Remove static initializers for llvm::StringRef (microsoft#6949)

Update llvm::StringRef constructor to be constexpr and replace static constant values in DxcOptToggles with constexpr variables to ensure non-trivial initializers are not generated in the final binary.

Fixes microsoft#6896

Author: chrdavis

include/dxc/Support/DxcOptToggles.h

@@ -27,27 +27,29 @@ namespace options {
 struct Toggle {
   llvm::StringRef Name;
   bool Default = false;
-  Toggle(llvm::StringRef Name, bool Default) : Name(Name), Default(Default) {}
+  constexpr Toggle(llvm::StringRef Name, bool Default)
+      : Name(Name), Default(Default) {}
 };
 
 enum {
   DEFAULT_ON = 1,
   DEFAULT_OFF = 0,
 };
 
-static const Toggle TOGGLE_GVN = {"gvn", DEFAULT_ON};
-static const Toggle TOGGLE_LICM = {"licm", DEFAULT_ON};
-static const Toggle TOGGLE_SINK = {"sink", DEFAULT_ON};
-static const Toggle TOGGLE_ENABLE_AGGRESSIVE_REASSOCIATION = {
+static constexpr Toggle TOGGLE_GVN = {"gvn", DEFAULT_ON};
+static constexpr Toggle TOGGLE_LICM = {"licm", DEFAULT_ON};
+static constexpr Toggle TOGGLE_SINK = {"sink", DEFAULT_ON};
+static constexpr Toggle TOGGLE_ENABLE_AGGRESSIVE_REASSOCIATION = {
     "aggressive-reassociation", DEFAULT_ON};
-static const Toggle TOGGLE_LIFETIME_MARKERS = {"lifetime-markers", DEFAULT_ON};
-static const Toggle TOGGLE_PARTIAL_LIFETIME_MARKERS = {
+static constexpr Toggle TOGGLE_LIFETIME_MARKERS = {"lifetime-markers",
+                                                   DEFAULT_ON};
+static constexpr Toggle TOGGLE_PARTIAL_LIFETIME_MARKERS = {
     "partial-lifetime-markers", DEFAULT_OFF};
-static const Toggle TOGGLE_STRUCTURIZE_LOOP_EXITS_FOR_UNROLL = {
+static constexpr Toggle TOGGLE_STRUCTURIZE_LOOP_EXITS_FOR_UNROLL = {
     "structurize-loop-exits-for-unroll", DEFAULT_ON};
-static const Toggle TOGGLE_DEBUG_NOPS = {"debug-nops", DEFAULT_ON};
-static const Toggle TOGGLE_STRUCTURIZE_RETURNS = {"structurize-returns",
-                                                  DEFAULT_OFF};
+static constexpr Toggle TOGGLE_DEBUG_NOPS = {"debug-nops", DEFAULT_ON};
+static constexpr Toggle TOGGLE_STRUCTURIZE_RETURNS = {"structurize-returns",
+                                                      DEFAULT_OFF};
 
 struct OptimizationToggles {
   // Optimization pass enables, disables and selects

include/llvm/ADT/StringRef.h

@@ -66,19 +66,20 @@ namespace llvm {
     /*implicit*/ StringRef() : Data(nullptr), Length(0) {}
     StringRef(std::nullptr_t) = delete; // HLSL Change - So we don't accidentally pass `false` again
 
+    // HLSL Change - Begin - Make StringRef constructors constexpr.
     /// Construct a string ref from a cstring.
-    /*implicit*/ StringRef(const char *Str)
-      : Data(Str) {
-        assert(Str && "StringRef cannot be built from a NULL argument");
-        Length = ::strlen(Str); // invoking strlen(NULL) is undefined behavior
-      }
+    /*implicit*/ constexpr StringRef(const char *Str)
+        : Data(Str), Length(Str ? std::char_traits<char>::length(Str) : 0) {
+      assert(Str && "StringRef cannot be built from a NULL argument");
+    }
 
     /// Construct a string ref from a pointer and length.
-    /*implicit*/ StringRef(const char *data, size_t length)
-      : Data(data), Length(length) {
-        assert((data || length == 0) &&
-        "StringRef cannot be built from a NULL argument with non-null length");
-      }
+    /*implicit*/ constexpr StringRef(const char *data, size_t length)
+        : Data(data), Length(length) {
+      assert((data || length == 0) && "StringRef cannot be built from a NULL "
+                                      "argument with non-null length");
+    }
+    // HLSL Change - End - Make StringRef constructors constexpr.
 
     /// Construct a string ref from an std::string.
     /*implicit*/ StringRef(const std::string &Str)