[libc][math] Fix subtraction of infinity in fputil::add_or_sub (#152017)

Signed-off-by: Krishna Pandey <[email protected]>

Author: krishna2803

libc/src/__support/FPUtil/generic/add_sub.h

@@ -87,8 +87,12 @@ add_or_sub(InType x, InType y) {
       return OutFPBits::inf(x_bits.sign()).get_val();
     }
 
-    if (y_bits.is_inf())
-      return OutFPBits::inf(y_bits.sign()).get_val();
+    if (y_bits.is_inf()) {
+      if constexpr (IsSub)
+        return OutFPBits::inf(y_bits.sign().negate()).get_val();
+      else
+        return OutFPBits::inf(y_bits.sign()).get_val();
+    }
 
     if (x_bits.is_zero()) {
       if (y_bits.is_zero()) {

libc/test/src/math/smoke/AddTest.h

@@ -47,6 +47,10 @@ class AddTest : public LIBC_NAMESPACE::testing::FEnvSafeTest {
     EXPECT_FP_EQ(neg_inf, func(in.neg_inf, in.zero));
     EXPECT_FP_EQ(inf, func(in.inf, in.neg_zero));
     EXPECT_FP_EQ(neg_inf, func(in.neg_inf, in.neg_zero));
+    EXPECT_FP_EQ(inf, func(in.zero, in.inf));
+    EXPECT_FP_EQ(inf, func(in.neg_zero, in.inf));
+    EXPECT_FP_EQ(neg_inf, func(in.zero, in.neg_inf));
+    EXPECT_FP_EQ(neg_inf, func(in.neg_zero, in.neg_inf));
   }
 
   void test_invalid_operations(AddFunc func) {

libc/test/src/math/smoke/SubTest.h

@@ -47,6 +47,10 @@ class SubTest : public LIBC_NAMESPACE::testing::FEnvSafeTest {
     EXPECT_FP_EQ(neg_inf, func(in.neg_inf, in.zero));
     EXPECT_FP_EQ(inf, func(in.inf, in.neg_zero));
     EXPECT_FP_EQ(neg_inf, func(in.neg_inf, in.neg_zero));
+    EXPECT_FP_EQ(neg_inf, func(in.zero, in.inf));
+    EXPECT_FP_EQ(neg_inf, func(in.neg_zero, in.inf));
+    EXPECT_FP_EQ(inf, func(in.zero, in.neg_inf));
+    EXPECT_FP_EQ(inf, func(in.neg_zero, in.neg_inf));
   }
 
   void test_invalid_operations(SubFunc func) {