math: fix scalbn and scalbnf on overflow/underflow

old code was correct only if the result was stored (without the
excess precision) or musl was compiled with -ffloat-store.
(see note 160 in n1570.pdf section 6.8.6.4)

diff --git a/src/math/scalbn.c b/src/math/scalbn.c
index c9c7af8..42a2b29 100644 (file)
--- a/src/math/scalbn.c
+++ b/src/math/scalbn.c
@@ -2,6 +2,8 @@
 
 double scalbn(double x, int n)
 {
+       /* make sure result is stored as double on overflow or underflow */
+       volatile double z;
        double scale;
 
        if (n > 1023) {
@@ -10,8 +12,10 @@ double scalbn(double x, int n)
                if (n > 1023) {
                        x *= 0x1p1023;
                        n -= 1023;
-                       if (n > 1023)
-                               return x * 0x1p1023;
+                       if (n > 1023) {
+                               z = x * 0x1p1023;
+                               return z;
+                       }
                }
        } else if (n < -1022) {
                x *= 0x1p-1022;
@@ -19,10 +23,13 @@ double scalbn(double x, int n)
                if (n < -1022) {
                        x *= 0x1p-1022;
                        n += 1022;
-                       if (n < -1022)
-                               return x * 0x1p-1022;
+                       if (n < -1022) {
+                               z = x * 0x1p-1022;
+                               return z;
+                       }
                }
        }
        INSERT_WORDS(scale, (uint32_t)(0x3ff+n)<<20, 0);
-       return x * scale;
+       z = x * scale;
+       return z;
 }

diff --git a/src/math/scalbnf.c b/src/math/scalbnf.c
index 243dafd..e5c76db 100644 (file)
--- a/src/math/scalbnf.c
+++ b/src/math/scalbnf.c
@@ -2,6 +2,8 @@
 
 float scalbnf(float x, int n)
 {
+       /* make sure result is stored as double on overflow or underflow */
+       volatile float z;
        float scale;
 
        if (n > 127) {
@@ -10,8 +12,10 @@ float scalbnf(float x, int n)
                if (n > 127) {
                        x *= 0x1p127f;
                        n -= 127;
-                       if (n > 127)
-                               return x * 0x1p127f;
+                       if (n > 127) {
+                               z = x * 0x1p127f;
+                               return z;
+                       }
                }
        } else if (n < -126) {
                x *= 0x1p-126f;
@@ -19,10 +23,13 @@ float scalbnf(float x, int n)
                if (n < -126) {
                        x *= 0x1p-126f;
                        n += 126;
-                       if (n < -126)
-                               return x * 0x1p-126f;
+                       if (n < -126) {
+                               z = x * 0x1p-126f;
+                               return z;
+                       }
                }
        }
        SET_FLOAT_WORD(scale, (uint32_t)(0x7f+n)<<23);
-       return x * scale;
+       z = x * scale;
+       return z;
 }