https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98251
Bug ID: 98251
Summary: libgcc on 32-bit soft-float ARM narrows -NaN
incorrectly
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgcc
Assignee: unassigned at gcc dot gnu.org
Reporter: bbarenblat at gmail dot com
Target Milestone: ---
Created attachment 49749
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49749&action=edit
preprocessed minimal example
On 32-bit ARM without hardware floating point, libgcc narrows a negative NaN
double to a positive NaN float. However, GCC's optimizer assumes that narrowing
conversions preserve sign even in the presence of NaN:
$ cat float.c
#include <math.h>
extern int printf(const char* format, ...);
static const char* pm(double d) {
return signbit(d) ? "-" : "+";
}
int main(void) {
double d = copysign(nan(""), -1.0);
float f = d;
printf("d is %sNaN; f is %sNaN\n", pm(d), pm(f));
}
$ gcc -O0 -o float float.c -lm
$ ./float
d is -NaN; f is +NaN
$ gcc -O1 -o float float.c -lm
$ ./float
d is -NaN; f is -NaN
Examining the assembly from the second invocation shows that GCC has optimized
out the entire computation. In the first invocation, however, the compiled
calls into libgcc, and libgcc/config/arm/ieee754-df.S:1475 always narrows NaNs
to +NaN:
3: @ chech for NAN
mvns r3, r2, asr #21
bne 5f @ simple overflow
orrs r3, xl, xh, lsl #12
do_it ne, tt
movne r0, #0x7f000000
orrne r0, r0, #0x00c00000
RETc(ne) @ return NAN
I've attached the preprocessed version of float.c. Output of 'gcc -v':
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-linux-gnueabi/10/lto-wrapper
Target: arm-linux-gnueabi
Configured with: ../src/configure -v --with-pkgversion='Debian 10.2.1-1'
--with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=arm-linux-gnueabi- --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-sjlj-exceptions --with-arch=armv5te --with-float=soft
--disable-werror --enable-checking=release --build=arm-linux-gnueabi
--host=arm-linux-gnueabi --target=arm-linux-gnueabi
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.1 20201207 (Debian 10.2.1-1)
