When x is not known to be nonzero, ctpop(x) == 1 is expanded to
x != 0 && (x & (x - 1)) == 0
resulting in codegen like
leal -1(%rdi), %eax
testl %eax, %edi
sete %cl
testl %edi, %edi
setne %al
andb %cl, %al
But another expression that works is
(x ^ (x - 1)) > x - 1
which has nicer codegen:
leal -1(%rdi), %eax
xorl %eax, %edi
cmpl %eax, %edi
seta %al
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