Windows/MSVC build fails because __builtin_clzll is not available

[SaurSum8]

Windows/MSVC build failure: __builtin_clzll is not available

Summary

RasterizeToPixelsFromWorld3DGS.cu uses __builtin_clzll inside ceil_log2_u64, but this builtin is not available when compiling with MSVC on Windows. This causes Windows builds to fail.

Relevant code

uint32_t ceil_log2_u64(uint64_t x) {
    if (x <= 1) {
        return 0;
    }
    return 64u - static_cast<uint32_t>(__builtin_clzll(x - 1));
}

Problem

__builtin_clzll is a GCC/Clang-specific builtin. On MSVC, the equivalent operation should use _BitScanReverse64 from <intrin.h>, or the code should use std::countl_zero when C++20 is available.

Suggested fix

One possible MSVC-compatible implementation is:

#if defined(_MSC_VER)
#include <intrin.h>
#endif

namespace {

uint32_t ceil_log2_u64(uint64_t x) {
    if (x <= 1) {
        return 0;
    }

#if defined(_MSC_VER)
    unsigned long index;
    _BitScanReverse64(&index, x - 1);
    return static_cast<uint32_t>(index) + 1u;
#else
    return 64u - static_cast<uint32_t>(__builtin_clzll(x - 1));
#endif
}

} // namespace

This is equivalent because _BitScanReverse64 returns the zero-based index of the most significant set bit, while 64 - __builtin_clzll(v) returns the bit width of v.

Notes

The zero case is already handled by the x <= 1 guard, so _BitScanReverse64 is only called with a nonzero value.