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* [Bug target/113578] New: Incorrect signbit for -nan on RISC-V
@ 2024-01-24 11:40 redi at gcc dot gnu.org
2024-01-24 11:41 ` [Bug target/113578] Incorrect sign printed " redi at gcc dot gnu.org
` (9 more replies)
0 siblings, 10 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-01-24 11:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
Bug ID: 113578
Summary: Incorrect signbit for -nan on RISC-V
Product: gcc
Version: 11.4.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
CC: doko at gcc dot gnu.org
Target Milestone: ---
Target: riscv64-linux-gnu
int main()
{
float f = std::bit_cast<float>(0xfff80000);
__builtin_printf("%f %d\n", f, __builtin_signbit(f));
}
With no optimization this prints:
nan 1
With optimization it prints:
-nan 1
i.e. signbit is always set, but sometimes the nan is printed as positive.
g++ -v:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/riscv64-linux-gnu/11/lto-wrapper
Target: riscv64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.4.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=riscv64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-libitm --disable-libquadmath
--disable-libquadmath-support --enable-plugin --enable-default-pie
--with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --disable-multilib --with-arch=rv64gc --with-abi=lp64d
--enable-checking=release --build=riscv64-linux-gnu --host=riscv64-linux-gnu
--target=riscv64-linux-gnu --with-build-config=bootstrap-lto-lean
--enable-link-serialization=4
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)
Originally reported to
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1059340
I've only tested this with Ubuntu's gcc, I haven't tried a build of upstream
GCC, nor a newer version. Sorry if it's Debian specific.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
@ 2024-01-24 11:41 ` redi at gcc dot gnu.org
2024-01-24 12:28 ` jsm28 at gcc dot gnu.org
` (8 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-01-24 11:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|Incorrect signbit for -nan |Incorrect sign printed for
|on RISC-V |-nan on RISC-V
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It could also be a glibc bug for RISC-V of course.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
2024-01-24 11:41 ` [Bug target/113578] Incorrect sign printed " redi at gcc dot gnu.org
@ 2024-01-24 12:28 ` jsm28 at gcc dot gnu.org
2024-01-24 17:34 ` pinskia at gcc dot gnu.org
` (7 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: jsm28 at gcc dot gnu.org @ 2024-01-24 12:28 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #2 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
If a conversion from float to double (for passing in variable arguments) occurs
at runtime on RISC-V, that will produce a positive-signed NaN (that's what the
RISC-V floating-point instructions do). Cf.
https://sourceware.org/bugzilla/show_bug.cgi?id=29501 (for strfrom functions).
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
2024-01-24 11:41 ` [Bug target/113578] Incorrect sign printed " redi at gcc dot gnu.org
2024-01-24 12:28 ` jsm28 at gcc dot gnu.org
@ 2024-01-24 17:34 ` pinskia at gcc dot gnu.org
2024-01-24 17:42 ` redi at gcc dot gnu.org
` (6 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-01-24 17:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
and since __builtin_printf is variadic function, floats are promoted to double
causing the removal of the payload of NaN on RISCV at runtime ...
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (2 preceding siblings ...)
2024-01-24 17:34 ` pinskia at gcc dot gnu.org
@ 2024-01-24 17:42 ` redi at gcc dot gnu.org
2024-01-24 17:56 ` redi at gcc dot gnu.org
` (5 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-01-24 17:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
So the reason the sign isn't lost for -O1 is that the conversion to double
happens at compile-time and a negative nan is compiled into the executable,
rather than being converted by the processor at runtime.
Huh, OK. I guess Matthias can close his Debian bug then.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (3 preceding siblings ...)
2024-01-24 17:42 ` redi at gcc dot gnu.org
@ 2024-01-24 17:56 ` redi at gcc dot gnu.org
2024-05-10 11:48 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-01-24 17:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Maybe we could make it work for std::ostream with something like:
--- a/libstdc++-v3/include/std/ostream
+++ b/libstdc++-v3/include/std/ostream
@@ -233,7 +233,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 117. basic_ostream uses nonexistent num_put member functions.
+#ifdef __riscv
+ return _M_insert(__builtin_copysign((double)__f,
+ (double)-!!__builtin_signbit(__f));
+#else
return _M_insert(static_cast<double>(__f));
+#endif
}
__ostream_type&
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (4 preceding siblings ...)
2024-01-24 17:56 ` redi at gcc dot gnu.org
@ 2024-05-10 11:48 ` redi at gcc dot gnu.org
2024-05-10 21:32 ` andrew at sifive dot com
` (3 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-05-10 11:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Joseph S. Myers from comment #2)
> If a conversion from float to double (for passing in variable arguments)
> occurs at runtime on RISC-V, that will produce a positive-signed NaN (that's
> what the RISC-V floating-point instructions do). Cf.
> https://sourceware.org/bugzilla/show_bug.cgi?id=29501 (for strfrom
> functions).
This seems to happen on Apple M1 chips too.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (5 preceding siblings ...)
2024-05-10 11:48 ` redi at gcc dot gnu.org
@ 2024-05-10 21:32 ` andrew at sifive dot com
2024-05-13 11:09 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: andrew at sifive dot com @ 2024-05-10 21:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
Andrew Waterman <andrew at sifive dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |andrew at sifive dot com
--- Comment #7 from Andrew Waterman <andrew at sifive dot com> ---
In the case of the M1, I don't think the problem is that the sign bit isn't
making it to printf; I think printf is choosing not to print the sign. The
ARMv8 fcvt instruction preserves the sign bit when converting NaNs (unless the
processor is in default-NaN mode, which Mac OS does not seem to use).
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (6 preceding siblings ...)
2024-05-10 21:32 ` andrew at sifive dot com
@ 2024-05-13 11:09 ` redi at gcc dot gnu.org
2024-05-13 22:36 ` andrew at sifive dot com
2024-05-15 11:13 ` redi at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-05-13 11:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Hmm, you could be right. It looks like the printf in Apple libc doesn't print
"-nan" even when not converting from float to double.
According to
https://developer.r-project.org/Blog/content/post/2020-11-02-apple-silicon.html
the Apple silicon NaN payloads are not propagated by default, but I don't know
if that means the signbit is not propagated. I assumed it meant they were
running in default NaN mode.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (7 preceding siblings ...)
2024-05-13 11:09 ` redi at gcc dot gnu.org
@ 2024-05-13 22:36 ` andrew at sifive dot com
2024-05-15 11:13 ` redi at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: andrew at sifive dot com @ 2024-05-13 22:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
--- Comment #9 from Andrew Waterman <andrew at sifive dot com> ---
For my M1 running Ventura 13.6, NaN payloads _are_ propagated, sign bit
included. This test prints fffc0080:
int main()
{
volatile long long ll = 0xffff801000000000;
volatile double d;
memcpy((char*)&d, (char*)&ll, sizeof(ll));
volatile float f = d;
volatile int i;
memcpy((char*)&i, (char*)&f, sizeof(i));
printf("%x\n", i);
return 0;
}
So perhaps Apple has switched from default-NaN mode to NaN-propagating mode in
the time since that R blog post was written.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug target/113578] Incorrect sign printed for -nan on RISC-V
2024-01-24 11:40 [Bug target/113578] New: Incorrect signbit for -nan on RISC-V redi at gcc dot gnu.org
` (8 preceding siblings ...)
2024-05-13 22:36 ` andrew at sifive dot com
@ 2024-05-15 11:13 ` redi at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2024-05-15 11:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113578
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2024-05-15
Resolution|INVALID |---
Status|RESOLVED |REOPENED
Ever confirmed|0 |1
--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm reopening this to deal with it as a libstdc++ bug, because LWG seems to
support changing the standard so that this Just Works.
https://cplusplus.github.io/LWG/issue4101
^ permalink raw reply [flat|nested] 11+ messages in thread
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