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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/102129] -ftrapping-math is broken or badly documented
Date: Mon, 30 Aug 2021 15:00:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102129

--- Comment #1 from Vincent Lef=C3=A8vre <vincent-gcc at vinc17 dot net> ---
Additional surprising behavior...

On the following C code:

void g (void);
double f (void)
{
  double x =3D 0.0, y =3D 0.0;
  double r =3D x / y;
  g ();
  return r;
}

one can see with -ftrapping-math (the default) that a 0.0 / 0.0 is done bef=
ore
the call to g, which is the expected behavior (this time, the order of the
operations has been preserved):

        pxor    %xmm0, %xmm0
        divsd   %xmm0, %xmm0
        movsd   %xmm0, 8(%rsp)
        call    g@PLT
        movsd   8(%rsp), %xmm0

(note that with -fno-trapping-math, a NaN is just generated at compile time=
).

But on the following C code:

void g (void);
double f (void)
{
  double x =3D 1.0, y =3D 3.0;
  double r =3D x / y;
  g ();
  return r;
}

GCC behaves as with -fno-trapping-math, i.e. g is called, then the rounded
value of 1/3 is returned, while there should have been an inexact exception
(the GCC manual explicitly mentions inexact as one of the possible traps).=