Add AoCO 2025 Day 05 Study Notes

[gapry]

These notes are based on the post ARM's barrel shifter tricks and the YouTube video [AoCO 5/25] Multiplying with a Constant which are Day 5 of the Advent of Compiler Optimisations 2025 Series by Matt Godbolt.

[gemini-code-assist]

Summary of Changes

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This pull request introduces a comprehensive study note document exploring how the LLVM compiler optimizes constant multiplication for the AArch64 architecture. The document systematically analyzes different multiplication scenarios, demonstrating the underlying ARM assembly instructions and optimization techniques employed, such as logical shifts, additions with shifted operands, and shift-and-subtract strategies. It also incorporates external technical discussions to enrich the context.

Highlights

• New Study Notes Document: A new document titled 'Study Notes: ARM's barrel shifter tricks, Advent of Compiler Optimisations 2025' has been added, focusing on compiler optimization strategies for constant multiplication on AArch64.
• AArch64 Compiler Optimizations: The notes detail six distinct compiler optimization strategies for constant multiplication (e.g., x * 2, x * 3, x * 6, x * 7, x * 11, x * 14) on the AArch64 target using the LLVM toolchain.
• Assembly Analysis: Each case includes the C code, compilation commands, and the resulting ARM assembly output, along with explanations of the ARM instructions used (e.g., lsl, add with shifted operands, sub with shifted operands, mul).
• External Insights and References: The document incorporates selected technical insights from YouTube comment sections related to the topic and provides a list of references for further reading on ARM architecture and barrel shifters.

Changelog

• _posts/2026-02-28-Advent-of-Compiler-Optimisations-Study-Notes-05.md
  • Added new study notes document.

Activity

• No specific pull request activity (comments, reviews, etc.) was provided in the context.

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[amazon-q-developer]

This PR adds comprehensive study notes on ARM barrel shifter optimization techniques. The content is well-structured educational material with accurate technical explanations and assembly code examples. No blocking issues found - the markdown is properly formatted, the technical content is accurate, and there are no security concerns or functional defects. Ready to merge.

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[gemini-code-assist]

Code Review

This pull request adds a well-researched and detailed blog post about compiler optimizations for constant multiplication on AArch64. The examples are clear and the explanations are insightful. I've added a few suggestions to improve consistency and clarity in the explanations and section headers throughout the document. Overall, this is a great contribution.

[gemini-code-assist]

For better clarity and to directly relate back to the C code return x * 3;, you could show the final result of the computation in the explanation.

Suggested change

	The operation represents the formula `w0 = w0 + (w0 << 1)`, which computes `x = x + x * 2`.
	The operation represents the formula `w0 = w0 + (w0 << 1)`, which computes `x + (x * 2) = 3x`.

[gemini-code-assist]

There's a minor inconsistency in this section header. In other cases, you've used "ARM Instruction:" or "ARM Instructions:". Here it's "ARM Instructions" without a colon. For consistency, I'd suggest adding a colon.

Suggested change

	ARM Instructions
	ARM Instructions:

[gemini-code-assist]

There's a minor inconsistency in this section header. In other cases, you've used "ARM Instruction:" or "ARM Instructions:". For consistency, I'd suggest changing "Instruction Format:" to "ARM Instructions:".

Suggested change

	Instruction Format:
	ARM Instructions: