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Questions and Answers
What is the main objective of the false dependencies solution?
What is the main objective of the false dependencies solution?
What is the technique used to avoid false WAR and WAW dependencies?
What is the technique used to avoid false WAR and WAW dependencies?
In the original code in Listing 5.4, what is the register involved in the false WAW dependency?
In the original code in Listing 5.4, what is the register involved in the false WAW dependency?
How is the false WAW dependency fixed in the modified code in Listing 5.5?
How is the false WAW dependency fixed in the modified code in Listing 5.5?
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What is the problem with the code in Listing 5.3?
What is the problem with the code in Listing 5.3?
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What is the register involved in the false WAR dependency in Listing 5.6?
What is the register involved in the false WAR dependency in Listing 5.6?
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How is the false WAR dependency fixed in Listing 5.7?
How is the false WAR dependency fixed in Listing 5.7?
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What is the purpose of register renaming?
What is the purpose of register renaming?
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What is the result of applying the register renaming technique?
What is the result of applying the register renaming technique?
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What is the goal of the false dependencies solution with respect to execution order?
What is the goal of the false dependencies solution with respect to execution order?
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What is the primary effect of out-of-order execution on the result of in-order execution?
What is the primary effect of out-of-order execution on the result of in-order execution?
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What type of dependencies do the techniques in the false dependencies solution aim to avoid?
What type of dependencies do the techniques in the false dependencies solution aim to avoid?
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In the context of the false dependencies solution, what is the purpose of identifying the next writing point of a register?
In the context of the false dependencies solution, what is the purpose of identifying the next writing point of a register?
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How does register renaming affect the dependency between instructions in a code snippet?
How does register renaming affect the dependency between instructions in a code snippet?
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What is the primary consequence of false dependencies on the execution of code?
What is the primary consequence of false dependencies on the execution of code?
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What is the relationship between the original code and the modified code in Listings 5.4 and 5.5?
What is the relationship between the original code and the modified code in Listings 5.4 and 5.5?
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What is the significance of the register F3 in the code snippet in Listing 5.6?
What is the significance of the register F3 in the code snippet in Listing 5.6?
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How does the register renaming technique affect the execution of instructions in a code snippet?
How does the register renaming technique affect the execution of instructions in a code snippet?
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What is the primary goal of the false dependencies solution in terms of instruction execution?
What is the primary goal of the false dependencies solution in terms of instruction execution?
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What is the relationship between the false dependencies solution and the concept of superscalar processors?
What is the relationship between the false dependencies solution and the concept of superscalar processors?
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Study Notes
Superscalar Processors
- Superscalar processors aim to exploit Instruction-Level Parallelism (ILP) to improve performance.
- There are two approaches to exploiting ILP: hardware-based and software-based.
- Hardware-based approaches focus on discovering and exploiting parallelism dynamically during runtime.
- Software-based approaches focus on finding parallelism statically at compile time.
Pipelining Limits
- The pipelining approach creates a theoretical barrier, where the clock cycles per instruction (CPI) is equal to 1.
- The actual pipeline CPI is greater than 1 due to stalls, which can be structural, data hazard, or control related.
- Equation 5.1 represents the CPI of a pipelined processor: CPI Pipeline = CPI IdealPipeline + Stalls Structural + Stalls DataHazard + Stalls Control.
Advances in ILP Exploration
- Techniques to advance ILP exploration include:
- Pipeline: dividing instructions into phases or stages and overlapping them to improve performance.
- Superpipeline: exploring the fact that various pipeline stages require less than half clock cycle to complete, enabling two same stages to run within one external clock cycle.
- Multiple issue: issuing multiple scalar instructions per cycle, allowing the processor to execute independent instructions in parallel.
Superscalar Processors
- Superscalar processors issue multiple scalar instructions per cycle, enabling the execution of independent instructions in parallel.
- This requires multiple functional units, out-of-order instructions execution, and handling of structural hazards, data hazards, and control hazards.
- Modern processors use one pipeline with multiple functional units, such as ALU, LOAD, STORE, and floating-point units.
Out-of-Order Execution
- Out-of-order execution allows the processor to execute instructions in a different order than they were issued, to improve performance.
- This is necessary because instructions have different execution times, and a simple pipeline does not work well with different instruction execution times.
- Out-of-order execution can be achieved through techniques like Tomasulo's algorithm.
WAR and WAW Dependencies and Out-of-Order Completion
- WAR (Write-After-Write) and WAW (Write-After-Write) dependencies occur when instructions access the same registers.
- Out-of-order completion can be achieved by allowing the execution stage to repeat as many times as required, and by using different functional units.
- Dynamic scheduling problems can be solved by modifying the code to accommodate out-of-order execution.
False Dependencies Solution
- False dependencies can be solved by using registers renaming, which replaces registers with new ones to avoid false dependencies.
- This technique is used to preserve the same result as in-order execution, while allowing out-of-order execution.
WAW and WAR Analysis
- WAW and WAR dependencies can be analyzed by examining the code and identifying dependencies.
- Register renaming can be used to fix false dependencies, allowing safe out-of-order execution.
Superscalar Processors
- There are two approaches to exploit Instruction-Level Parallelism (ILP): hardware-based and software-based.
- Hardware-based approaches aim to discover and exploit parallelism during runtime, while software-based approaches aim to find parallelism at compile time.
Pipelining Limits
- The pipelining approach creates a theoretical barrier, with a Clock Cycles Per Instruction (CPI) value of 1.
- The actual pipeline CPI is greater than 1 due to stalls, including structural stalls, data hazard stalls, and control stalls.
- To achieve a CPI less than 1, techniques such as pipeline, superpipeline, and multiple issue can be used.
Pipeline Technique
- The pipeline technique divides instructions into phases or stages and overlaps these stages to improve performance.
- Pipeline is suitable for superscalar processors and Very Long Instruction Word (VLIW) approach.
Very Long Instruction Word (VLIW)
- VLIW encodes multiple instructions in one long word, including opcodes and operands.
- Packing is handled at design time by the compiler, and the processor fetches the package and issues it entirely at once.
- Modern processors have an "end of group" bit in the packing to identify a group of instructions comprising a package.
Out-of-Order Execution
- Out-of-order execution allows more than one instruction to be issued, accommodating different instruction execution times.
- This approach remains with the in-order issue, accommodates different execution times, and has a potential out-of-order instruction completion.
- Tomasulo's algorithm is used for out-of-order execution in superscalar processors.
WAR and WAW Dependencies and Out-of-Order Completion
- WAR (Write-After-Read) and WAW (Write-After-Write) dependencies can occur in out-of-order execution.
- False dependencies can be solved by register renaming to avoid WAR and WAW dependencies.
False Dependencies Solution
- Register renaming is a technique to avoid false WAR and WAW dependencies in out-of-order execution.
- This technique replaces a register with another register not used or read until its next writing point.
WAW and WAR Analysis
- WAW dependency analysis involves identifying false dependencies, and register renaming is used to fix them.
- WAR dependency verification involves identifying false dependencies, and register renaming is used to fix them.
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Description
Explore the concepts of superscalar processors, instruction-level parallelism, and hardware and software approaches to exploiting parallelism.
