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From: "dlong at cadence dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105071] New: Incorrect code with -Os and complex<double>
Date: Sun, 27 Mar 2022 23:22:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105071

            Bug ID: 105071
           Summary: Incorrect code with -Os and complex<double>
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dlong at cadence dot com
  Target Milestone: ---

Created attachment 52691
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52691&action=3Dedit
.ii file for test case

64-bit Intel, RedHat Enterprise Linux 7.4

$ cat foo.cc
#include <complex>
#include <stdio.h>

using namespace std;

void broken(complex<double> x)
{
  complex<double> x2=3Dx*x;
  complex<double> x3=3Dx2*x;
  complex<double> x4=3Dx3*x;
  complex<double> x5=3Dx4*x;
  complex<double> d=3D1.0+2.0/x+4.0/x2+8.0/x3+16.0/x4+32.0/x5;
  fprintf(stderr, "%e%+ej\n", real(d), imag(d));
}

int main(int argc, char **argv) {
  broken(complex<double>(1.0, 0.000000e+00));
  return 0;
}
$ g++-9 -Wall -o foo foo.cc && ./foo  # correct
6.300000e+01+0.000000e+00j
$ g++-9 -Wall -O2 -o foo foo.cc && ./foo  # also correct
6.300000e+01+0.000000e+00j
$ g++-9 -Wall -Os -o foo foo.cc && ./foo  # wrong
1.000000e+00+0.000000e+00j

If the +32.0/x5 at the end of the computation of d is
omitted, then -Os also gives the correct result.

$ g++-9 -v
Using built-in specs.
COLLECT_GCC=3Dg++-9
COLLECT_LTO_WRAPPER=3D/grid/common/pkgsData/gcc-v9.3.0p3/Linux/RHEL7.0-2017=
-x86_64/libexec/gcc/x86_64-redhat-linux/9.3.0/lto-wrapper
Target: x86_64-redhat-linux
Configured with: /tmp/gcc-v9.3.0p3/gcc.source/configure
--prefix=3D/grid/common/pkgsData/gcc-v9.3.0p3/Linux/RHEL7.0-2017-x86_64
--with-pkgversion=3DCadence --disable-libgcj --enable-threads=3Dposix
--enable-shared --enable-checking=3Drelease --with-system-zlib
--enable-__cxa_atexit --disable-libunwind-exceptions
--with-linker-hash-style=3Dgnu --enable-languages=3Dc,c++,fortran --disable=
-nls
--enable-gnu-unique-object --enable-bootstrap --enable-plugin
--enable-initfini-array --enable-linker-build-id --enable-gnu-indirect-func=
tion
--enable-install-libiberty
--with-ld=3D/grid/common/pkgsData/gcc-v9.3.0p3/Linux/RHEL7.0-2017-x86_64/bi=
n/ld
--with-as=3D/grid/common/pkgsData/gcc-v9.3.0p3/Linux/RHEL7.0-2017-x86_64/bi=
n/as
--build=3Dx86_64-redhat-linux --host=3Dx86_64-redhat-linux --with-tune=3Dge=
neric
--enable-multiarch
Thread model: posix
gcc version 9.3.0 (Cadence)=