在 2024-03-14 23:33, Andrew Cooper via Gcc 写道:
And for x86's arch_ffs(),unsigned int arch_ffs(unsigned int x) { unsigned int res; if ( __builtin_constant_p(x > 0) && x > 0 ) { // Well defined when x is known non-zero asm("bsf %1, %0" : "=r"(res) : "rm"(x));
Even if you may assume that the destination operand is always destroyed, the hardware has no knowledge about that, so this statement has a dependency on `res`, and shouldn't be declared `=r`.
I think it's better to remove this `if`. The other branch below clearly eliminates the dependency.
}
else
{
// The architects say this is safe even for 0.
res = -1;
asm("bsf %1, %0" : "+r"(res) : "rm"(x));
}
return res + 1;
}
-- Best regards, LIU Hao
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