Ethereum: Inlining Assembly for Array Retrieval in Mapping with Yul
As you get into Ethereum development, it’s crucial to understand how to manipulate data structures. One common task is retrieving arrays from structures using inline assembly with Yul. This article will walk you through the process of achieving this goal.
What are structures and mappings?
In Ethereum, a structure represents an object with multiple fields. Mappings (also known as enums in some contexts) are used to store references to these structures, allowing for efficient lookups and manipulation.
Let’s say your structure looks like this:
struct MyStruct {
uint256 id;
array of data;
};
The mapping might look like this:
July
mapping(address => address) “my_structures” {
“0x1234567890abcdef” => “0x1234567890abcdef”,
}
Inline array fetcher
To retrieve a data field from a structure using the built-in assembler, you need to use the asm keyword followed by the assembler instructions. The general pattern is:
c
asm volatile(“mov %0, %1” : “=r”(struct field)) : “r”(field);
In this example:
%0 and %1 are the registers that will contain the addresses of the structural fields.
=r specifies that register values should be used as memory addresses.
For our case MyStruct, the assembly instructions would look like this:
c
asm volatile(“mov %0, %1” : “=r”(struct.data)) : “r”(data);
This instruction will set the value "%1" (the address of the "data" field) to the string stored in our mapping.
Step-by-step solution
Here is a step-by-step example:
c
// Define your structure and mapping
struct MyStruct {
uint256 id;
data string;
};
mapping(address => address) “my_structs” {
“0x1234567890abcdef” => “0x1234567890abcdef”,
}
// Retrieving an array from a structure using built-in assembler
asm volatile(“mov %0, %1” : “=r”(struct.data)) : “r”(id);
The resulting assembly code would be:
c
asm volatile(“mov %0, %1” : “=r”(my_structs[“0x1234567890abcdef”])) : “r”(id);
What's next?
After you retrieve an array from a structure using built-in assembler, you can use it as needed. If you're working with mapping, you can retrieve a reference to the structure using its address.
However, if you need to iterate over structures in your mapping and perform operations on each one, consider using Yul's struct function for more convenience:
July
function MyStruct(id: uint256) public view return(String memoryData) {
return struct("MyStruct", {id})[data];
}
This will allow you to retrieve an array value from a structure without the need for inline assembler.
Following this tutorial, you should now have a good understanding of how to use the built-in assembler to retrieve strings from structures in Ethereum mappings using Yula.