math: add fma TODO comments about the underflow issue

The underflow exception is not raised correctly in some
cornercases (see previous fma commit), added comments
with examples for fmaf, fmal and non-x86 fma.

In fmaf store the result before returning so it has the
correct precision when FLT_EVAL_METHOD!=0

diff --git a/src/math/fma.c b/src/math/fma.c
index 89def79..850a4a6 100644 (file)
--- a/src/math/fma.c
+++ b/src/math/fma.c
@@ -441,6 +441,8 @@ double fma(double x, double y, double z)
                /*
                 * There is no need to worry about double rounding in directed
                 * rounding modes.
                /*
                 * There is no need to worry about double rounding in directed
                 * rounding modes.

+                * TODO: underflow is not raised properly, example in downward rounding:
+                * fma(0x1.000000001p-1000, 0x1.000000001p-30, -0x1p-1066)

                 */
                fesetround(oround);
                adj = r.lo + xy.lo;
                 */
                fesetround(oround);
                adj = r.lo + xy.lo;

diff --git a/src/math/fmaf.c b/src/math/fmaf.c
index a1c7f4f..745ee39 100644 (file)
--- a/src/math/fmaf.c
+++ b/src/math/fmaf.c
@@ -49,7 +49,14 @@ float fmaf(float x, float y, float z)
                (hr & 0x7ff00000) == 0x7ff00000 ||  /* NaN */
                result - xy == z ||                 /* exact */
                fegetround() != FE_TONEAREST)       /* not round-to-nearest */
                (hr & 0x7ff00000) == 0x7ff00000 ||  /* NaN */
                result - xy == z ||                 /* exact */
                fegetround() != FE_TONEAREST)       /* not round-to-nearest */

-               return (result);
+       {
+               /*
+               TODO: underflow is not raised correctly, example in
+               downward rouding: fmaf(0x1p-120f, 0x1p-120f, 0x1p-149f)
+               */
+               z = result;
+               return z;
+       }

 
        /*
         * If result is inexact, and exactly halfway between two float values,
 
        /*
         * If result is inexact, and exactly halfway between two float values,


@@ -63,5 +70,6 @@ float fmaf(float x, float y, float z)
        fesetround(FE_TONEAREST);
        if (result == adjusted_result)
                SET_LOW_WORD(adjusted_result, lr + 1);
        fesetround(FE_TONEAREST);
        if (result == adjusted_result)
                SET_LOW_WORD(adjusted_result, lr + 1);

-       return (adjusted_result);
+       z = adjusted_result;
+       return z;

 }
 }

diff --git a/src/math/fmal.c b/src/math/fmal.c
index ccbe434..87e30fc 100644 (file)
--- a/src/math/fmal.c
+++ b/src/math/fmal.c
@@ -262,6 +262,8 @@ long double fmal(long double x, long double y, long double z)
                /*
                 * There is no need to worry about double rounding in directed
                 * rounding modes.
                /*
                 * There is no need to worry about double rounding in directed
                 * rounding modes.

+                * TODO: underflow is not raised correctly, example in downward rounding:
+                * fmal(0x1.0000000001p-16000L, 0x1.0000000001p-400L, -0x1p-16440L)

                 */
                fesetround(oround);
                adj = r.lo + xy.lo;
                 */
                fesetround(oround);
                adj = r.lo + xy.lo;