Conditional Execution

Tom Kelliher, CS 220

Dec. 8, 2003

Administrivia

Announcements

Assignment

Nothing new.

From Last Time

Operands and instruction formats.

Outline

1. Immediate operands.

2. Branch and jump instructions.

3. Compiling HLL control structures.

4. Class teamwork assignment.

Coming Up

Procedure invocation I.

Immediate Operands

1. Operand types (addressing modes) we've seen so far: registers, memory.

2. What about constants? Where have we already seen immediates? Arithmetic example:

            addi $t0, $s0, 8   # An immediate operand.
   

   Why no subi?

3. Immediate operand: found within the instruction itself.

4. Small immediates occur frequently, so...

5. Design principle 4: Make the common case fast.

6. But, how do I load a 32-bit immediate? lui followed by addi (whoops, sign extension) or ori:

            lui $s0, 0x5555
            ori $s0, $s0, 0xaaaa
   

7. How does the assembler manufacture 32-bit immediates for us? Register $at.

8. Programmer conventions. ``Enhanced'' assembly language. Simplicity.

Branch and Jump Instructions

1. I-format instructions.

2. The idea behind a branch or jump:

   

3. Branch forward or backward words.

The complete set, all synthesized from beq, bne, and slt.

Branch instructions use a signed 16-bit offset field; hence they can jump instructions (not bytes) forward or instructions backwards. The jump instruction contains a 26 bit address field (the third instruction format).

b label instruction
Unconditionally branch to the instruction at the label.

beq Rsrc1, Src2, label on Equal
Conditionally branch to the instruction at the label if the contents of register Rsrc1 equals Src2.

beqz Rsrc, label on Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc equals 0.

bge Rsrc1, Src2, label on Greater Than Equal
bgeu Rsrc1, Src2, label on GTE Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than or equal to Src2.

bgez Rsrc, label on Greater Than Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are greater than or equal to 0.

bgt Rsrc1, Src2, label on Greater Than
bgtu Rsrc1, Src2, label on Greater Than Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than Src2.

bgtz Rsrc, label on Greater Than Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are greater than 0.

ble Rsrc1, Src2, label on Less Than Equal
bleu Rsrc1, Src2, label on LTE Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than or equal to Src2.

blez Rsrc, label on Less Than Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are less than or equal to 0.

blt Rsrc1, Src2, label on Less Than
bltu Rsrc1, Src2, label on Less Than Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than Src2.

bltz Rsrc, label on Less Than Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are less than 0.

bne Rsrc1, Src2, label on Not Equal
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are not equal to Src2.

bnez Rsrc, label on Not Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are not equal to 0.

j label
Unconditionally jump to the instruction at the label.

jal label and Link
jalr Rsrc and Link Register
Unconditionally jump to the instruction at the label or whose address is in register Rsrc. Save the address of the next instruction in register 31.

jr Rsrc Register
Unconditionally jump to the instruction whose address is in register Rsrc.

Compiling HLL Control Structures

Write MIPS code fragments corresponding to the following:

1. Compiling an if:

   

   if (i < 12)
      ++i;
   else
      --j;
   

2. Compiling a loop:

   

   i = 1;
   j = 0;
   while (i < 200)
   {
      j += i;
      i *= i;
   }
   

Class Teamwork Assignment

Working in groups of 2--3, solve as many of the following as possible. Turn in your solutions.

1. j = 0;
   for (i = 0; i < 10; ++i)
      j += i;
   

2. j = 0;
   for (i = 0; i < 10; ++i)
      if (i > 5)
         j += i;
   

3. while (i > 0 && i < 10)
      ++i;
   

4. if (i < 12 && j > 3 || k != 0)
      ++i;
   else if (i == 33)
      --j;
   else
      k += 2;
   

5. (3.9 from the text) The naive way of compiling

   while (save[i] == k)
      i += k;
   

   requires execution of both a conditional branch and an unconditional jump each time through the loop. Produce the naive code.

   Optimize the naive code so that only a conditional branch is executed each time through the loop.

6. (3.24 from the text, a variation) Write a code segment which takes two ``parameters:''
   1. An ASCII character in $a0.

   2. A pointer to a NULL-terminated string in $a1.

   and ``returns'' a count of the number of occurrences of the character in the string in $v0.