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Remove volatile keyword from ARM inline assembler#1908

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Dead2 merged 1 commit intozlib-ng:developfrom
ccawley2011:arm-asm-volatile
Jul 11, 2025
Merged

Remove volatile keyword from ARM inline assembler#1908
Dead2 merged 1 commit intozlib-ng:developfrom
ccawley2011:arm-asm-volatile

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@ccawley2011 ccawley2011 commented Apr 29, 2025 •
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Specifying the volatile keyword prevents the inlined assembler from being reordered by the compiler.

Summary by CodeRabbit

  • Refactor
    • Updated inline assembly statements by removing the __volatile__ qualifier in various ARM intrinsic functions and configuration checks. No changes to user-facing features or exported interfaces.

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coderabbitai bot commented Apr 29, 2025 •
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Walkthrough

The changes systematically remove the __volatile__ qualifier from inline assembly statements across several components related to ARM architecture support. This adjustment is applied in the ARM ACLE intrinsic function definitions, CMake compiler detection macros, and embedded C test code within the configure script. The logic, operands, and outputs of the inline assembly instructions remain unchanged, and there are no modifications to function signatures, control flow, or error handling.

Changes

File(s) Change Summary
arch/arm/acle_intrins.h Removed __volatile__ from all inline assembly statements in ARM ACLE intrinsic functions.
cmake/detect-intrinsics.cmake Removed __volatile__ from inline assembly in macros checking ARMv8 and ARMv6 compiler intrinsics.
configure Removed __volatile__ from inline assembly in C test code for ARMv8 and ARMv6 instruction checks.

Sequence Diagram(s)

Not applicable; the changes only affect inline assembly qualifiers and do not alter control flow or introduce new features.

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Reviewing files that changed from the base of the PR and between dd15e04 and 15e0cff.

📒 Files selected for processing (3)
  • arch/arm/acle_intrins.h (2 hunks)
  • cmake/detect-intrinsics.cmake (2 hunks)
  • configure (2 hunks)
⏰ Context from checks skipped due to timeout of 90000ms (24)
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  • GitHub Check: Ubuntu GCC No CTZLL
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  • GitHub Check: Ubuntu GCC AARCH64 Compat No Opt UBSAN
  • GitHub Check: Ubuntu GCC No CTZLL
  • GitHub Check: Ubuntu GCC Compat Symbol Prefix
  • GitHub Check: Ubuntu GCC AARCH64 Compat No Opt UBSAN
  • GitHub Check: Ubuntu GCC No CTZLL
  • GitHub Check: Ubuntu GCC Compat Symbol Prefix
  • GitHub Check: Ubuntu GCC AARCH64 Compat No Opt UBSAN
  • GitHub Check: Ubuntu GCC No CTZLL
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🔇 Additional comments (3)
arch/arm/acle_intrins.h (1)

22-22: Removing __volatile__ qualifier from ARM inline assembly is appropriate.

The removal of the __volatile__ qualifier from ARM inline assembly statements in these ACLE intrinsic functions allows the compiler more freedom to optimize the code. This change is consistent with modern compiler optimization practices, as the volatile keyword unnecessarily restricts compiler optimizations by forcing it to maintain the exact instruction sequence as written.

Also applies to: 28-28, 34-34, 40-40, 46-46, 52-52, 58-58, 79-79

cmake/detect-intrinsics.cmake (1)

34-37: Consistent removal of __volatile__ qualifier in compiler detection code.

Removing the __volatile__ qualifier from the ARM inline assembly in these compiler detection macros ensures consistency with the changes in the actual implementation. This change in the detection code is important to match the behavior in the main codebase, ensuring that tests accurately reflect how the code will be compiled.

Also applies to: 78-78

configure (1)

1291-1294: Correctly aligned __volatile__ removal in configure script.

The removal of the __volatile__ qualifier from the ARM inline assembly in the configure script's test code maintains consistency with the changes in the main header files and CMake detection code. This ensures that the runtime detection matches the compilation behavior.

Also applies to: 1389-1389

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codecov bot commented Apr 29, 2025 •
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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.72%. Comparing base (dd15e04) to head (15e0cff).
Report is 14 commits behind head on develop.

Additional details and impacted files 
@@           Coverage Diff            @@
##           develop    #1908   +/-   ##
========================================
  Coverage    81.71%   81.72%           
========================================
  Files          147      147           
  Lines        13308    13308           
  Branches      2951     2951           
========================================
+ Hits         10875    10876    +1     
+ Misses        1526     1486   -40     
- Partials       907      946   +39

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mtl1979 commented Apr 30, 2025

The whole point of using inline assembly is that compiler doesn't understand the intrinsics, so it shouldn't try to reorder the instruction(s) either. Any compiler that has knowledge of the effects of the instruction(s) should already implement the intrinsics correctly.

@nmoinvaz nmoinvaz added cleanup Improving maintainability or removing code. Architecture Architecture specific labels May 12, 2025
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ccawley2011 commented Jul 2, 2025

I encountered this while working on new optimisations, where using volatile causes it to load and store temporary variables on the stack during a loop, while removing it allows instructions to be reordered to avoid that. volatile isn't necessary here, I just didn't realise what it was for when I wrote the ARMv6 slide_hash code.

Dead2
Dead2 approved these changes Jul 10, 2025
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LGTM

@Dead2 Dead2 merged commit f7ea8cc into zlib-ng:develop Jul 11, 2025
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@ccawley2011 ccawley2011 deleted the arm-asm-volatile branch July 11, 2025 14:53
@Dead2 Dead2 mentioned this pull request Nov 5, 2025
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