[flang] Correct off-by-one error in SET_EXPONENT

[klausler]

Summary

SET_EXPONENT is returning values that are too large by a factor of two.

[klausler]

Fix both SET_EXPONENT and test.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/runtime/numeric.cpp

 template <typename T> inline T SetExponent(T x, std::int64_t p) {
   if (std::isnan(x)) {
     return x; // NaN -> same NaN
   } else if (std::isinf(x)) {
     return std::numeric_limits<T>::quiet_NaN(); // +/-Inf -> NaN
   } else if (x == 0) {
     return 0; // 0 -> 0
   } else {
-    int expo{std::ilogb(x)};
+    int expo{std::ilogb(x) + 1};
     auto ip{static_cast<int>(p - expo)};
     if (ip != p - expo) {
       ip = p < 0 ? std::numeric_limits<int>::min()
                  : std::numeric_limits<int>::max();
     }
     return std::ldexp(x, ip); // x*2**(p-e)
   }
 }

flang/unittests/Runtime/Numeric.cpp

EXPECT_TRUE(
       std::isinf(RTNAME(Scale4)(std::numeric_limits<Real<4>>::infinity(), 1)));
   EXPECT_TRUE(
       std::isnan(RTNAME(Scale8)(std::numeric_limits<Real<8>>::quiet_NaN(), 1)));
 }
 
 TEST(Numeric, SetExponent) {
   EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{0}, 0), 0);
   EXPECT_EQ(RTNAME(SetExponent8)(Real<8>{0}, 666), 0);
-  EXPECT_EQ(RTNAME(SetExponent8)(Real<8>{3.0}, 0), 1.5);
-  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, 0), 1.0);
-  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, 1), 2.0);
-  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, -1), 0.5);
+  EXPECT_EQ(RTNAME(SetExponent8)(Real<8>{3.0}, 0), 0.75);
+  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, 0), 0.5);
+  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, 1), 1.0);
+  EXPECT_EQ(RTNAME(SetExponent4)(Real<4>{1.0}, -1), 0.25);
   EXPECT_TRUE(std::isnan(
       RTNAME(SetExponent4)(std::numeric_limits<Real<4>>::infinity(), 1)));
   EXPECT_TRUE(std::isnan(
       RTNAME(SetExponent8)(std::numeric_limits<Real<8>>::quiet_NaN(), 1)));
 }
 
 TEST(Numeric, Spacing) {
   EXPECT_EQ(RTNAME(Spacing8)(Real<8>{0}), std::numeric_limits<Real<8>>::min());
   EXPECT_EQ(RTNAME(Spacing4)(Real<4>{3.0}), std::ldexp(Real<4>{1.0}, -22));
   EXPECT_TRUE(
       std::isnan(RTNAME(Spacing4)(std::numeric_limits<Real<4>>::infinity())));
   EXPECT_TRUE(
       std::isnan(RTNAME(Spacing8)(std::numeric_limits<Real<8>>::quiet_NaN())));
 }