/* * The C99 standard intends x+I*y to be used for this, but x+I*y is * currently unusable because gcc introduces many overflow, * underflow, sign and efficiency bugs by rewriting I*y as * (0.0+I)*(y+0.0*I) and laboriously computing the full complex product. * In particular, I*Inf is corrupted to NaN+I*Inf, and I*-0 is corrupted * to -0.0+I*0.0. */ #include <complex.h> #include <math.h> #include <stdio.h> int main(void) { double complex z; double x, y; x = 0.; y = 1. / x; x = copysign(x, -1.); /* z = 0 + i (-0) */ z = I * x; printf("%e %e\n", creal(z), cimag(z)); /* z = 0 + i Inf */ z = I * y; printf("%e %e\n", creal(z), cimag(z)); } kargl[223] gcc -o z a.c -lm kargl[224] ./z -0.000000e+00 0.000000e+00 nan inf This bug is in 3.4.4 up to an including mainline.

Subject: Re: New: Complex arithmetic on special cases is incorrect. On Sat, 29 Oct 2005, kargl at gcc dot gnu dot org wrote: > * underflow, sign and efficiency bugs by rewriting I*y as > * (0.0+I)*(y+0.0*I) and laboriously computing the full complex product. Note that the correct form is (0.0+I)*y, since I is (per C99+TC1+TC2) _Complex_I, of complex type (Annex G imaginary types conflicting with the normative standard, unless and until anything changes in this respect following DR#323). But the usual arithmetic conversions as specified in the standard do not convert both operands to complex, so one can be real and one complex. I suspect there are lots of presumptions in the compiler that arithmetic operations such as PLUS_EXPR and MULT_EXPR have both operands of the same type, which would need to be fixed to represent a real*complex product properly.

Hmm, I want to say this is a defect in the standard but what do I know.

(In reply to comment #1) > I suspect there are lots of presumptions in the compiler that arithmetic > operations such as PLUS_EXPR and MULT_EXPR have both operands of the same > type, which would need to be fixed to represent a real*complex product > properly. Or get the front-end emitting the correct code in the first place and not worry about real*complex in the middle-end which is a much better option and safer one.

Subject: Re: Complex arithmetic on special cases is incorrect. On Sat, Oct 29, 2005 at 08:09:45PM -0000, joseph at codesourcery dot com wrote: > > > * underflow, sign and efficiency bugs by rewriting I*y as > > * (0.0+I)*(y+0.0*I) and laboriously computing the full complex product. > > Note that the correct form is (0.0+I)*y, since I is (per C99+TC1+TC2) > _Complex_I, of complex type (Annex G imaginary types conflicting with the > normative standard, unless and until anything changes in this respect > following DR#323). But the usual arithmetic conversions as specified in > the standard do not convert both operands to complex, so one can be real > and one complex. > If I read Annex G correctly, the z = I*inf = NaN + I inf is going to really bad things because the NaN is going to propagate if z is used in further computations. Annex G says z is an infinity.

Confirmed.

Subject: Bug 24581 Author: jsm28 Date: Fri May 8 10:22:08 2009 New Revision: 147281 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=147281 Log: PR c/24581 * c-typeck.c (build_binary_op): Handle arithmetic between one real and one complex operand specially. * tree-complex.c (some_nonzerop): Do not identify a real value as zero if flag_signed_zeros. testsuite: * gcc.dg/torture/complex-sign.h: New header. * gcc.dg/torture/complex-sign-add.c, gcc.dg/torture/complex-sign-mixed-add.c, gcc.dg/torture/complex-sign-mixed-div.c, gcc.dg/torture/complex-sign-mixed-mul.c, gcc.dg/torture/complex-sign-mixed-sub.c, gcc.dg/torture/complex-sign-mul.c, gcc.dg/torture/complex-sign-sub.c: New tests. Added: trunk/gcc/testsuite/gcc.dg/torture/complex-sign-add.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-mixed-add.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-mixed-div.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-mixed-mul.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-mixed-sub.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-mul.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign-sub.c trunk/gcc/testsuite/gcc.dg/torture/complex-sign.h Modified: trunk/gcc/ChangeLog trunk/gcc/c-typeck.c trunk/gcc/testsuite/ChangeLog trunk/gcc/tree-complex.c

Mixed real/complex arithmetic now handles signed zeros properly and GCC will no longer try to second-guess complex/complex arithmetic as having one half real or imaginary just because the imaginary or real part of that half is zero, so signed zeros should be handled correctly within the constraints of not having imaginary types. This may of course not be what you want in that I is of complex type, not imaginary, but imaginary types have ABI implications and are of very doubtful utility apart from these corner cases.

*** Bug 43251 has been marked as a duplicate of this bug. ***

*** Bug 43639 has been marked as a duplicate of this bug. ***

the compiler produce incorrect output also when multiplying pure complex numbers (but not adding them). Using gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4) on x86_64 The outcome of the following code is (inf,0) (-nan,inf) (inf,-nan) instead of the expected (inf,0) (0,inf) (inf,0) --------------------------------------------------- #include <iostream> #include <complex> using namespace std; int main() { complex<double> z; complex<double> z2; complex<double> z3; double a = 0; double b = 1. / a; z = complex<double> (b,a); z2 = complex<double> (0,1); z3 = complex<double> (1,0); std::cout << z << '\n'; z2 = z * z2 ; std::cout << z2 << '\n'; z3 = z * z3 ; }

Note that this specific PR is about *C* not C++. And the issue is supposed to be "RESOLVED FIXED". Thus, I would suggest first trying to reproduce the problem in C too and then either reopen this one or a C++ version (search Bugzilla first for duplicates).

(In reply to comment #12) > Note that this specific PR is about *C* not C++. And the issue is supposed to > be "RESOLVED FIXED". Thus, I would suggest first trying to reproduce the > problem in C too and then either reopen this one or a C++ version (search > Bugzilla first for duplicates). Sorry Paolo, I am a bit confused. If the bug is "RESOLVED FIXED" why on 4.5.1 the outcome of the original program is still -0.000000e+00 0.000000e+00 nan inf

Yes I'm also a bit puzzled, either is just expected behavior or isn't really fixed ;) Myself I was surprised to see you just adding something to the audit trail as if it was just yet another testcase. Anyway, in the meanwhile I double checked that C does exactly the same (in the C++ front-end we have a completely similar piece of code, I'm not surprised), thus let's add in CC Joseph, and ask his opinion before re-opening.

For the original program I get -0.000000e+00 -0.000000e+00 -nan inf which appears correct (if one part of a complex number is an infinity, anything is valid for the other part and the overall value is still an infinity).

On Sun, Nov 21, 2010 at 11:34:46PM +0000, joseph at codesourcery dot com wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24581 > > --- Comment #15 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-11-21 23:33:48 UTC --- > For the original program I get > > -0.000000e+00 -0.000000e+00 > -nan inf > > which appears correct (if one part of a complex number is an infinity, > anything is valid for the other part and the overall value is still an > infinity). > The '-nan inf' is incorrect. The correct answer is '0 inf'.

On Sun, 21 Nov 2010, sgk at troutmask dot apl.washington.edu wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24581 > > --- Comment #16 from Steve Kargl <sgk at troutmask dot apl.washington.edu> 2010-11-21 23:43:10 UTC --- > On Sun, Nov 21, 2010 at 11:34:46PM +0000, joseph at codesourcery dot com wrote: > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24581 > > > > --- Comment #15 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-11-21 23:33:48 UTC --- > > For the original program I get > > > > -0.000000e+00 -0.000000e+00 > > -nan inf > > > > which appears correct (if one part of a complex number is an infinity, > > anything is valid for the other part and the overall value is still an > > infinity). > > > > The '-nan inf' is incorrect. The correct answer is '0 inf'. Annex G does not define the results for complex*complex multiplication to that level of detail, and for the complex*real multiplication we have here it seems entirely correct to have a NaN (sign unspecified) as the real part.

On Sun, Nov 21, 2010 at 11:53:50PM +0000, joseph at codesourcery dot com wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24581 > > Annex G does not define the results for complex*complex multiplication to > that level of detail, and for the complex*real multiplication we have here > it seems entirely correct to have a NaN (sign unspecified) as the real > part. > We've had this discussion before. Annex G, which I do acknowledge as informative, states: The * and / operators satisfy the following infinity properties for all real, imaginary, and complex operands:319) -- if one operand is an infinity and the other operand is a nonzero finite number or an infinity, then the result of the * operator is an infinity; I'll note the above comes from n1124.pdf (page 468). Perhaps, the actual C99 standard says something else. -nan is not an infinity.

On Mon, 22 Nov 2010, sgk at troutmask dot apl.washington.edu wrote: > We've had this discussion before. Annex G, which I do acknowledge > as informative, states: > > The * and / operators satisfy the following infinity properties for > all real, imaginary, and complex operands:319) > > -- if one operand is an infinity and the other operand is a nonzero > finite number or an infinity, then the result of the * operator > is an infinity; > > I'll note the above comes from n1124.pdf (page 468). Perhaps, > the actual C99 standard says something else. > > -nan is not an infinity. That -nan is not an infinity is true but irrelevant, because "A complex or imaginary value with at least one infinite part is regarded as an infinity (even if its other part is a NaN)." (G.3), so the complex result of the multiplication *is* an infinity (with one part NaN and one part infinity, which is a valid representation of complex infinity).

(In reply to comment #19) > On Mon, 22 Nov 2010, sgk at troutmask dot apl.washington.edu wrote: > > > That -nan is not an infinity is true but irrelevant, because "A complex or > imaginary value with at least one infinite part is regarded as an infinity > (even if its other part is a NaN)." (G.3), so the complex result of the > multiplication *is* an infinity (with one part NaN and one part infinity, > which is a valid representation of complex infinity). I guess that I was misleaded by the status FIXED. Following your reasoning INVALID or WONTFIX are probably more accurate STATUS as the behaviour is not a BUG but a possible implementation. As 0 * Inf = NaN on real/double, it follows that for complex ( 0 + I ) * Inf = 0 * Inf + I * Inf = NaN + I * Inf however the implementation is not symmetric as ( 1 + I*0) * Inf = Inf + 0 * I Of course (Inf + 0 * I) and (NaN + I * Inf) are both complex infinities, but the lack of symmetry is inelegant ;-) The table at C99 G.5.1-2 seems to suggest a symmetric behaviour, of course IMHO

If Joseph's comments are correct, and I trust him, then FIXED is the right status, because his patch actually fixed long standing serious issues vs the letter of Annex G.