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Check the proper bit for BMI2 #1947

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Merged
merged 1 commit into from
Aug 18, 2025
Merged

Check the proper bit for BMI2 #1947

merged 1 commit into from
Aug 18, 2025

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@KungFuJesus KungFuJesus commented Aug 16, 2025 •
edited by coderabbitai bot
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We were actually checking for BMI1 support here. This is unlikely to have caused any issues because to date there have not been any x86 CPUs with AVX2 support but no BMI2 support.

Summary by CodeRabbit

  • Bug Fixes
    • Corrected CPU capability detection on x86 systems to accurately recognize BMI2 and AVX-512 support. This ensures features are enabled only when supported, reducing potential illegal-instruction errors and improving stability.
    • Users on affected CPUs may see more reliable startup, fewer compatibility issues, and performance improvements where advanced instructions are now correctly utilized.

@KungFuJesus
Check the proper bit for BMI2
dd216de 
We were actually checking for BMI1 support here. This is unlikely to have
caused any issues because to date there have not been any x86 CPUs with
AVX2 support but no BMI2 support.
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coderabbitai bot commented Aug 16, 2025 •
edited
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Walkthrough

Updated x86 BMI2 feature detection in arch/x86/x86_features.c by changing the CPUID leaf 7, subleaf 0 EBX bitmask from 0x8 to 0x100, shifting the tested bit from 3 to 8. This affects has_bmi2 and any dependent feature gating.

Changes

Cohort / File(s) Change Summary
x86 feature detection
arch/x86/x86_features.c		 Adjusted CPUID EBX bitmask for BMI2 from 0x8 to 0x100; updates has_bmi2 calculation and impacts dependent feature checks.

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🎯 2 (Simple) | ⏱️ ~10 minutes

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  • Dead2
  • nmoinvaz

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coderabbitai bot reviewed Aug 16, 2025
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Actionable comments posted: 0

🧹 Nitpick comments (1)
arch/x86/x86_features.c (1)

103-103: Clarify the BMI2 comment with exact CPUID bit to prevent future regressions

Make the comment explicit about the leaf/subleaf/bit position for quick verification.

Apply this diff:

-        // check BMI2 bit
+        // check BMI2 bit: CPUID.(EAX=07H, ECX=0):EBX[8]
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📥 Commits

Reviewing files that changed from the base of the PR and between 3a52db3 and dd216de.

📒 Files selected for processing (1)
  • arch/x86/x86_features.c (1 hunks)
🧰 Additional context used
🧬 Code Graph Analysis (1)
arch/x86/x86_features.c (1)
arch/x86/x86_features.h (2)
  • x86_cpu_features (9-26)
  • x86_cpu_features (28-28)
⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (70)
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
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  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
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  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
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  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
  • GitHub Check: macOS Clang ASAN (ARM64)
  • GitHub Check: Windows MSVC 2022 v143 Win64 Native Instructions (AVX)
  • GitHub Check: EL9 Clang S390X DFLTCC MSAN
  • GitHub Check: Windows MSVC 2022 v143 Win64
  • GitHub Check: Ubuntu GCC AARCH64 ASAN
  • GitHub Check: Ubuntu GCC ARM HF No ARMv8 ASAN
  • GitHub Check: Ubuntu GCC PPC64
🔇 Additional comments (1)
arch/x86/x86_features.c (1)

104-104: Correct BMI2 bit (EBX[8]) is now used — fix is accurate

CPUID.(EAX=07H, ECX=0):EBX[8] denotes BMI2; previous 0x8 was BMI1 (EBX[3]). This aligns detection with Intel’s SDM and fixes the intended feature gate. No OSXSAVE dependency needed for BMI2.

@Dead2 Dead2 added the Architecture Architecture specific label Aug 16, 2025
Dead2
Dead2 approved these changes Aug 16, 2025
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LGTM

@Dead2 Dead2 merged commit f8bcae0 into zlib-ng:develop Aug 18, 2025
144 of 148 checks passed
@coderabbitai coderabbitai bot mentioned this pull request Oct 5, 2025
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@Dead2 Dead2 mentioned this pull request Nov 5, 2025
Merged
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