Fix: Set SNAPPY_HAVE_BMI2 to 0 to avoid crashes on older CPUs

[EllieTellie]

Description

Fixes EXCEPTION_ILLEGAL_INSTRUCTION crash on CPUs without BMI2 support.
Snappy compression library detects BMI2 support at compile time based on the build machine. When built on a machine with BMI2, Snappy uses instructions like BZHI which crashes.

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Testing

Tested the change on devices that have older CPUs and confirmed it no longer crashes.
To reproduce the machine you are building on must support BMI2.
To reproduce the machine you are running Firebase on must not support BMI2, for example: Intel Celeron/Pentium Silver (Gemini Lake), Intel Core i-series pre-Haswell or AMD pre-Zen.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

crash.dmp

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Summary of Changes

Hello @EllieTellie, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug that caused application crashes on older CPUs by preventing the Snappy compression library from using advanced instruction sets (BMI2) that are not universally supported. By explicitly disabling these optimizations during the build process, the change significantly enhances the application's stability and compatibility across a wider range of hardware, particularly for devices with pre-Haswell Intel or pre-Zen AMD processors.

Highlights

• Crash Fix: Addresses an EXCEPTION_ILLEGAL_INSTRUCTION crash that occurred on CPUs lacking BMI2 support when the Snappy compression library was compiled on a machine with BMI2 capabilities.
• CPU Compatibility: Ensures broader CPU compatibility by explicitly disabling Snappy's BMI2 optimizations at compile time.
• Build Configuration: Modifies CMakeLists.txt to set the SNAPPY_HAVE_BMI2 flag to 0, preventing the use of BMI2 instructions like BZHI on incompatible hardware.

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[gemini-code-assist]

Code Review

This pull request aims to fix a crash on older CPUs by disabling BMI2 optimizations in the Snappy compression library. The approach of setting SNAPPY_HAVE_BMI2 to 0 is correct. However, the implementation has a potential issue where the setting might not be applied in existing build directories due to CMake's caching behavior. I've added a specific comment with a suggestion to ensure the fix is always effective.

[adabru]

I confirm that this patch resolved a crash I was experiencing with the firebase-unity-sdk on a Windows machine with an Intel Celeron N4120.