[libc][math] Fix subtraction of infinity in fputil::add_or_sub

[krishna2803]

No description provided.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Krishna Pandey (krishna2803)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/152017.diff

2 Files Affected:

• (modified) libc/src/__support/FPUtil/generic/add_sub.h (+6-2)
• (modified) libc/test/src/math/smoke/SubTest.h (+2)

diff --git a/libc/src/__support/FPUtil/generic/add_sub.h b/libc/src/__support/FPUtil/generic/add_sub.h
index 7205d8d41f5bd..d4a4129496640 100644
--- a/libc/src/__support/FPUtil/generic/add_sub.h
+++ b/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()) {
diff --git a/libc/test/src/math/smoke/SubTest.h b/libc/test/src/math/smoke/SubTest.h
index c344db28b7fc6..5fafde00778c0 100644
--- a/libc/test/src/math/smoke/SubTest.h
+++ b/libc/test/src/math/smoke/SubTest.h
@@ -47,6 +47,8 @@ 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(inf, func(in.zero, in.neg_inf));
   }
 
   void test_invalid_operations(SubFunc func) {