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Re: [PULL 08/13] softfloat: Fix BAD_SHIFT from normalizeFloatx80Subnorma
From: |
Richard Henderson |
Subject: |
Re: [PULL 08/13] softfloat: Fix BAD_SHIFT from normalizeFloatx80Subnormal |
Date: |
Fri, 10 Apr 2020 08:17:36 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
On 4/10/20 2:38 AM, Aleksandar Markovic wrote:
> 17:55 Uto, 07.04.2020. Alex Bennée <address@hidden
> <mailto:address@hidden>> је написао/ла:
>>
>> From: Richard Henderson <address@hidden
> <mailto:address@hidden>>
>>
>> All other calls to normalize*Subnormal detect zero input before
>> the call -- this is the only outlier. This case can happen with
>> +0.0 + +0.0 = +0.0 or -0.0 + -0.0 = -0.0, so return a zero of
>> the correct sign.
>>
>> Reported-by: Coverity (CID 1421991)
>> Signed-off-by: Richard Henderson <address@hidden
> <mailto:address@hidden>>
>> Signed-off-by: Alex Bennée <address@hidden
> <mailto:address@hidden>>
>> Message-Id: <address@hidden
> <mailto:address@hidden>>
>> Message-Id: <address@hidden
> <mailto:address@hidden>>
>>
>> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
>> index 301ce3b537b..ae6ba718540 100644
>> --- a/fpu/softfloat.c
>> +++ b/fpu/softfloat.c
>> @@ -5856,6 +5856,9 @@ static floatx80 addFloatx80Sigs(floatx80 a, floatx80 b,
> flag zSign,
>> zSig1 = 0;
>> zSig0 = aSig + bSig;
>> if ( aExp == 0 ) {
>> + if (zSig0 == 0) {
>> + return packFloatx80(zSign, 0, 0);
>> + }
>> normalizeFloatx80Subnormal( zSig0, &zExp, &zSig0 );
>> goto roundAndPack;
>> }
>> --
>> 2.20.1
>>
>>
>
> We in MIPS have extensive FP tests, that certainly include many cases of
> operations with +0 and -0. And they are all correct even before this patch.
This is for the 80-bit extended-double type, only used on x86 and m68k. You
will not execute this path using MIPS.
> Alex, from the commit message, it not clear if this is a fix of a bug (in
> which
> case a test example would be useful to have, and the assesment on what
> scenarios could be affected), or just a correction for some rare condition
> that
> practically for all intents and purposes was never triggered, or perhaps
> something third.
This only avoids a Coverity out-of-range shift warning.
Beforehand, we executed 0 << 64, got 0 as the result (regardless of whether or
not the host truncates the shift count), and constructed the correctly signed
fp zero in the end.
There was more discussion about this in an earlier thread, associated with a
different patch for this same problem:
https://lists.nongnu.org/archive/html/qemu-devel/2020-03/msg08278.html
> Secondly, and not related to this patch only, I see more and more patches
> integrated into the main tree without "Reviewed-by:" tag. I don't think this
> is
> the best way an open source community works. In my personal opinion, this must
> stop.
The only way to avoid this is to have more developers review code outside their
own bailiwick. The patch has been on the list for two weeks and was pinged
twice.
Although why Alex didn't add his own R-b to my patch when merging it to his
branch, I don't know.
r~
- [PULL 02/13] elf-ops: bail out if we have no function symbols, (continued)
- [PULL 02/13] elf-ops: bail out if we have no function symbols, Alex Bennée, 2020/04/07
- [PULL 03/13] linux-user: protect fcntl64 with an #ifdef, Alex Bennée, 2020/04/07
- [PULL 04/13] tests/tcg: remove extraneous pasting macros, Alex Bennée, 2020/04/07
- [PULL 01/13] .github: Enable repo-lockdown bot to refuse GitHub pull requests, Alex Bennée, 2020/04/07
- [PULL 05/13] linux-user: more debug for init_guest_space, Alex Bennée, 2020/04/07
- [PULL 06/13] target/xtensa: add FIXME for translation memory leak, Alex Bennée, 2020/04/07
- [PULL 07/13] gdbstub: fix compiler complaining, Alex Bennée, 2020/04/07
- [PULL 10/13] linux-user: clean-up padding on /proc/self/maps, Alex Bennée, 2020/04/07
- [PULL 08/13] softfloat: Fix BAD_SHIFT from normalizeFloatx80Subnormal, Alex Bennée, 2020/04/07
[PULL 09/13] linux-user: factor out reading of /proc/self/maps, Alex Bennée, 2020/04/07
[PULL 12/13] configure: Add -Werror to PIE probe, Alex Bennée, 2020/04/07
[PULL 11/13] hw/core: properly terminate loading .hex on EOF record, Alex Bennée, 2020/04/07
[PULL 13/13] tcg/i386: Fix %r12 guest_base initialization, Alex Bennée, 2020/04/07