byte)

Chapter 2 — Instructions: Language of the Computer — 8 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

More Branch Instructions Can use slt, beq, bne and the fixed value of 0 in regitster $zero to create other conditions  Less than  Less than or equal to  Greater than  Greater than or equal to Such branches are included in the instruction set as pseudo-instructions. These are recognized and expanded) by the assembler  Its why the assembler needs a reserved register ($at) blt $s1, $s2, Label slt $at, $s1, $s2 #$at set to 1 if bne $at, $zero, Label #$s1<$s2 ble $s1, $s2, Label bgt $s1, $s2, Label bge $s1, $s2, Label

Chapter 2 — Instructions: Language of the Computer — 10 Signed vs. Unsigned Signed comparison: slt, slti Unsigned comparison: sltu, sltui Example  $s0 =  $s1 =  slt $t0, $s0, $s1 # signed  –1 < +1  $t0 = 1  sltu $t0, $s0, $s1 # unsigned  +4,294,967,295 > +1  $t0 = 0

Chapter 2 — Instructions: Language of the Computer — 11 Branch Instruction Design Why not blt, bge, etc? Hardware for <, ≥, … slower than =, ≠  Combining with branch involves more work per instruction, requiring a slower clock  All instructions penalized! beq and bne are the common case This is a good design compromise

Bounds Check Shortcut Treating signed numbers as if they were unsigned gives a low cost way of checking if 0≤x<y (index out of bounds for arrays) The key is that negative integers in 2’s complement look like large numbers in unsigned notation. Thus, an unsigned comparison of x<y also checks if x is negative as well as if x is less than y. sltu $t0, $s1, $s2 #$t0 set to 1 if beq $t0, $zero, IOOB #go to IOOB if $t0=0 #$s1 > $s2 (max) #or $s1 <0 (min)