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Date: Mon, 3 Jun 2019 13:10:57 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: Agner Fog <agner AT agner DOT org>
Cc: cygwin AT cygwin DOT com
Subject: Re: Bug report. Clang sqrtl(-1) causes access violation
Message-ID: <20190603111057.GE3437@calimero.vinschen.de>
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Mail-Followup-To: Agner Fog <agner AT agner DOT org>, cygwin AT cygwin DOT com
References: <c80240c2-a30e-41b5-7635-cf31f54741d1 AT agner DOT org>
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On May 10 14:57, Agner Fog wrote:
> Bug description:
>=20
> The sqrtl function under Clang causes an access violation when the argume=
nt
> is negative.
>=20
> This error occurs only under Cygwin.
>=20
> This error occurs only with the sqrtl function, not with sqrt or sqrtf
>=20
> Attached:
>=20
> sqrt.cpp: program to reproduce the error. Compile clang sqrt.cpp and run
> ./a.exe

Thanks for the testcase.

This looks like a bug in clang-5.0.  It does not occur when building the
testcase with gcc, nor does it occur when building with clang-4.0.

Looking with GDB, it seems the conversion from float to long double is
screwed up.  The sequence converting float to long double prior to
calling sqrtl is very different:

With gcc:

  Thread 1 "sqrtl" hit Temporary breakpoint 1, main () at sqrtl.c:6
  6           float b =3D sqrtl(a);
  (gdb) s
  sqrtl (x=3D-1)
      at /home/corinna/src/cygwin/vanilla/winsup/cygwin/math/sqrt.def.h:69
  69        __FLT_TYPE res =3D __FLT_CST (0.0);
  (gdb) n
  70        int x_class =3D fpclassify (x);
  (gdb) p x
  $1 =3D -1
  (gdb) x/4x &x
  0xffffcaf0:     0x00000000      0x80000000      0x0004bfff      0x00000006

  =3D> 0x100401093 <main+19>:       lea    -0x20(%rbp),%rax
  =3D> 0x100401097 <main+23>:       fstpt  -0x30(%rbp)
  =3D> 0x10040109a <main+26>:       lea    -0x30(%rbp),%rdx
  =3D> 0x10040109e <main+30>:       mov    %rax,%rcx
  =3D> 0x1004010a1 <main+33>:       callq  0x100401110 <sqrtl>

With clang:

  Thread 1 "sqrtl" hit Temporary breakpoint 1, main () at sqrtl.c:6
  6           float b =3D sqrtl(a);
  (gdb) s
  sqrtl (x=3D<invalid float value>)
      at /home/corinna/src/cygwin/vanilla/winsup/cygwin/math/sqrt.def.h:69
  69        __FLT_TYPE res =3D __FLT_CST (0.0);
  (gdb) n
  70        int x_class =3D fpclassify (x);
  (gdb) p x
  $1 =3D <invalid float value>
  (gdb) x/4x &x
  0xffffcb10:     0x00000000      0x00000000      0x00040524      0x00000006

  =3D> 0x100401096 <main+22>:       movabs $0x100402010,%rax
  =3D> 0x1004010a0 <main+32>:       flds   (%rax)
  =3D> 0x1004010a2 <main+34>:       mov    %rsp,%rax
  =3D> 0x1004010a5 <main+37>:       fstpt  0x20(%rax)
  =3D> 0x1004010a8 <main+40>:       callq  0x100401120 <sqrtl>


Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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