When doing real by complex multiplications in C++ using std::complex<double>,
the multiplication is transformed to a complex by complex multiplication, even
though the imaginary part is zero for one of the arguments.
This is similar to TR 19953, however there is still a problem even though it is
supposed to be fixed.
Here is a small test program:
#include <complex>
std::complex<double> a, b;
double c;
void f()
{
a = b * c;
}
When compiled with "g++ -O3 -march=pentium4 -mfpmath=sse -S -c test.C", the
following is produced (from test.s):
_Z1fv:
.LFB1857:
pushl %ebp
.LCFI0:
movl %esp, %ebp
.LCFI1:
movsd b+8, %xmm3
movsd b, %xmm2
movsd c, %xmm4
pxor %xmm5, %xmm5
movapd %xmm2, %xmm0
mulsd %xmm5, %xmm0
movapd %xmm4, %xmm1
mulsd %xmm3, %xmm1
addsd %xmm1, %xmm0
movsd %xmm0, a+8
mulsd %xmm4, %xmm2
mulsd %xmm5, %xmm3
subsd %xmm3, %xmm2
movsd %xmm2, a
popl %ebp
ret
I.e, the real value c is still converted to a complex value and a full
multiplication is done.
I'm using the following version of gcc:
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.0-20050312/configure --prefix=/home/fredrik/gcc
--enable-languages=c,c++
Thread model: posix
gcc version 4.0.0 20050312 (prerelease)
--
Summary: Real by complex multiplications perform unnecessary
operations
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: fredrik dot huss at home dot se
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20610
