VmaCountBitsSet crashes on CPUs without support for MSVC __popcnt (ARM, older x86)

[akien-mga]

__popcnt is used here unconditionally for MSVC without checking that it's actually supported by the CPU:

VulkanMemoryAllocator/include/vk_mem_alloc.h

Lines 3080 to 3081 in dc3f6bb

	#ifdef _MSC_VER
	return __popcnt(v);

This has been reported to triggering a crash in Godot: godotengine/godot#58463

I don't know much about this instruction myself but Stackoverflow seems to have decent advice on checking capability with cpuid here:
https://stackoverflow.com/a/42913358

[adam-sawicki-a]

Thank you for reporting this issue. I removed usage of __popcnt intrinsic function. User can still provide optimized implementation by defining macro VMA_COUNT_BITS_SET.

I hope that functions like _BitScanForward are not affected in the same way?

[akien-mga]

Thank you for reporting this issue. I removed usage of __popcnt intrinsic function. User can still provide optimized implementation by defining macro VMA_COUNT_BITS_SET.

Thanks! That works, but if that optimization is important, I think you should be able to improve things to actually check CPU capabilities. The MSDN docs provide some guidance on how to check for Advanced Bit Manipulation support: https://docs.microsoft.com/en-us/cpp/intrinsics/popcnt16-popcnt-popcnt64?view=msvc-170

So far I didn't find information that would indicate that the GCC/Clang __builtin_popcount has a similar requirement, so you might be able to always use it, and only need to check CPU capabilities (or add a macro entry point) for MSVC. Might need double checking though.

I hope that functions like _BitScanForward are not affected in the same way?

Based on https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64?view=msvc-170 I think it might be fine, it seems supported without specific requirements on all architectures supported by MSVC:

[akien-mga]

Reading https://stackoverflow.com/questions/60165922/stdbitsetncount-vs-builtin-popcount it sounds like std::bitset<T>::count might be a good cross-platform alternative? http://www.cplusplus.com/reference/bitset/bitset/count/

[adam-sawicki-a]

std::bitset is an STL container, so something different than we need here, where we want to just calculate bit count in a single number.

There is a nice header <bit> that offers what we need: https://en.cppreference.com/w/cpp/numeric/popcount but it is available only since C++20, while the library uses C++14.

So I am going to leave it as-is for now.