Control: Condition codes
Processor State
-
Information about currently executing program
- Temporary data
- Location of runtime stack
- Location of current code control point
- Status of recent tests
Condition Codes (Implicit Setting)
-
Single bit registers
- CF, SF, ZF, OF
-
Implicitly set by arithmetic operations
-
Not set by leaq instruction
Condition Codes (Explicit Setting)
-
Explicit Setting by Compare Instruction
- cmpq b,a like computing a-b without setting destiation
-
Explicit Setting by Test instruction
- testq b,a like computing a&b without setting destination
- Sets condition codes based on value of a&b
Reading Condition Codes
-
SetX Instructions
- Set low-order byte of destination to 0 or 1 based on combinations of condition codes
-
One of addressable byte registers
- Does not alter remaining bytes
- Typically use movzbl to finish job
Conditional branches
Jumping
-
jX Instructions
- Jump to different part of code depending on condition codes
Expressing with Goto Code
- C allows goto statement
- Jump to position designated by label
Using Conditional Moves
-
Conditional Move Instructions
- Instruction supports
- Supported in post-1995x86 processors
- GCC tries to use them
Bad Cases for Conditional Move
- Expensive Computations
- Risky Computations
- Computations with side effects
Loops
Do-While Loop
- Use conditional branch
While Loop
- Initial goto starts loop at test
- Initial conditional guards entrance to loop
For Loop
- Init Test Update Body
Switch Statements
Prototype
- Multiple case labels
- Fall through cases
- Missing cases
Structure
- From Jump Table to Jump Targets
