Скачать или смотреть Understanding Inline ASM in C++: Why is the First Operand Added Twice?

A beginner's guide to understanding inline assembly in C++, addressing the issue of operand duplication in assembly operations.
---
This video is based on the question https://stackoverflow.com/q/77852804/ asked by the user 'MAA1117' ( https://stackoverflow.com/u/22937669/ ) and on the answer https://stackoverflow.com/a/77853073/ provided by the user 'Chris Dodd' ( https://stackoverflow.com/u/16406/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, comments, revision history etc. For example, the original title of the Question was: Why is this inline ASM function adding the first input operand twice?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Inline ASM in C++: Why is the First Operand Added Twice?

Working with inline assembly in C++ can be an exciting challenge, particularly for those just starting their programming journey. For novices, the intricacies of how assembly language interacts with C++ can be perplexing. A common question that often arises is: Why is this inline ASM function adding the first input operand twice? Let's delve into this issue systematically.

The Problem at Hand

In the provided C++ code, a function is implemented to calculate the sum of three integers using inline assembly. Here's a brief overview of the function:

[[See Video to Reveal this Text or Code Snippet]]

When the function sum(4, 10, 23) is called, instead of returning the expected sum of a + b + c, the result surprisingly adds a twice, resulting in 37 (i.e., 4 + 4 + 10 + 23). This unexpected behavior leads to confusion, especially for beginner programmers trying to learn the foundational concepts of inline assembly.

Understanding the Assembly Code

To understand why the first operand (a) is added twice, we need to explore what the inline assembly code is doing and how constraints in assembly play a crucial role. The way operands are bound to registers can lead to overlapping if not handled properly. Here's an analysis:

The Assembly Constraints

Output Constraint:

: "=r" (result): This designates result as an output variable, which means it will receive its final value from the assembly code.

Input Constraints:

: "r" (a), "r" (b), "r" (c): This registers a, b, and c as input variables. Each of these inputs will be associated with separate registers.

What Happens During the Execution?

The assembly code snippet:

[[See Video to Reveal this Text or Code Snippet]]

translates to:

addl %1, %0;: Add the value of a (from %1) to result (in %0).

addl %2, %0;: Add the value of b (from %2) to the updated result.

addl %3, %0;: Add the value of c (from %3) to the updated result.

The Overlap Issue

The crucial part here is that the compiler can assign the same register for both result and a:

If result and a are bound to the same register, the first addition effectively results in result holding the value of a again, leading to a + a + b + c.

A Simple Solution

To fix the problem of operand duplication, we need to ensure that the register assigned to result is separate from the registers for a, b, and c.

Using the + Constraint

One straightforward fix is to change the output constraint for result to "+r":

[[See Video to Reveal this Text or Code Snippet]]

What Does This Change Do?

The "+r" constraint binds the register to result for both input and output, ensuring it is distinct and initialized correctly (starting at 0).

Conclusion

Understanding inline assembly functions can be daunting, especially when the expected outcomes don't match reality. By breaking down the structure and constraints of inline assembly, we can solve issues like operand duplication effectively.

If you are new to programming or assembly, it's essential to experiment and learn how registers and constraints interact. With time and practice, these concepts will become clearer.

Happy coding, and remember: in assembly, every little detail counts!