[Attr] Add "willreturn" function attribute

This patch introduces a new function attribute, willreturn, to indicate
that a call of this function will either exhibit undefined behavior or
comes back and continues execution at a point in the existing call stack
that includes the current invocation.

This attribute guarantees that the function does not have any endless
loops, endless recursion, or terminating functions like abort or exit.

Patch by Hideto Ueno (@uenoku)

Reviewers: jdoerfert

Subscribers: mehdi_amini, hiraditya, steven_wu, dexonsmith, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62801

llvm-svn: 364555

llvm/docs/LangRef.rst

@@ -1475,6 +1475,13 @@ example:
     This function attribute indicates that the function does not call itself
     either directly or indirectly down any possible call path. This produces
     undefined behavior at runtime if the function ever does recurse.
+``willreturn``
+    This function attribute indicates that a call of this function will
+    either exhibit undefined behavior or comes back and continues execution
+    at a point in the existing call stack that includes the current invocation.
+    Annotated functions may still raise an exception, i.a., ``nounwind`` is not implied.
+    If an invocation of an annotated function does not return control back
+    to a point in the call stack, the behavior is undefined.
 ``nounwind``
     This function attribute indicates that the function never raises an
     exception. If the function does raise an exception, its runtime

llvm/include/llvm/Bitcode/LLVMBitCodes.h

@@ -606,7 +606,8 @@ enum AttributeKindCodes {
   ATTR_KIND_OPT_FOR_FUZZING = 57,
   ATTR_KIND_SHADOWCALLSTACK = 58,
   ATTR_KIND_SPECULATIVE_LOAD_HARDENING = 59,
-  ATTR_KIND_IMMARG = 60
+  ATTR_KIND_IMMARG = 60,
+  ATTR_KIND_WILLRETURN = 61,
 };
 
 enum ComdatSelectionKindCodes {

llvm/include/llvm/IR/Attributes.td

@@ -196,6 +196,9 @@ def SwiftSelf : EnumAttr<"swiftself">;
 /// Function must be in a unwind table.
 def UWTable : EnumAttr<"uwtable">;
 
+/// Function always comes back to callsite.
+def WillReturn : EnumAttr<"willreturn">;
+
 /// Function only writes to memory.
 def WriteOnly : EnumAttr<"writeonly">;
 

llvm/lib/AsmParser/LLLexer.cpp

@@ -683,6 +683,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(swifterror);
   KEYWORD(swiftself);
   KEYWORD(uwtable);
+  KEYWORD(willreturn);
   KEYWORD(writeonly);
   KEYWORD(zeroext);
   KEYWORD(immarg);

llvm/lib/AsmParser/LLParser.cpp

@@ -1315,6 +1315,7 @@ bool LLParser::ParseFnAttributeValuePairs(AttrBuilder &B,
       break;
     case lltok::kw_strictfp: B.addAttribute(Attribute::StrictFP); break;
     case lltok::kw_uwtable: B.addAttribute(Attribute::UWTable); break;
+    case lltok::kw_willreturn: B.addAttribute(Attribute::WillReturn); break;
     case lltok::kw_writeonly: B.addAttribute(Attribute::WriteOnly); break;
 
     // Error handling.

llvm/lib/AsmParser/LLToken.h

@@ -225,6 +225,7 @@ enum Kind {
   kw_swifterror,
   kw_swiftself,
   kw_uwtable,
+  kw_willreturn,
   kw_writeonly,
   kw_zeroext,
   kw_immarg,

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1272,6 +1272,8 @@ static uint64_t getRawAttributeMask(Attribute::AttrKind Val) {
     return 1ULL << 60;
   case Attribute::ImmArg:
     return 1ULL << 61;
+  case Attribute::WillReturn:
+    return 1ULL << 62;
   case Attribute::Dereferenceable:
     llvm_unreachable("dereferenceable attribute not supported in raw format");
     break;
@@ -1510,6 +1512,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::SwiftSelf;
   case bitc::ATTR_KIND_UW_TABLE:
     return Attribute::UWTable;
+  case bitc::ATTR_KIND_WILLRETURN:
+    return Attribute::WillReturn;
   case bitc::ATTR_KIND_WRITEONLY:
     return Attribute::WriteOnly;
   case bitc::ATTR_KIND_Z_EXT:

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -710,6 +710,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_SWIFT_SELF;
   case Attribute::UWTable:
     return bitc::ATTR_KIND_UW_TABLE;
+  case Attribute::WillReturn:
+    return bitc::ATTR_KIND_WILLRETURN;
   case Attribute::WriteOnly:
     return bitc::ATTR_KIND_WRITEONLY;
   case Attribute::ZExt:

llvm/lib/IR/Attributes.cpp

@@ -333,6 +333,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     return "noredzone";
   if (hasAttribute(Attribute::NoReturn))
     return "noreturn";
+  if (hasAttribute(Attribute::WillReturn))
+    return "willreturn";
   if (hasAttribute(Attribute::NoCfCheck))
     return "nocf_check";
   if (hasAttribute(Attribute::NoRecurse))

llvm/lib/IR/Verifier.cpp

@@ -1486,6 +1486,7 @@ void Verifier::visitModuleFlagCGProfileEntry(const MDOperand &MDO) {
 static bool isFuncOnlyAttr(Attribute::AttrKind Kind) {
   switch (Kind) {
   case Attribute::NoReturn:
+  case Attribute::WillReturn:
   case Attribute::NoCfCheck:
   case Attribute::NoUnwind:
   case Attribute::NoInline:

llvm/lib/Transforms/Utils/CodeExtractor.cpp

@@ -801,6 +801,7 @@ Function *CodeExtractor::constructFunction(const ValueSet &inputs,
       case Attribute::StructRet:
       case Attribute::SwiftError:
       case Attribute::SwiftSelf:
+      case Attribute::WillReturn:
       case Attribute::WriteOnly:
       case Attribute::ZExt:
       case Attribute::ImmArg:

llvm/test/Bitcode/attributes.ll

@@ -204,7 +204,7 @@ define void @f34()
 ; CHECK: define void @f34()
 {
         call void @nobuiltin() nobuiltin
-; CHECK: call void @nobuiltin() #36
+; CHECK: call void @nobuiltin() #37
         ret void;
 }
 
@@ -351,6 +351,12 @@ define void @f59() shadowcallstack
   ret void
 }
 
+; CHECK: define void @f60() #36
+define void @f60() willreturn
+{
+  ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }
@@ -387,4 +393,5 @@ define void @f59() shadowcallstack
 ; CHECK: attributes #33 = { speculatable }
 ; CHECK: attributes #34 = { sanitize_hwaddress }
 ; CHECK: attributes #35 = { shadowcallstack }
-; CHECK: attributes #36 = { nobuiltin }
+; CHECK: attributes #36 = { willreturn }
+; CHECK: attributes #37 = { nobuiltin }