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Re: [patch] std::experimental::gcd and std::experimental::lcd
- From: Marc Glisse <marc dot glisse at inria dot fr>
- To: Jonathan Wakely <jwakely at redhat dot com>
- Cc: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Sat, 2 May 2015 19:07:14 +0200 (CEST)
- Subject: Re: [patch] std::experimental::gcd and std::experimental::lcd
- Authentication-results: sourceware.org; auth=none
- References: <20150502151849 dot GZ3618 at redhat dot com> <alpine dot DEB dot 2 dot 11 dot 1505021813010 dot 4001 at laptop-mg dot saclay dot inria dot fr> <20150502164728 dot GA3618 at redhat dot com>
- Reply-to: libstdc++ at gcc dot gnu dot org
On Sat, 2 May 2015, Jonathan Wakely wrote:
On 02/05/15 18:27 +0200, Marc Glisse wrote:
On Sat, 2 May 2015, Jonathan Wakely wrote:
These where simple to implement (almost too simple ... I probably
got something wrong!)
I didn't remember that std::abs works for unsigned. It will need more work
for performance, but that can certainly be done later (I didn't look at the
code beyond checking what you meant by "simple").
std::abs seems to work fine for unsigned, the overload in <cmath> for
integral types just uses __builtin_fabs.
That's bad! You don't want to go through floating point, that cannot even
represent int64_t accurately.
Maybe it would be better for
gcd() to just use that directly instead of including <cmath> (as
attached, which also removes the qualification on the call to gcd
because the functions only work for integral types which have no
associated namespaces anyway).
Actually you use __builtin_abs (no 'f') in the patch, which apparently
casts to int, that's even worse...
<ratio> was added to libstdc++ in 2008 so I think the constexpr
support was not good enough at the time.
Yes, but we modified a few pieces with Paolo in 2011, and those could have
used constexpr I guess.
--
Marc Glisse