Index: libc/src/__support/integer_operations.h
===================================================================
--- libc/src/__support/integer_operations.h
+++ libc/src/__support/integer_operations.h
@@ -14,10 +14,9 @@
 namespace __llvm_libc {
 
 template <typename T>
-static inline cpp::EnableIfType<cpp::IsIntegral<T>::Value, T> integerAbs(T n) {
-  if (n < 0)
-    return -n;
-  return n;
+static constexpr cpp::EnableIfType<cpp::IsIntegral<T>::Value, T>
+integerAbs(T n) {
+  return (n < 0) ? -n : n;
 }
 
 } // namespace __llvm_libc
Index: libc/src/ctype/ctype_utils.h
===================================================================
--- libc/src/ctype/ctype_utils.h
+++ libc/src/ctype/ctype_utils.h
@@ -18,17 +18,17 @@
 // of a function call by inlining them.
 // ------------------------------------------------------
 
-static inline int isalpha(unsigned ch) { return (ch | 32) - 'a' < 26; }
+static constexpr int isalpha(unsigned ch) { return (ch | 32) - 'a' < 26; }
 
-static inline int isdigit(unsigned ch) { return (ch - '0') < 10; }
+static constexpr int isdigit(unsigned ch) { return (ch - '0') < 10; }
 
-static inline int isalnum(unsigned ch) { return isalpha(ch) || isdigit(ch); }
+static constexpr int isalnum(unsigned ch) { return isalpha(ch) || isdigit(ch); }
 
-static inline int isgraph(unsigned ch) { return 0x20 < ch && ch < 0x7f; }
+static constexpr int isgraph(unsigned ch) { return 0x20 < ch && ch < 0x7f; }
 
-static inline int islower(unsigned ch) { return (ch - 'a') < 26; }
+static constexpr int islower(unsigned ch) { return (ch - 'a') < 26; }
 
-static inline int isupper(unsigned ch) { return (ch - 'A') < 26; }
+static constexpr int isupper(unsigned ch) { return (ch - 'A') < 26; }
 
 } // namespace internal
 } // namespace __llvm_libc
Index: libc/src/ctype/isalnum.h
===================================================================
--- libc/src/ctype/isalnum.h
+++ libc/src/ctype/isalnum.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isalnum(int c);
+constexpr int isalnum(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isalnum.cpp
===================================================================
--- libc/src/ctype/isalnum.cpp
+++ libc/src/ctype/isalnum.cpp
@@ -15,6 +15,8 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isalnum, (int c)) { return internal::isalnum(c); }
+constexpr LLVM_LIBC_FUNCTION(int, isalnum, (int c)) {
+  return internal::isalnum(c);
+}
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isalpha.h
===================================================================
--- libc/src/ctype/isalpha.h
+++ libc/src/ctype/isalpha.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isalpha(int c);
+constexpr int isalpha(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isalpha.cpp
===================================================================
--- libc/src/ctype/isalpha.cpp
+++ libc/src/ctype/isalpha.cpp
@@ -15,6 +15,8 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isalpha, (int c)) { return internal::isalpha(c); }
+constexpr LLVM_LIBC_FUNCTION(int, isalpha, (int c)) {
+  return internal::isalpha(c);
+}
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isascii.h
===================================================================
--- libc/src/ctype/isascii.h
+++ libc/src/ctype/isascii.h
@@ -13,7 +13,7 @@
 
 namespace __llvm_libc {
 
-int isascii(int c);
+constexpr int isascii(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isascii.cpp
===================================================================
--- libc/src/ctype/isascii.cpp
+++ libc/src/ctype/isascii.cpp
@@ -12,6 +12,8 @@
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, isascii, (int c)) { return (c & (~0x7f)) == 0; }
+constexpr LLVM_LIBC_FUNCTION(int, isascii, (int c)) {
+  return (c & (~0x7f)) == 0;
+}
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isblank.h
===================================================================
--- libc/src/ctype/isblank.h
+++ libc/src/ctype/isblank.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isblank(int c);
+constexpr int isblank(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isblank.cpp
===================================================================
--- libc/src/ctype/isblank.cpp
+++ libc/src/ctype/isblank.cpp
@@ -14,7 +14,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isblank, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, isblank, (int c)) {
   const unsigned char ch = static_cast<char>(c);
   return ch == ' ' || ch == '\t';
 }
Index: libc/src/ctype/iscntrl.h
===================================================================
--- libc/src/ctype/iscntrl.h
+++ libc/src/ctype/iscntrl.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int iscntrl(int c);
+constexpr int iscntrl(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/iscntrl.cpp
===================================================================
--- libc/src/ctype/iscntrl.cpp
+++ libc/src/ctype/iscntrl.cpp
@@ -14,7 +14,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, iscntrl, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, iscntrl, (int c)) {
   const unsigned char ch = static_cast<char>(c);
   return ch < 0x20 || ch == 0x7f;
 }
Index: libc/src/ctype/isdigit.h
===================================================================
--- libc/src/ctype/isdigit.h
+++ libc/src/ctype/isdigit.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isdigit(int c);
+constexpr int isdigit(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isdigit.cpp
===================================================================
--- libc/src/ctype/isdigit.cpp
+++ libc/src/ctype/isdigit.cpp
@@ -14,6 +14,8 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isdigit, (int c)) { return internal::isdigit(c); }
+constexpr LLVM_LIBC_FUNCTION(int, isdigit, (int c)) {
+  return internal::isdigit(c);
+}
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isgraph.h
===================================================================
--- libc/src/ctype/isgraph.h
+++ libc/src/ctype/isgraph.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isgraph(int c);
+constexpr int isgraph(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isgraph.cpp
===================================================================
--- libc/src/ctype/isgraph.cpp
+++ libc/src/ctype/isgraph.cpp
@@ -15,6 +15,8 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isgraph, (int c)) { return internal::isgraph(c); }
+constexpr LLVM_LIBC_FUNCTION(int, isgraph, (int c)) {
+  return internal::isgraph(c);
+}
 
 } // namespace __llvm_libc
Index: libc/src/ctype/islower.h
===================================================================
--- libc/src/ctype/islower.h
+++ libc/src/ctype/islower.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int islower(int c);
+constexpr int islower(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/islower.cpp
===================================================================
--- libc/src/ctype/islower.cpp
+++ libc/src/ctype/islower.cpp
@@ -15,6 +15,6 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, islower, (int c)) { return internal::islower(c); }
+constexpr LLVM_LIBC_FUNCTION(int, islower, (int c)) { return internal::islower(c); }
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isprint.h
===================================================================
--- libc/src/ctype/isprint.h
+++ libc/src/ctype/isprint.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isprint(int c);
+constexpr int isprint(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isprint.cpp
===================================================================
--- libc/src/ctype/isprint.cpp
+++ libc/src/ctype/isprint.cpp
@@ -14,7 +14,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isprint, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, isprint, (int c)) {
   const unsigned ch = c;
   return (ch - ' ') < 95;
 }
Index: libc/src/ctype/ispunct.h
===================================================================
--- libc/src/ctype/ispunct.h
+++ libc/src/ctype/ispunct.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int ispunct(int c);
+constexpr int ispunct(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/ispunct.cpp
===================================================================
--- libc/src/ctype/ispunct.cpp
+++ libc/src/ctype/ispunct.cpp
@@ -15,7 +15,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, ispunct, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, ispunct, (int c)) {
   return !internal::isalnum(c) && internal::isgraph(c);
 }
 
Index: libc/src/ctype/isspace.h
===================================================================
--- libc/src/ctype/isspace.h
+++ libc/src/ctype/isspace.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isspace(int c);
+constexpr int isspace(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isspace.cpp
===================================================================
--- libc/src/ctype/isspace.cpp
+++ libc/src/ctype/isspace.cpp
@@ -14,7 +14,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isspace, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, isspace, (int c)) {
   const unsigned ch = c;
   return ch == ' ' || (ch - '\t') < 5;
 }
Index: libc/src/ctype/isupper.h
===================================================================
--- libc/src/ctype/isupper.h
+++ libc/src/ctype/isupper.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isupper(int c);
+constexpr int isupper(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isupper.cpp
===================================================================
--- libc/src/ctype/isupper.cpp
+++ libc/src/ctype/isupper.cpp
@@ -15,6 +15,6 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isupper, (int c)) { return internal::isupper(c); }
+constexpr LLVM_LIBC_FUNCTION(int, isupper, (int c)) { return internal::isupper(c); }
 
 } // namespace __llvm_libc
Index: libc/src/ctype/isxdigit.h
===================================================================
--- libc/src/ctype/isxdigit.h
+++ libc/src/ctype/isxdigit.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int isxdigit(int c);
+constexpr int isxdigit(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/isxdigit.cpp
===================================================================
--- libc/src/ctype/isxdigit.cpp
+++ libc/src/ctype/isxdigit.cpp
@@ -15,7 +15,7 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, isxdigit, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, isxdigit, (int c)) {
   const unsigned ch = c;
   return internal::isdigit(ch) || (ch | 32) - 'a' < 6;
 }
Index: libc/src/ctype/toascii.h
===================================================================
--- libc/src/ctype/toascii.h
+++ libc/src/ctype/toascii.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int toascii(int c);
+constexpr int toascii(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/toascii.cpp
===================================================================
--- libc/src/ctype/toascii.cpp
+++ libc/src/ctype/toascii.cpp
@@ -12,6 +12,6 @@
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, toascii, (int c)) { return (c & 0x7f); }
+constexpr LLVM_LIBC_FUNCTION(int, toascii, (int c)) { return (c & 0x7f); }
 
 } // namespace __llvm_libc
Index: libc/src/ctype/tolower.h
===================================================================
--- libc/src/ctype/tolower.h
+++ libc/src/ctype/tolower.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int tolower(int c);
+constexpr int tolower(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/tolower.cpp
===================================================================
--- libc/src/ctype/tolower.cpp
+++ libc/src/ctype/tolower.cpp
@@ -15,9 +15,9 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, tolower, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, tolower, (int c)) {
   if (internal::isupper(c))
-    return c + 'a' - 'A';
+    c += 'a' - 'A';
   return c;
 }
 
Index: libc/src/ctype/toupper.h
===================================================================
--- libc/src/ctype/toupper.h
+++ libc/src/ctype/toupper.h
@@ -11,7 +11,7 @@
 
 namespace __llvm_libc {
 
-int toupper(int c);
+constexpr int toupper(int c);
 
 } // namespace __llvm_libc
 
Index: libc/src/ctype/toupper.cpp
===================================================================
--- libc/src/ctype/toupper.cpp
+++ libc/src/ctype/toupper.cpp
@@ -15,9 +15,9 @@
 
 // TODO: Currently restricted to default locale.
 // These should be extended using locale information.
-LLVM_LIBC_FUNCTION(int, toupper, (int c)) {
+constexpr LLVM_LIBC_FUNCTION(int, toupper, (int c)) {
   if (internal::islower(c))
-    return c + 'A' - 'a';
+    c -= 'a' - 'A';
   return c;
 }
 
Index: libc/utils/HdrGen/Command.h
===================================================================
--- libc/utils/HdrGen/Command.h
+++ libc/utils/HdrGen/Command.h
@@ -36,7 +36,7 @@
   public:
     ErrorReporter(llvm::SMLoc L, llvm::SourceMgr &SM) : Loc(L), SrcMgr(SM) {}
 
-    void printFatalError(llvm::Twine Msg) const {
+    [[noreturn]] void printFatalError(llvm::Twine Msg) const {
       SrcMgr.PrintMessage(Loc, llvm::SourceMgr::DK_Error, Msg);
       std::exit(1);
     }