[libc] Fix the overflow check condition of ldexp.

Targeted tests have been added.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D91752

libc/test/src/math/LdExpTest.h

@@ -112,6 +112,30 @@ class LdExpTestTemplate : public __llvm_libc::testing::Test {
     T x = NormalFloat(-FPBits::exponentBias + 1, 2 * NormalFloat::one - 1, 0);
     ASSERT_FP_EQ(func(x, -1), x / 2);
     ASSERT_FP_EQ(func(-x, -1), -x / 2);
+
+    // Start with a normal number high exponent but pass a very low number for
+    // exp. The result should be a subnormal number.
+    x = NormalFloat(FPBits::exponentBias, NormalFloat::one, 0);
+    int exp = -FPBits::maxExponent - 5;
+    T result = func(x, exp);
+    FPBits resultBits(result);
+    ASSERT_FALSE(resultBits.isZero());
+    // Verify that the result is indeed subnormal.
+    ASSERT_EQ(resultBits.exponent, uint16_t(0));
+    // But if the exp is so less that normalization leads to zero, then
+    // the result should be zero.
+    result = func(x, -FPBits::maxExponent - int(mantissaWidth) - 5);
+    ASSERT_TRUE(FPBits(result).isZero());
+
+    // Start with a subnormal number but pass a very high number for exponent.
+    // The result should not be infinity.
+    x = NormalFloat(-FPBits::exponentBias + 1, NormalFloat::one >> 10, 0);
+    exp = FPBits::maxExponent + 5;
+    ASSERT_EQ(isinf(func(x, exp)), 0);
+    // But if the exp is large enough to oversome than the normalization shift,
+    // then it should result in infinity.
+    exp = FPBits::maxExponent + 15;
+    ASSERT_NE(isinf(func(x, exp)), 0);
   }
 };
 

libc/utils/FPUtil/ManipulationFunctions.h

@@ -124,14 +124,17 @@ static inline T ldexp(T x, int exp) {
     return x;
 
   // NormalFloat uses int32_t to store the true exponent value. We should ensure
-  // that adding |exp| to it does not lead to integer rollover. But, we |exp|
+  // that adding |exp| to it does not lead to integer rollover. But, if |exp|
   // value is larger the exponent range for type T, then we can return infinity
-  // early.
-  if (exp > FPBits<T>::maxExponent)
+  // early. Because the result of the ldexp operation can be a subnormal number,
+  // we need to accommodate the (mantissaWidht + 1) worth of shift in
+  // calculating the limit.
+  int expLimit = FPBits<T>::maxExponent + MantissaWidth<T>::value + 1;
+  if (exp > expLimit)
     return bits.sign ? FPBits<T>::negInf() : FPBits<T>::inf();
 
-  // Similarly on the negative side.
-  if (exp < -FPBits<T>::maxExponent)
+  // Similarly on the negative side we return zero early if |exp| is too small.
+  if (exp < -expLimit)
     return bits.sign ? FPBits<T>::negZero() : FPBits<T>::zero();
 
   // For all other values, NormalFloat to T conversion handles it the right way.