[PATCH v3] Fix for powerpc64 long double complex divide failure

Patrick McGehearty patrick.mcgehearty@oracle.com
Thu Aug 12 16:03:13 GMT 2021

This patch resolves the failure of powerpc64 long double complex divide
in native ibm long double format after the patch "Practical improvement
to libgcc complex divide".

The new code uses the following macros which are intended to be mapped
to appropriate values according to the underlying hardware representation.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101104

RBIG     a value near the maximum representation
RMIN     a value near the minimum representation
         (but not in the subnormal range)
RMIN2    a value moderately less than 1
RMINSCAL the inverse of RMIN2
RMAX2    RBIG * RMIN2  - a value to limit scaling to not overflow

When "long double" values were not using the IEEE 128-bit format but
the traditional IBM 128-bit, the previous code used the LDBL values
which caused overflow for RMINSCAL. The new code uses the DBL values.

RBIG  LDBL_MAX = 0x1.fffffffffffff800p+1022
      DBL_MAX  = 0x1.fffffffffffff000p+1022

RMIN  LDBL_MIN = 0x1.0000000000000000p-969
RMIN  DBL_MIN  = 0x1.0000000000000000p-1022

RMIN2 LDBL_EPSILON = 0x0.0000000000001000p-1022 = 0x1.0p-1074
RMIN2 DBL_EPSILON  = 0x1.0000000000000000p-52

RMINSCAL 1/LDBL_EPSILON = inf (1.0p+1074 does not fit in IBM 128-bit).
         1/DBL_EPSILON  = 0x1.0000000000000000p+52

RMAX2 = RBIG * RMIN2 = 0x1.fffffffffffff800p-52
        RBIG * RMIN2 = 0x1.fffffffffffff000p+970

The MAX and MIN values have only modest changes since the maximum and
minimum values are about the same as for double precision.  The
EPSILON field is considerably different. Due to how very small values
can be represented in the lower 64 bits of the IBM 128-bit floating
point, EPSILON is extremely small, so far beyond the desired value
that inversion of the value overflows and even without the overflow,
the RMAX2 is so small as to eliminate most usage of the test.

Instead of just replacing the use of KF_EPSILON with DF_EPSILON, we
replace all uses of KF_* with DF_*. Since the exponent fields are
essentially the same, we gain the positive benefits from the new
formula while avoiding all under/overflow issues in the #defines.

The change has been tested on gcc135.fsffrance.org and gains the
expected improvements in accuracy for long double complex divide.

	PR target/101104
	* config/rs6000/_divkc3.c (RBIG, RMIN, RMIN2, RMINSCAL, RMAX2):
	Use more correct values for native IBM 128-bit.
 libgcc/config/rs6000/_divkc3.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libgcc/config/rs6000/_divkc3.c b/libgcc/config/rs6000/_divkc3.c
index a1d29d2..2b229c8 100644
--- a/libgcc/config/rs6000/_divkc3.c
+++ b/libgcc/config/rs6000/_divkc3.c
@@ -38,10 +38,10 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #ifndef __LONG_DOUBLE_IEEE128__
-#define RBIG   (__LIBGCC_KF_MAX__ / 2)
-#define RMIN   (__LIBGCC_KF_MIN__)
-#define RMIN2  (__LIBGCC_KF_EPSILON__)
+#define RBIG   (__LIBGCC_DF_MAX__ / 2)
+#define RMIN   (__LIBGCC_DF_MIN__)
+#define RMIN2  (__LIBGCC_DF_EPSILON__)
 #define RMAX2  (RBIG * RMIN2)
 #define RBIG   (__LIBGCC_TF_MAX__ / 2)

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