S390: Fix handling of DXC-byte in FPC-register.

On s390, the DXC(data-exception-code)-byte in FPC(floating-point-control)-
register contains a code of the last occured exception.
If bits 6 and 7 of DXC-byte are zero, the bits 0-5 correspond to the
ieee-exception flag bits.
The current implementation always uses these bits as ieee-exception flag bits.
fetestexcept() reports any exception after the first usage of a
vector-instruction in a process, because it raises an "vector instruction
exception" with DXC-code 0xFE.
This patch fixes the handling of the DXC-byte. The DXC-Byte is only handled
if bits 6 and 7 are zero.

The #define _FPU_RESERVED is extended by the DXC-Byte.
Otherwise the tests math/test-fpucw-static and math/test-fpucw-ieee-static
fails, because DXC-Byte contains the vector instruction exception when reaching
main(). This exception was triggered by strrchr() call in __init_misc().
__init_misc() is called after __setfpucw () in __libc_init_first().

The field __ieee_instruction_pointer in struct fenv_t is renamed to __unused
because it is a relict from commit "Remove PTRACE_PEEKUSER"
(87b9b50f0d4b92248905e95a06a13c513dc45e59) and isn´t used anymore.

ChangeLog:

	[BZ #18610]
	* sysdeps/s390/fpu/bits/fenv.h (fenv_t): Rename
	__ieee_instruction_pointer to __unused.
	* sysdeps/s390/fpu/fesetenv.c (__fesetenv): Remove usage of
	__ieee_instruction_pointer.
	* sysdeps/s390/fpu/fclrexcpt.c (feclearexcept): Fix dxc-field handling.
	* sysdeps/s390/fpu/fgetexcptflg.c (fegetexceptflag): Likewise.
	* sysdeps/s390/fpu/fsetexcptflg.c (fesetexceptflag): Likewise.
	* sysdeps/s390/fpu/ftestexcept.c (fetestexcept): Likewise.
	* sysdeps/s390/fpu/fpu_control.h (_FPU_RESERVED):
	Mark dxc-field as reserved.

1 files changed, 7 insertions, 1 deletions

diff --git a/sysdeps/s390/fpu/fgetexcptflg.c b/sysdeps/s390/fpu/fgetexcptflg.c
index c14dc15c25..0a5a081d30 100644
--- a/sysdeps/s390/fpu/fgetexcptflg.c
+++ b/sysdeps/s390/fpu/fgetexcptflg.c
@@ -27,7 +27,13 @@ fegetexceptflag (fexcept_t *flagp, int excepts)
 
   /* Get the current exceptions.  */
   _FPU_GETCW (temp);
-  newexcepts = (excepts << FPC_DXC_SHIFT) | (excepts << FPC_FLAGS_SHIFT);
+  newexcepts = excepts << FPC_FLAGS_SHIFT;
+  if ((temp & FPC_NOT_FPU_EXCEPTION) == 0)
+    /* Bits 6, 7 of dxc-byte are zero,
+       thus bits 0-5 of dxc-byte correspond to the flag-bits.
+       Evaluate flags and last dxc-exception-code.  */
+    newexcepts |= excepts << FPC_DXC_SHIFT;
+
   *flagp = temp & newexcepts;
 
   /* Success.  */