https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114050
Bug ID: 114050
Summary: Inconsistency in double/float constant evaluation
between 32 and 64 bit
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: sjh at schilling dot dk
Target Milestone: ---
Created attachment 57491
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57491&action=edit
The test application
In GCC version 13 and later, we've found some inconsistency in the way the
compiler calculates double constants in our code. We originally discovered this
in a rounding unit test in our code, and have extracted the specific
inconsistency in attached double-test.cpp.
When compiling in default 64-bit mode on the x86 platform, we get the
consistent result of previous GCC versions (the output from the test code is
the same constant double value calculated in three different ways, and dumped
as hex as well):
g++ -std=c++23 -m64 -o double-test double-test.cpp
./double-test
Output:
GCC version: 13.2.0
a = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
b = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
c = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
However, compiling the same application in 32-bit mode yields a different
result:
g++ -std=c++23 -m32 -o double-test double-test.cpp
./double-test
Output:
GCC version: 13.2.0
a = -4.06576e-20 ( 00 00 00 00 00 00 e8 bb )
b = 0 ( 00 00 00 00 00 00 00 00 )
c = -4.06576e-20 ( 00 00 00 00 00 00 e8 bb )
If using gnu extensions as the selected standard, we're back to the old,
consistent behavior, also for 32 bits:
g++ -std=gnu++23 -m32 -o double-test double-test.cpp
./double-test
Output:
GCC version: 13.2.0
a = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
b = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
c = -5.42101e-20 ( 00 00 00 00 00 00 f0 bb )
We have only seen the problem in 32 bit mode with one of the strict c++
standards selected (-std=c++*), not with gnu extensions (-std=gnu++*).
Testing various versions on the Compiler Explorer site, the problem seems to
occur as of version 13 of GCC.
g++ -v output:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/schilling/gcc-13.2.0/libexec/gcc/x86_64-pc-linux-gnu/13.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-13.2.0/configure
--prefix=/usr/local/schilling/gcc-13.2.0 --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-languages=c,c++
--enable-multilib --disable-bootstrap --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.2.0 (GCC)
