[Bug target/60181] constant folding of complex number incorrect

[zimmerma+gcc at loria dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60181

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zimmerma+gcc at loria dot fr

--- Comment #9 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
Here is another example:

$ cat t.c
#include <complex.h>
#include <stdio.h>
#include <stdlib.h>

_Complex double __attribute__ ((noinline))
calc_div (_Complex double a, _Complex double b)
{
  return a / b;
}

_Complex double __attribute__ ((noinline))
calc_mul (_Complex double a, _Complex double b)
{
  return a * b;
}


int
main (int argc, char **argv)
{
  double x, y, z, t;
  _Complex double a, b, c, d;

  x = -1.0689346237980500e+252; y = 7.8846096489686452e-145;
  z = 5.1152592576527620e-318;  t = 6.7327111521288819e+78;
  a = calc_div (x + y * I, z + t * I);
  b = (x + y * I) / (z + t * I);

  printf ("calc_div: %ap + %ap * i\n", creal (a), cimag (a));
  printf ("div:      %ap + %ap * i\n", creal (b), cimag (b));

  x = -4.8798814420289904e-22;  y = -6.4569205261627488e+209;
  z = -1.0918976936190148e+147; t = -8.2942999263695497e-85;
  c = calc_mul (x + y * I, z + t * I);
  d = (x + y * I) * (z + t * I);

  printf ("calc_mul: %ap + %ap * i\n", creal (c), cimag (c));
  printf ("mul:      %ap + %ap * i\n", creal (d), cimag (d));

  return 0;
}

With gcc version 9.2.1 20200123 (Debian 9.2.1-25) on x86_64 I get:

$ gcc -O0 t.c; ./a.out
calc_div: 0x1.5ac8471ecdabp-741p + 0x1.48aa457eda0eep+575p * i
div:      0x1.5ac8471ecdabp-741p + 0x1.48aa457eda0eep+575p * i
calc_mul: -0x1.07a6046b053p+410p + infp * i
mul:      -0x1.07a6046b053p+410p + infp * i

$ gcc -O2 t.c; ./a.out
calc_div: 0x1.5ac8471ecdabp-741p + 0x1.48aa457eda0eep+575p * i
div:      -0x1.4d35f7c831ef5p-746p + 0x1.48aa457eda0eep+575p * i
calc_mul: -0x1.07a6046b053p+410p + infp * i
mul:      -0x1.07a6046b053p+410p + infp * i

For division, the correctly rounded result is the one for "div" with -O2
(negative real part).

For multiplication, all results are incorrectly rounded, we should get
-0x1.07a6046b05347p410 if I'm correct.

I understand there is no accuracy requirement, but I thought with -O2 gcc was
using GNU MPC to get correct rounding, thus with -O2 both the "div" and "mul"
results should be correctly rounded. Why isn't it the case for mul?