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powerpc: fix check-before-set in SET_RESTORE_ROUND

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A performance regression was introduced by commit
84d74e427a "powerpc: Cleanup fenv_private.h".

In the powerpc implementation of SET_RESTORE_ROUND, there is the
following code in the "SET" function (slightly simplified):
--
  old.fenv = fegetenv_register ();

  new.l = (old.l & _FPU_MASK_TRAPS_RN) | r; (1)

  if (new.l != old.l)                       (2)
    {
      if ((old.l & _FPU_ALL_TRAPS) != 0)
        (void) __fe_mask_env ();
      fesetenv_register (new.fenv);         (3)
--

Line (1) sets the value of "new" to the current value of FPSCR,
but masks off summary bits, exceptions, non-IEEE mode, and
rounding mode, then ORs in the new rounding mode.

Line (2) compares this new value to the current value in order to
avoid setting a new value in the FPSCR (line (3)) unless something
significant has changed (exception enables or rounding mode).

The summary bits are not germane to the comparison, but are cleared
in "new" and preserved in "old", resulting in false negative
comparisons, and unnecessarily setting the FPSCR in those cases
with associated negative performance impacts.

The solution is to treat the summaries identically for "new" and "old":
- save them in SET
- leave them alone otherwise
- restore the saved values in RESTORE

Also minor changes:
- expand _FPU_MASK_RN to 64bit hex, to match other MASKs
- treat bit 52 (left-to-right) as reserved (since it is)

	* sysdeps/powerpc/fpu/fenv_private.h (_FPU_MASK_TRAPS_RN):
	(_FPU_MASK_FRAC_INEX_RET_CC): Fix masks to more properly handle
	summary bits.
	(_FPU_MASK_RN): Expand _FPU_MASK_RN to 64bit hex.
	(_FPU_MASK_NOT_RN_NI): Treat bit 52 (left-to-right) as reserved.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
This commit is contained in:
Paul Clarke 2017-10-18 12:02:02 -02:00 committed by Tulio Magno Quites Machado Filho
parent 508b1e71a3
commit 346729f66b
2 changed files with 14 additions and 7 deletions
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8
ChangeLog
View File

@ -1,3 +1,11 @@
2017-10-18 Paul A. Clarke <pc@us.ibm.com>
* sysdeps/powerpc/fpu/fenv_private.h (_FPU_MASK_TRAPS_RN):
(_FPU_MASK_FRAC_INEX_RET_CC): Fix masks to more properly handle
summary bits.
(_FPU_MASK_RN): Expand _FPU_MASK_RN to 64bit hex.
(_FPU_MASK_NOT_RN_NI): Treat bit 52 (left-to-right) as reserved.
2017-10-18 Mike FABIAN <mfabian@redhat.com>
[BZ #16777]

13
sysdeps/powerpc/fpu/fenv_private.h
View File

@ -28,17 +28,16 @@
| _FPU_MASK_XM | _FPU_MASK_IM)
/* Mask the rounding mode bits. */
#define _FPU_MASK_RN (~0x3)
#define _FPU_MASK_RN 0xfffffffffffffffcLL
/* Mask everything but the rounding moded and non-IEEE arithmetic flags. */
#define _FPU_MASK_NOT_RN_NI 0xffffffff00000007LL
/* Mask everything but the rounding modes and non-IEEE arithmetic flags. */
#define _FPU_MASK_NOT_RN_NI 0xffffffff00000807LL
/* Mask restore rounding mode and exception enabled. */
#define _FPU_MASK_TRAPS_RN 0xffffffff1fffff00LL
#define _FPU_MASK_TRAPS_RN 0xffffffffffffff00LL
/* Mask exception enable but fraction rounded/inexact and FP result/CC
bits. */
#define _FPU_MASK_FRAC_INEX_RET_CC 0xffffffff1ff80fff
/* Mask FP result flags, preserve fraction rounded/inexact bits. */
#define _FPU_MASK_FRAC_INEX_RET_CC 0xfffffffffff80fffLL
static __always_inline void
__libc_feholdbits_ppc (fenv_t *envp, unsigned long long mask,