diff --git a/libc/utils/FPUtil/x86_64/FEnv.h b/libc/utils/FPUtil/x86_64/FEnv.h
--- a/libc/utils/FPUtil/x86_64/FEnv.h
+++ b/libc/utils/FPUtil/x86_64/FEnv.h
@@ -43,7 +43,10 @@
 // encoding as well as the same bit positions.
 struct ExceptionFlags {
   static constexpr uint16_t Invalid = 0x1;
-  static constexpr uint16_t Denormal = 0x2; // This flag is not used
+  // Some libcs define __FE_DENORM corresponding to the denormal input
+  // exception and include it in FE_ALL_EXCEPTS. We define and use it to
+  // support compiling against headers provided by such libcs.
+  static constexpr uint16_t Denormal = 0x2;
   static constexpr uint16_t DivByZero = 0x4;
   static constexpr uint16_t Overflow = 0x8;
   static constexpr uint16_t Underflow = 0x10;
@@ -62,6 +65,9 @@
   // We will make use of the fact that exception control bits are single
   // bit flags in the control registers.
   return (excepts & FE_INVALID ? ExceptionFlags::Invalid : 0) |
+#ifdef __FE_DENORM
+         (excepts & __FE_DENORM ? ExceptionFalgs::Denormal : 0) |
+#endif // __FE_DENORM
          (excepts & FE_DIVBYZERO ? ExceptionFlags::DivByZero : 0) |
          (excepts & FE_OVERFLOW ? ExceptionFlags::Overflow : 0) |
          (excepts & FE_UNDERFLOW ? ExceptionFlags::Underflow : 0) |
@@ -70,6 +76,9 @@
 
 static inline int exceptionStatusToMacro(uint16_t status) {
   return (status & ExceptionFlags::Invalid ? FE_INVALID : 0) |
+#ifdef __FE_DENORM
+         (status & ExceptionFalgs::Denormal ? __FE_DENORM : 0) |
+#endif // __FE_DENORM
          (status & ExceptionFlags::DivByZero ? FE_DIVBYZERO : 0) |
          (status & ExceptionFlags::Overflow ? FE_OVERFLOW : 0) |
          (status & ExceptionFlags::Underflow ? FE_UNDERFLOW : 0) |