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Re: [Qemu-devel] [PATCH v4 1/9] softfloat: Implement run-time-configurab

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH v4 1/9] softfloat: Implement run-time-configurable meaning of signaling NaN bit
Date: Tue, 12 Apr 2016 15:45:51 -0300
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Apr 12, 2016 at 02:58:03PM +0200, Aleksandar Markovic wrote:
>   4) Updated code for all platforms to reflect changes in SoftFloat library.
>      This change is twofolds: it includes modifications of SoftFloat library
>      functions invocations, and an addition of invocation of function
>      set_snan_bit_is_one() during CPU initialization, with arguments that
>      are appropriate for each particular platform.
>      In order to be at the same time accurate in relation to floating point
>      arithmetics and consistent with a particular platform code, following
>      principle is adopted related to invocations of new function
>      set_snan_bit_is_one():
>      1. If a target platform doesn't use SoftFloat library, nothing is added.
>             (cris, lm32, moxie)
>      2. Else, if a target platform's signaling bit meaning is "1 is
>         signaling", explicit invocation of set_snan_bit_is_one(1) is added.
>             (mips, sh4, unicore32)
>               (for mips this will change in the second patch of this
>               series, but this very patch leaves mips features as
>               they currently are.)
>      3. Else, if a target platform explicitly sets other fields of its
>         float_status structure(s), explicit invocation of
>         set_snan_bit_is_one(0) is added.
>             (ppc, s390x, tricore)
>      4. Else, if a target platform doesn't explicitly set its structure
>         CPUXXXState, explicit invocation of set_snan_bit_is_one(0) is added.
>             (alpha, xtensa)

The CPU struct is zeroed by object_new() when the CPU object is
created, so the field is already set to 0 by default on all
architectures. You shouldn't need set_snan_bit_is_one(0) calls on
CPU initialization on any architecture.

>      5. For remaining cases, nothing is added. (Those cases explicitly set
>         their CPUXXXState/float_status structures to 0, and thus implicitly
>         perform set_snan_bit_is_one(0).)
>             (arm, i386, m68k, microblaze, openrisc, sparc)


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