[clang] [Clang][AArch64] Include SME attributes in the name mangling of function types (PR #114209)

[Kerry McLaughlin via cfe-commits]

================
@@ -3536,35 +3536,64 @@ void CXXNameMangler::mangleExtFunctionInfo(const FunctionType *T) {
   // FIXME: noreturn
 }
 
-bool hasSharedState(unsigned SMEAttrs) {
+unsigned getZAState(unsigned SMEAttrs) {
   switch (SMEAttrs) {
   case FunctionType::ARM_In:
+    return 1;
   case FunctionType::ARM_Out:
+    return 2;
   case FunctionType::ARM_InOut:
+    return 3;
   case FunctionType::ARM_Preserves:
-    return true;
+    return 4;
   default:
-    return false;
+    return 0;
   }
 }
 
+// The mangling scheme for function types which have SME attributes is implemented as
+// a "pseudo" template:
+//
+//   '__SME_ATTRS<<normal_function_type>, <sme_state>>'
+//
+// Combining the function type with a bitmask representing the streaming and ZA properties
+// of the function's interface. The bits of sme_state are defined as follows:
+//    0:  Streaming Mode
+//    1:  Streaming Compatible
+//    2:  ZA Agnostic
+//  3-5:  ZA State
+//  6-8:  ZT0 State
+//  9-63: 0, reserved for future type attributes.
+//
+// For example:
+//  void f(svint8_t (*fn)() __arm_streaming_compatible __arm_inout("za")) { fn(); }
+//
+// The function fn is described as '__SME_ATTRS<Fu10__SVInt8_tvE, 26u>' and mangled as:
+//
+//  "11__SME_ATTRSI" + function type mangling + "Lj" + bitmask + "EE"
+//
+//  i.e. "11__SME_ATTRSIFu10__SVInt8_tvELj26EE"
+//
 void CXXNameMangler::mangleSMEAttrs(unsigned SMEAttrs) {
   if (!SMEAttrs)
     return;
 
   // Streaming Mode
+  unsigned Bitmask = 0;
   if (SMEAttrs & FunctionType::SME_PStateSMEnabledMask)
-    Out << "Lj1E";
+    Bitmask |= 1;
----------------
kmclaughlin-arm wrote:

I've removed details of what each bit represents from the comment now that there is an enum with this information. I've still left in a short description of the template, similar to what has been written for the `arm_sve_vector_bits` attribute.

https://github.com/llvm/llvm-project/pull/114209