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Re: [PATCH, RFC] fma vs gcc 4.9
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: Richard Henderson <rth at twiddle dot net>
- Cc: libc-alpha <libc-alpha at sourceware dot org>
- Date: Wed, 16 Jul 2014 20:56:33 +0000
- Subject: Re: [PATCH, RFC] fma vs gcc 4.9
- Authentication-results: sourceware.org; auth=none
- References: <53B59CDF dot 1010604 at twiddle dot net>
On Thu, 3 Jul 2014, Richard Henderson wrote:
> It seems to me that there's a typo on that exact zero test: a2 should be used,
> not m2. Correct, or have I mis-read the code?
The existing exact zero test seems correct to me. The conditions for such
an exact zero are that the result of the multiplication is exactly
representable in 53 bits (i.e., m2 == 0, with m1 being the exact result of
the multiplication), and that the result of the addition (of z to the high
part of the multipliation result) is an exact zero (i.e. a1 == 0, which
implies a2 == 0).
--
Joseph S. Myers
joseph@codesourcery.com