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Re: ieee754[sd]p_neg workaround

To: ralf@linux-mips.org
Subject: Re: ieee754[sd]p_neg workaround
From: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Date: Mon, 02 Jan 2006 21:59:49 +0900 (JST)
Cc: dom@mips.com, macro@linux-mips.org, linux-mips@linux-mips.org
In-reply-to: <20050421.161945.79301612.nemoto@toshiba-tops.co.jp>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <16998.20933.14301.397793@arsenal.mips.com> <20050420131304.GF5212@linux-mips.org> <20050421.161945.79301612.nemoto@toshiba-tops.co.jp>
Sender: linux-mips-bounce@linux-mips.org
>>>>> On Thu, 21 Apr 2005 16:19:45 +0900 (JST), Atsushi Nemoto 
>>>>> <anemo@mba.ocn.ne.jp> said:

>>> So file a bug against glibc, but we should fix the emulator so it
>>> correctly imitates the MIPS instruction set...

ralf> As a matter of defensive design I think we should try to follow
ralf> the establish behaviour if nothing more specific is defined
ralf> anywhere.

anemo> OK, I sent a bug reoport to glibc bugzilla. (Bug# 864)

The bug was resolved ... marked as WONTFIX.

glibc developers said math-emu should be fixed.  Please look at:

http://sourceware.org/bugzilla/show_bug.cgi?id=864

for their comments.

So I send a patch for math-emu again.

Description:

It looks glibc's pow() assume an unary '-' operation for any number
(including NaNs) always invert its sign bit (though IEEE754 does not
specify the sign bit for NaNs).  This patch make the kernel math-emu
emulates real MIPS neg.[ds] instruction.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>

diff --git a/arch/mips/math-emu/dp_simple.c b/arch/mips/math-emu/dp_simple.c
index 495c1ac..1c555e6 100644
--- a/arch/mips/math-emu/dp_simple.c
+++ b/arch/mips/math-emu/dp_simple.c
@@ -48,16 +48,22 @@ ieee754dp ieee754dp_neg(ieee754dp x)
        CLEARCX;
        FLUSHXDP;
 
+       /*
+        * Invert the sign ALWAYS to prevent an endless recursion on
+        * pow() in libc.
+        */
+       /* quick fix up */
+       DPSIGN(x) ^= 1;
+
        if (xc == IEEE754_CLASS_SNAN) {
+               ieee754dp y = ieee754dp_indef();
                SETCX(IEEE754_INVALID_OPERATION);
-               return ieee754dp_nanxcpt(ieee754dp_indef(), "neg");
+               DPSIGN(y) = DPSIGN(x);
+               return ieee754dp_nanxcpt(y, "neg");
        }
 
        if (ieee754dp_isnan(x)) /* but not infinity */
                return ieee754dp_nanxcpt(x, "neg", x);
-
-       /* quick fix up */
-       DPSIGN(x) ^= 1;
        return x;
 }
 
diff --git a/arch/mips/math-emu/sp_simple.c b/arch/mips/math-emu/sp_simple.c
index c809830..770f0f4 100644
--- a/arch/mips/math-emu/sp_simple.c
+++ b/arch/mips/math-emu/sp_simple.c
@@ -48,16 +48,22 @@ ieee754sp ieee754sp_neg(ieee754sp x)
        CLEARCX;
        FLUSHXSP;
 
+       /*
+        * Invert the sign ALWAYS to prevent an endless recursion on
+        * pow() in libc.
+        */
+       /* quick fix up */
+       SPSIGN(x) ^= 1;
+
        if (xc == IEEE754_CLASS_SNAN) {
+               ieee754sp y = ieee754sp_indef();
                SETCX(IEEE754_INVALID_OPERATION);
-               return ieee754sp_nanxcpt(ieee754sp_indef(), "neg");
+               SPSIGN(y) = SPSIGN(x);
+               return ieee754sp_nanxcpt(y, "neg");
        }
 
        if (ieee754sp_isnan(x)) /* but not infinity */
                return ieee754sp_nanxcpt(x, "neg", x);
-
-       /* quick fix up */
-       SPSIGN(x) ^= 1;
        return x;
 }
 

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