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I suppose you're using (assuming 32-bit)
ctz(x) := 31 - clz(x & -x)
now, which gives -1 for 0; and the version you're looking for is
ctz(x) := 32 - clz(~x & (x-1))
which gives 32 for 0.
(Straight from the venerable PowerPC Compiler Writer's Guide, btw).
What does the popcount version look like? Never seen that before, but I think it will be really expensive on PowerPC.
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