I’m not at my computer, so I can’t double-check this, but IIRC the issue was that ((long long)0xffffffff << 32) is undefined because it shifts past 2^63-1, which causes an overflow of the long long. (I know the undefined behavior sanitizer flagged some equivalent code in a Linux reimplementation I was working on, which is how I noticed it in the kernel.)
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I’m not at my computer, so I can’t double-check this, but IIRC the issue was that ((long long)0xffffffff << 32) is undefined because it shifts past 2^63-1, which causes an overflow of the long long. (I know the undefined behavior sanitizer flagged some equivalent code in a Linux reimplementation I was working on, which is how I noticed it in the kernel.)