Fix incorrect hypotl(3) result with subnormal numbers

This adjusts the factor used to scale the subnormal numbers, so it
becomes the right value after adjusting its exponent. Thanks to Steve
Kargl for finding the most elegant fix.

Also enable the hypot tests, and add a test case for this bug.

PR:		253313

(cherry picked from commit d3338f3)

Fix lib/msun/test builds on platforms without 80-bit long doubles

After d3338f3, the lib/msun test case
'hypotl_near_underflow' would fail to compile on platforms where long
doubles weren't 80 bit, like on x86. Disable this particular test on
such platforms for now.

PR:		253313

(cherry picked from commit 2512066)

contrib/netbsd-tests/lib/libm/t_hypot.c

@@ -70,12 +70,36 @@ ATF_TC_BODY(pr50698, tc)
 	ATF_CHECK(!isnan(val));
 }
 
+#if __LDBL_MANT_DIG__ == 64
+ATF_TC(hypotl_near_underflow);
+ATF_TC_HEAD(hypotl_near_underflow, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test hypotl near underflow");
+}
+
+ATF_TC_BODY(hypotl_near_underflow, tc)
+{
+	volatile long double a = 0x1.b2933cafa0bb7p-16383L;
+	volatile long double b = 0x1.fffffffffffffp-16351L;
+	volatile long double e = 0x1.fffffffffffffp-16351L;
+	volatile long double ulp = __LDBL_EPSILON__;
+
+	volatile long double val = hypotl(a, b);
+
+	ATF_CHECK(!isinf(val));
+	ATF_CHECK(fabsl(val - e) <= 2 * ulp);
+}
+#endif /* __LDBL_MANT_DIG__ == 64 */
+
 ATF_TP_ADD_TCS(tp)
 {
 
 	ATF_TP_ADD_TC(tp, hypot_integer);
 	ATF_TP_ADD_TC(tp, hypotf_integer);
 	ATF_TP_ADD_TC(tp, pr50698);
+#if __LDBL_MANT_DIG__ == 64
+	ATF_TP_ADD_TC(tp, hypotl_near_underflow);
+#endif /* __LDBL_MANT_DIG__ == 64 */
 
 	return atf_no_error();
 }

lib/msun/src/e_hypotl.c

@@ -82,7 +82,7 @@ hypotl(long double x, long double y)
 	        man_t manh, manl;
 		GET_LDBL_MAN(manh,manl,b);
 		if((manh|manl)==0) return a;
-		t1=0;
+		t1=1;
 		SET_HIGH_WORD(t1,ESW(MAX_EXP-2));	/* t1=2^(MAX_EXP-2) */
 		b *= t1;
 		a *= t1;

lib/msun/tests/Makefile

@@ -31,6 +31,7 @@ NETBSD_ATF_TESTS_C+=	erf_test
 NETBSD_ATF_TESTS_C+=	exp_test
 NETBSD_ATF_TESTS_C+=	fmod_test
 NETBSD_ATF_TESTS_C+=	fe_round_test
+NETBSD_ATF_TESTS_C+=	hypot_test
 NETBSD_ATF_TESTS_C+=	infinity_test
 NETBSD_ATF_TESTS_C+=	ilogb_test
 NETBSD_ATF_TESTS_C+=	ldexp_test