[IR][ModRef] Introduce errno memory location

Model C/C++ `errno` macro by adding a corresponding `errno`
memory location kind to the IR. Preliminary work to separate
`errno` writes from other memory accesses, to the benefit of
alias analyses and optimization correctness.

Previous discussion: https://discourse.llvm.org/t/rfc-modelling-errno-memory-effects/82972.

Author: antoniofrighetto

llvm/include/llvm/AsmParser/LLToken.h

@@ -201,11 +201,13 @@ enum Kind {
   kw_readwrite,
   kw_argmem,
   kw_inaccessiblemem,
+  kw_errnomem,
 
   // Legacy memory attributes:
   kw_argmemonly,
   kw_inaccessiblememonly,
   kw_inaccessiblemem_or_argmemonly,
+  kw_errnomemonly,
 
   // nofpclass attribute:
   kw_all,

llvm/include/llvm/IR/Function.h

@@ -575,6 +575,10 @@ class LLVM_ABI Function : public GlobalObject, public ilist_node<Function> {
   bool onlyAccessesInaccessibleMemory() const;
   void setOnlyAccessesInaccessibleMemory();
 
+  /// Determine if the function may only access errno memory.
+  bool onlyAccessesErrnoMemory() const;
+  void setOnlyAccessesErrnoMemory();
+
   /// Determine if the function may only access memory that is
   ///  either inaccessible from the IR or pointed to by its arguments.
   bool onlyAccessesInaccessibleMemOrArgMem() const;

llvm/include/llvm/IR/InstrTypes.h

@@ -1907,6 +1907,10 @@ class CallBase : public Instruction {
   bool onlyAccessesInaccessibleMemory() const;
   void setOnlyAccessesInaccessibleMemory();
 
+  /// Determine if the function may only access errno memory.
+  bool onlyAccessesErrnoMemory() const;
+  void setOnlyAccessesErrnoMemory();
+
   /// Determine if the function may only access memory that is
   /// either inaccessible from the IR or pointed to by its arguments.
   bool onlyAccessesInaccessibleMemOrArgMem() const;

llvm/include/llvm/Support/ModRef.h

@@ -61,8 +61,10 @@ enum class IRMemLocation {
   ArgMem = 0,
   /// Memory that is inaccessible via LLVM IR.
   InaccessibleMem = 1,
+  /// Errno memory.
+  ErrnoMem = 2,
   /// Any other memory.
-  Other = 2,
+  Other = 3,
 
   /// Helpers to iterate all locations in the MemoryEffectsBase class.
   First = ArgMem,
@@ -139,6 +141,11 @@ template <typename LocationEnum> class MemoryEffectsBase {
     return MemoryEffectsBase(Location::InaccessibleMem, MR);
   }
 
+  /// Create MemoryEffectsBase that can only access errno memory.
+  static MemoryEffectsBase errnoMemOnly(ModRefInfo MR = ModRefInfo::ModRef) {
+    return MemoryEffectsBase(Location::ErrnoMem, MR);
+  }
+
   /// Create MemoryEffectsBase that can only access inaccessible or argument
   /// memory.
   static MemoryEffectsBase
@@ -207,11 +214,21 @@ template <typename LocationEnum> class MemoryEffectsBase {
     return isModOrRefSet(getModRef(Location::ArgMem));
   }
 
+  /// Whether this function may access errno memory.
+  bool doesAccessErrnoMem() const {
+    return isModOrRefSet(getModRef(Location::ErrnoMem));
+  }
+
   /// Whether this function only (at most) accesses inaccessible memory.
   bool onlyAccessesInaccessibleMem() const {
     return getWithoutLoc(Location::InaccessibleMem).doesNotAccessMemory();
   }
 
+  /// Whether this function only (at most) accesses errno memory.
+  bool onlyAccessesErrnoMem() const {
+    return getWithoutLoc(Location::ErrnoMem).doesNotAccessMemory();
+  }
+
   /// Whether this function only (at most) accesses argument and inaccessible
   /// memory.
   bool onlyAccessesInaccessibleOrArgMem() const {

llvm/lib/AsmParser/LLLexer.cpp

@@ -701,6 +701,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(readwrite);
   KEYWORD(argmem);
   KEYWORD(inaccessiblemem);
+  KEYWORD(errnomem);
   KEYWORD(argmemonly);
   KEYWORD(inaccessiblememonly);
   KEYWORD(inaccessiblemem_or_argmemonly);

llvm/lib/AsmParser/LLParser.cpp

@@ -1668,6 +1668,9 @@ static bool upgradeMemoryAttr(MemoryEffects &ME, lltok::Kind Kind) {
   case lltok::kw_inaccessiblememonly:
     ME &= MemoryEffects::inaccessibleMemOnly();
     return true;
+  case lltok::kw_errnomemonly:
+    ME &= MemoryEffects::errnoMemOnly();
+    return true;
   case lltok::kw_inaccessiblemem_or_argmemonly:
     ME &= MemoryEffects::inaccessibleOrArgMemOnly();
     return true;
@@ -2488,6 +2491,8 @@ static std::optional<MemoryEffects::Location> keywordToLoc(lltok::Kind Tok) {
     return IRMemLocation::ArgMem;
   case lltok::kw_inaccessiblemem:
     return IRMemLocation::InaccessibleMem;
+  case lltok::kw_errnomem:
+    return IRMemLocation::ErrnoMem;
   default:
     return std::nullopt;
   }
@@ -2536,7 +2541,7 @@ std::optional<MemoryEffects> LLParser::parseMemoryAttr() {
     std::optional<ModRefInfo> MR = keywordToModRef(Lex.getKind());
     if (!MR) {
       if (!Loc)
-        tokError("expected memory location (argmem, inaccessiblemem) "
+        tokError("expected memory location (argmem, inaccessiblemem, errnomem) "
                  "or access kind (none, read, write, readwrite)");
       else
         tokError("expected access kind (none, read, write, readwrite)");

llvm/lib/IR/Attributes.cpp

@@ -637,6 +637,9 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
       case IRMemLocation::InaccessibleMem:
         OS << "inaccessiblemem: ";
         break;
+      case IRMemLocation::ErrnoMem:
+        OS << "errnomem: ";
+        break;
       case IRMemLocation::Other:
         llvm_unreachable("This is represented as the default access kind");
       }

llvm/lib/IR/Function.cpp

@@ -922,6 +922,14 @@ void Function::setOnlyAccessesInaccessibleMemory() {
   setMemoryEffects(getMemoryEffects() & MemoryEffects::inaccessibleMemOnly());
 }
 
+/// Determine if the function may only access errno memory.
+bool Function::onlyAccessesErrnoMemory() const {
+  return getMemoryEffects().onlyAccessesErrnoMem();
+}
+void Function::setOnlyAccessesErrnoMemory() {
+  setMemoryEffects(getMemoryEffects() & MemoryEffects::errnoMemOnly());
+}
+
 /// Determine if the function may only access memory that is
 ///  either inaccessible from the IR or pointed to by its arguments.
 bool Function::onlyAccessesInaccessibleMemOrArgMem() const {

llvm/lib/IR/Instructions.cpp

@@ -665,6 +665,14 @@ void CallBase::setOnlyAccessesInaccessibleMemory() {
   setMemoryEffects(getMemoryEffects() & MemoryEffects::inaccessibleMemOnly());
 }
 
+/// Determine if the function may only access errno memory.
+bool CallBase::onlyAccessesErrnoMemory() const {
+  return getMemoryEffects().onlyAccessesErrnoMem();
+}
+void CallBase::setOnlyAccessesErrnoMemory() {
+  setMemoryEffects(getMemoryEffects() & MemoryEffects::errnoMemOnly());
+}
+
 /// Determine if the function may only access memory that is
 ///  either inaccessible from the IR or pointed to by its arguments.
 bool CallBase::onlyAccessesInaccessibleMemOrArgMem() const {

llvm/lib/Support/ModRef.cpp

@@ -42,6 +42,9 @@ raw_ostream &llvm::operator<<(raw_ostream &OS, MemoryEffects ME) {
     case IRMemLocation::InaccessibleMem:
       OS << "InaccessibleMem: ";
       break;
+    case IRMemLocation::ErrnoMem:
+      OS << "ErrnoMem: ";
+      break;
     case IRMemLocation::Other:
       OS << "Other: ";
       break;