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From: "ams at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/105246] [amdgcn] Use library call for SQRT with -ffast-math + provide additional option to use single-precsion opcode
Date: Wed, 13 Apr 2022 08:29:52 +0000	[thread overview]
Message-ID: <bug-105246-4-GonA4asvIr@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-105246-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105246

--- Comment #2 from Andrew Stubbs <ams at gcc dot gnu.org> ---
When we first coded this we only had the GCN3 ISA manual, which says nothing
about the accuracy.

Now I look in the Vega manual (GCN5) I see:

  Square root with perhaps not the accuracy you were hoping for --
  (2**29)ULP accuracy. On the upside, denormals are supported.

The most recent CDNA2 manual is a bit less verbose:

  Square root. Precision is (2**29) ULP, and supports denormals.

The compiler already emits Newton Raphson iterations for division with
-ffast-math, so I'm sure it can be done, but I'm not too clear on the
mathematics myself.

      parent reply	other threads:[~2022-04-13  8:29 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-12 17:29 [Bug target/105246] New: " burnus at gcc dot gnu.org
2022-04-13  6:35 ` [Bug target/105246] " rguenth at gcc dot gnu.org
2022-04-13  8:29 ` ams at gcc dot gnu.org [this message]

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