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From: Peter Maydell <peter.maydell@linaro.org>
To: Avi Kivity <avi@redhat.com>
Cc: Laurent Desnogues <laurent.desnogues@gmail.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Mulyadi Santosa <mulyadi.santosa@gmail.com>,
	qemu-devel <qemu-devel@nongnu.org>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH] tcg: Remove stack protection from helper functions
Date: Mon, 26 Sep 2011 21:19:15 +0100	[thread overview]
Message-ID: <CAFEAcA-kkobH763sPVs3zbgh6J9LTsJjo5XkEqHC5g3TDwaMMg@mail.gmail.com> (raw)
In-Reply-To: <4E80D7E9.4050203@redhat.com>

On 26 September 2011 20:52, Avi Kivity <avi@redhat.com> wrote:
> Why do floating point ops need helpers?  At least if all the edge cases
> match? (i.e. NaNs and denormals)

The answer is that the edge cases basically never match. No CPU
architecture does handling of NaNs and input denormals and output
denormals and underflow checks and all the rest of it in exactly
the same way as anybody else. (In particular x86 is pretty crazy,
which is unfortunate given that it's the most significant host
arch at the moment.) So any kind of TCG native floating point
support would probably have to translate to "check if either
input is a special case; if not, try the op; check if the output
was a special case; if any of those checks fired, back off to
the softfloat helper function". Which is quite a lot of inline
code, and also annoyingly bouncing between fp ops and integer
bitpattern checks on the fp values.

-- PMM

  parent reply	other threads:[~2011-09-26 20:19 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-09-26  7:46 [Qemu-devel] [PATCH] tcg: Remove stack protection from helper functions Jan Kiszka
2011-09-26  8:01 ` Mulyadi Santosa
2011-09-26  8:15   ` Laurent Desnogues
2011-09-26 17:41     ` Avi Kivity
2011-09-26 19:43       ` Richard Henderson
2011-09-26 19:52         ` Avi Kivity
2011-09-26 19:53           ` Richard Henderson
2011-09-26 20:20             ` Avi Kivity
2011-09-26 20:19           ` Peter Maydell [this message]
2011-09-26 20:26             ` Avi Kivity
2011-09-27  4:29             ` Andi Kleen
2011-09-27  7:58               ` Peter Maydell
2011-09-26 10:51 ` [Qemu-devel] [PATCH v2] " Jan Kiszka
2011-09-26 11:33   ` Peter Maydell
2011-09-26 11:43     ` Jan Kiszka
2011-09-26 11:56       ` Peter Maydell
2011-09-26 17:22         ` Blue Swirl
2011-09-26 17:33           ` Jan Kiszka
2011-09-26 18:20             ` Blue Swirl
2011-09-26 18:25               ` Jan Kiszka
2011-09-26 18:40                 ` Blue Swirl
2011-09-26 19:08               ` Peter Maydell

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