[libc] Clear all exceptions before setting in fesetexceptflag.

Previously, exceptions from the flag were being added. This patch
changes it such that only the exceptions in the flag will be set.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D105085

libc/src/fenv/fesetexceptflag.cpp

@@ -21,6 +21,7 @@ LLVM_LIBC_FUNCTION(int, fesetexceptflag,
   static_assert(sizeof(int) >= sizeof(fexcept_t),
                 "fexcept_t value cannot fit in an int value.");
   int excepts_to_set = static_cast<const int>(*flagp) & excepts;
+  fputil::clearExcept(FE_ALL_EXCEPT);
   return fputil::setExcept(excepts_to_set);
 }
 

libc/test/src/fenv/exception_flags_test.cpp

@@ -42,4 +42,15 @@ TEST(LlvmLibcFenvTest, GetExceptFlagAndSetExceptFlag) {
     // Cleanup
     __llvm_libc::fputil::clearExcept(e);
   }
+
+  // Next, we will raise one exception and save the flags.
+  __llvm_libc::fputil::raiseExcept(FE_INVALID);
+  fexcept_t eflags;
+  __llvm_libc::fegetexceptflag(&eflags, FE_ALL_EXCEPT);
+  // Clear all exceptions and raise two other exceptions.
+  __llvm_libc::fputil::clearExcept(FE_ALL_EXCEPT);
+  __llvm_libc::fputil::raiseExcept(FE_OVERFLOW | FE_INEXACT);
+  // When we set the flags and test, we should only see FE_INVALID.
+  __llvm_libc::fesetexceptflag(&eflags, FE_ALL_EXCEPT);
+  EXPECT_EQ(__llvm_libc::fputil::testExcept(FE_ALL_EXCEPT), FE_INVALID);
 }