Re: [Tinycc-devel] x86_64 tcc doesn't set sign bit on NaNs

[Christian Jullien]

Also on OpenBSD x64 with clang 10

./configure --strip-binaries --with-selinux --cc=clang 
--prefix=/home/jullien/tinycc/static
Binary directory    /home/jullien/tinycc/static/bin
TinyCC directory    /home/jullien/tinycc/static/lib/tcc
Library directory   /home/jullien/tinycc/static/lib
Include directory   /home/jullien/tinycc/static/include
Manual directory    /home/jullien/tinycc/static/share/man
Info directory      /home/jullien/tinycc/static/share/info
Doc directory       /home/jullien/tinycc/static/share/doc
Source path         /home/jullien/tinycc
C compiler          clang (10.0)
Target OS           OpenBSD
CPU                 x86_64
Config              BSD ldl=no strip selinux
Creating config.mak and config.h
...
------------ test3 ------------
--- test.ref    Mon Jan  4 08:35:32 2021
+++ test.out3   Mon Jan  4 08:35:32 2021
@@ -573,7 +573,7 @@
 Test 1.0 / x != 1.0 / -x returns 1 (should be 1).
 Test 1.0 / x != 1.0 / +y returns 1 (should be 1).
 Test 1.0 / x != 1.0 / -y returns 0 (should be 0).
-nantest: nan -nan
+nantest: -nan nan
 testing 'double'
 0 1 1 0 0 1
 1.000000 2.500000 3.500000 -1.500000 2.500000 0.400000 -1.000000
@@ -613,7 +613,7 @@
 Test 1.0 / x != 1.0 / -x returns 1 (should be 1).
 Test 1.0 / x != 1.0 / +y returns 1 (should be 1).
 Test 1.0 / x != 1.0 / -y returns 0 (should be 0).
-nantest: nan -nan
+nantest: -nan nan
 testing 'long double'
 0 1 1 0 0 1
 1.000000 2.500000 3.500000 -1.500000 2.500000 0.400000 -1.000000
@@ -653,7 +653,7 @@
 Test 1.0 / x != 1.0 / -x returns 1 (should be 1).
 Test 1.0 / x != 1.0 / +y returns 1 (should be 1).
 Test 1.0 / x != 1.0 / -y returns 0 (should be 0).
-nantest: nan -nan
+nantest: -nan nan
 1.200000 3.400000 -5.600000
 2.120000 0.500000 23000000000.000000
 da=123.000000
gmake[2]: *** [Makefile:128: test3] Error 1
------------ memtest ------------

-----Original Message-----
From: Tinycc-devel [mailto:tinycc-devel-bounces+eligis=orange.fr@nongnu.org] On 
Behalf Of Christian Jullien
Sent: Monday, January 04, 2021 07:18
To: tinycc-devel@nongnu.org
Subject: Re: [Tinycc-devel] x86_64 tcc doesn't set sign bit on NaNs

Hi Michael,
The fix does not make the test suite happy, at least, on Linux Aarch64 when 
clang is used (gcc and tcc are Ok):

./configure --strip-binaries --with-selinux --cc=clang 
--prefix=/home/jullien/tinycc/static
Binary directory    /home/jullien/tinycc/static/bin
TinyCC directory    /home/jullien/tinycc/static/lib/tcc
Library directory   /home/jullien/tinycc/static/lib
Include directory   /home/jullien/tinycc/static/include
Manual directory    /home/jullien/tinycc/static/share/man
Info directory      /home/jullien/tinycc/static/share/info
Doc directory       /home/jullien/tinycc/static/share/doc
Source path         /home/jullien/tinycc
C compiler          clang (11.0)
Target OS           Linux
CPU                 aarch64
Config              strip selinux
Creating config.mak and config.h

...

------------ test3 ------------
--- test.ref    2021-01-04 06:54:01.523848942 +0100
+++ test.out3   2021-01-04 06:54:03.523846911 +0100
@@ -653,7 +653,7 @@
 Test 1.0 / x != 1.0 / -x returns 1 (should be 1).
 Test 1.0 / x != 1.0 / +y returns 1 (should be 1).
 Test 1.0 / x != 1.0 / -y returns 0 (should be 0).
-nantest: nan nan
+nantest: nan -nan
 1.200000 3.400000 -5.600000
 2.120000 0.500000 23000000000.000000
 da=123.000000
gmake[2]: *** [Makefile:128: test3] Error 1


-----Original Message-----
From: Tinycc-devel [mailto:tinycc-devel-bounces+eligis=orange.fr@nongnu.org] On 
Behalf Of Michael Matz
Sent: Monday, January 04, 2021 04:59
To: tinycc-devel@nongnu.org
Subject: Re: [Tinycc-devel] x86_64 tcc doesn't set sign bit on NaNs

Hello,

On Mon, 4 Jan 2021, Vincent Lefevre wrote:

>> -----------------------------
>> #include <stdio.h>
>> #include <math.h>
>> #include <stdlib.h>
>> 
>> int main(int argc, char **argv)
>> {
>>      double d = strtod("-nan", NULL);
>>      d = -d;
>>      printf("%g, signbit(d) = %d\n", d, signbit(d));
>>      return 0;
>> }
>> -----------------------------
>> 
>> Results:
>>
>>      $ gcc foo.c -o foo && ./foo
>>      -nan, signbit(d) = 1
>>
>>      $ tcc foo.c -o foo2 && ./foo2
>>      nan, signbit(d) = 0
>> 
>> I get the same results as gcc with clang and pcc. tcc is the outlier.
>
> AFAIK, the status of the sign bit of a NaN is unspecified, except
> for some particular functions, but not strtod. So I don't see a
> bug in tcc.
>
> Note: for GCC, there's an inconsistency between your testcase
> and the result.

Yeah, I think that's merely a typo in Arnolds email.  The inconsistency is 
there, applying unary '-' to a NaN doesn't change the sign of it in TCC.

While the interpretation of sign bits in NaNs isn't specified in 
IEEE754/854/P754, its existence is a given (in particular it talks about 
"the sign of a NaN", in order to say that their interpretation isn't 
determined :) )

Further IEEE754 recommends implementations to provide a negate(x) 
operation that copies x with reversed sign, that is to work on NaNs (and 
due to copysign needs to have observable behaviour).  C99 and up specify 
that the unary '-' operator maps to that operation.

So, I think it's pretty clear, that whatever the sign bit of NaNs is 
supposed to mean, it must be switchable by unary '-' when IEEE754 
conformance is claimed.  We currently don't claim so, but we aim for it if 
possible :)

So the current "-0.0-x" expansion of unary '-' needs a change.  It turned 
out to be a bit uglier than I wished for, but alas, fixed in mob.

Thanks for the report, Arnold.


Ciao,
Michael.

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