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When an expression is previously computed, and the values of the variables in an expression have not changed since the previous computation, it is known as a common subexpression. Copy propagation is related to common subexpressions where copies are introduced during common expression elimination.
Copy Propagation in compiler design is an optimization technique that enhances code efficiency by replacing the irrelevant variable assignments with the previously calculated values that remain unchanged. This approach improves memory utilization and requires less time for overall execution.
In this article, we will discuss about copy propagation in compiler design. We will discuss its example, uses, advantages, and disadvantages. Moving forward, let’s understand about Copy Propagation more briefly.
What is copy propagation with example?
Copy propagation is the process in which the occurrence of targets of direct assignments(e.g.,b=5, a=b) is replaced directly with their values. Copy propagation is related to common subexpressions where copies are introduced during common expression elimination.
While designing a compiler, we might encounter certain expressions that are used repeatedly. An occurrence of an expression ‘e’ is known as a common subexpression if ‘e’ was previously computed and the values of the variables in ‘e’ stays the same since the previous computation.
In the above example, a+b is a common subexpression as a+b is already computed and has not been changed after the first commutation. Therefore, storing this repeated value in a separate variable, say, temp, and using it instead is known as common subexpression elimination. It is a compiler optimization process that looks for instances of identical expressions (in this case, a+b) and analyses whether it is worth replacing it with a single variable that holds the computed value.
So we will introduce copies whenever we try to eliminate the common sub-expressions. A copy instruction is generally in the form of a=b and is called copy statement. Through copy propagation, we replace the later uses of a with b, provided the value of b remains unchanged during the program.
Steps of Copy Propagation
Here is the flow chart that depicts the process involved in copy propagation.
As a first step, the compiler identifies the variables in the program.
The compiler checks for variables with a constant value throughout the program. The compiler keeps track of such variables with their values using a data structure.
In the next step, the compiler replaces those variables directly with the constant value throughout the program
The last step involves the removal of dead code. It refers to the code that is not required in the program after replacement and serves no purpose.
The steps above are repeated until further changes in the code are not required.
Example
Let's see an example of how copy propagation reduces memory access and simplifies the code. Compiler eliminates copies that are not further needed in the program, for example eliminating temporary variables.
NinjaVar1 = a+b // assigning the value to the variable
NinjaVar2 = NinjaVar1 //here Ninja1=NinjaVar is known as copy
A = NinjaVar2+B
//After copy propagation by the compiler
NinjaVar1 = a+b
A = NinjaVar1+B
Implementation of copy propagation
Before copy propagation
Code
C++
C++
#include <iostream> using namespace std;
int main() {
int NinjaVar1 = 2+3; //assigning the value to the variable int NinjaVar2 = NinjaVar1; //here Ninja1=NinjaVar is known as copy int A = NinjaVar2+5; cout<<"Value before copy propagation:"<<A ;
return 0; }
You can also try this code with Online C++ Compiler
So a compiler can directly eliminate that statement if a variable is just copying the value, and those instructions are not used for any helpful computation in the program. Wherever NinjaVar2 is used further in a program, for example, A=NinjaVar2+5, here compiler can directly replace it with NinjaVar1 through copy propagation.
Copy propagation is an optimization technique used in compilers. To make other optimizations more effective, it optimizes the code. It usually operates on low-level intermediate representations like register transfer level (RTL) or quads.
Types of Copy Propagation
The types of copy propagation are:
Simple or Local Copy Propagation
Simple or local Copy Propagation involves replacing uses of variables with their most recent definitions within the same basic block. This optimization is limited to the scope of a single basic block and does not propagate across basic block boundaries.
Global Copy Propagation
Global Copy Propagation extends the concept beyond basic blocks to encompass the entire program. It analyzes data flow through the entire program to identify opportunities for replacing variables with their most recent definitions.
Sparse Conditional Constant Propagation (SCCP)
Sparse Conditional Constant Propagation (SCCP) is a specialized form focusing on propagating constants through conditional branches. It analyzes conditions and branch outcomes to determine if variables are assigned constant values along certain paths and replaces variable uses with constant values where possible.
Dead Code Elimination with Copy Propagation
Dead Code Elimination with Copy Propagation combines copy propagation with dead code elimination to remove redundant assignments and computations. It identifies assignments to variables whose values are never used subsequently and removes both the assignment and any uses of the assigned variable.
Partial Redundancy Elimination (PRE)
Partial Redundancy Elimination (PRE) targets expressions partially redundant across program paths. It analyzes expressions producing the same value along some but not all paths and replaces uses of redundant expressions with computed results, thereby reducing redundant computations.
Uses of Copy Propagation
Below are some of the uses of copy propagation.
Copy propagation is generally used for “cleaning up” optimizations. It is usually used when other compiler passes have already been performed.
Optimizations like classical implementations of elimination of common sub-expressions need that specific copy propagation to run afterward to achieve greater efficiency.
It is performed at an early level of optimization processes and results in smaller code.
Copy propagation is used in eliminating dead code. Dead code is the code that is not further used in the program after doing proper replacements.
Loop unrolling is commonly followed by copy propagation. It is used to reduce the number of iterations.
Advantages of Copy Propagation
Some of the advantages of copy propagation are mentioned below.
Through copy propagation, we can save the number of computations.
Copy propagation helps in reducing space and enables other transformations.
Copy propagation turns the copy statement into dead code.
Disadvantages of Copy Propagation
Some of the disadvantages of copy propagation are mentioned below.
Copy propagation may generate code that we may not require further or need not be evaluated.
Sometimes, the change through copy propagation may not appear to be an improvement.
Copy propagation is the process in which the occurrence of targets of direct assignments(e.g., a=b) is replaced directly with their values. A copy instruction is generally in the form of a=b and is called copy statement. Through copy propagation, we replace the later uses of a with b, provided the value of b remains unchanged during the program.
What is the difference between copy propagation and constant propagation?
In copy propagation, A copy instruction is generally in the form of a=b and is called copy statement. While in constant propagation, statements are in the form a=c, where c is a constant. In constant propagation, constant ‘c’ replaces the variable ‘a.
What are the advantages of copy propagation?
Copy propagation is generally used for “cleaning up” optimizations. It is usually used when other compiler passes have already been performed. It is used to eliminate dead code. Dead code is the code that is not further used in the program after doing proper replacements.
How copy propagation leads to dead code?
Copy propagation can lead to dead code by replacing variables with their definitions, potentially eliminating assignments or computations that are no longer used, rendering them dead code.
How do you implement copy propagation?
Implement copy propagation by analyzing data flow, replacing uses of variables with their most recent definitions, and updating program code accordingly, enhancing efficiency and reducing redundancy.
Conclusion
We hope this article helped you in understanding copy propagation. In this article, we have discussed copy propagation in compiler design, its example, its uses, and its advantages and disadvantages. You can read more such articles on our platform, Coding Ninjas Studio.
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