[PATCH] Teach builtin clz, ctz, and popcount to be const

Arthur O'Dwyer arthur.j.odwyer at gmail.com
Tue Jun 11 13:14:30 PDT 2013

On Tue, Jun 11, 2013 at 12:26 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> > On Tue, Jun 11, 2013 at 12:18:55AM -0600, Justin Bogner wrote:
>> >> The clz, ctz, and popcount builtins are trivial to add constant folded
>> >> implementations of, and gcc accepts them in const contexts.
> A testcase which is less likely to pass by accident would be better (for
> instance, your popcount tests would succeed if the builtin just returned its
> argument, and tests with a '1' in the sign bit would be useful).
> You should also ensure that __builtin_clz(0) and __builtin_ctz(0) are not
> treated as constants.

I may be displaying my ignorance here, but is it also necessary to test
that __builtin_clz(0x123456789ABCDEF0LL) even in a constant context
returns (int)0 and not (int)3?  (Because it truncates its argument.)


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