[AllocToken] Fix and clarify -falloc-token-max=0 (#168689)

The option -falloc-token-max=0 is supposed to be usable to override
previous settings back to the target default max tokens (SIZE_MAX).

This did not work for the builtin:
```
| executed command: clang -cc1 [..] -nostdsysteminc -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify clang/test/SemaCXX/alloc-token.cpp -falloc-token-max=0
| clang: llvm/lib/Support/AllocToken.cpp:38: std::optional<uint64_t> llvm::getAllocToken(AllocTokenMode, const AllocTokenMetadata &, uint64_t): Assertion `MaxTokens && "Must provide non-zero max tokens"' failed.
```

Fix it by also picking the default if "0" is passed.

Improve the documentation to be clearer what the value of "0" means.

clang/docs/AllocToken.rst

@@ -52,8 +52,8 @@ change or removal. These may (experimentally) be selected with ``-Xclang
 The following command-line options affect generated token IDs:
 
 * ``-falloc-token-max=<N>``
-    Configures the maximum number of tokens. No max by default (tokens bounded
-    by ``SIZE_MAX``).
+    Configures the maximum number of token IDs. By default the number of tokens
+    is bounded by ``SIZE_MAX``.
 
 Querying Token IDs with ``__builtin_infer_alloc_token``
 =======================================================
@@ -129,7 +129,7 @@ Fast ABI
 --------
 
 An alternative ABI can be enabled with ``-fsanitize-alloc-token-fast-abi``,
-which encodes the token ID hint in the allocation function name.
+which encodes the token ID in the allocation function name.
 
 .. code-block:: c
 

clang/include/clang/Basic/LangOptions.h

@@ -566,8 +566,8 @@ public:
   bool AtomicFineGrainedMemory = false;
   bool AtomicIgnoreDenormalMode = false;
 
-  /// Maximum number of allocation tokens (0 = no max), nullopt if none set (use
-  /// target default).
+  /// Maximum number of allocation tokens (0 = target SIZE_MAX), nullopt if none
+  /// set (use target SIZE_MAX).
   std::optional<uint64_t> AllocTokenMax;
 
   /// The allocation token mode.

clang/include/clang/Options/Options.td

@@ -2758,7 +2758,7 @@ defm sanitize_alloc_token_extended : BoolOption<"f", "sanitize-alloc-token-exten
 def falloc_token_max_EQ : Joined<["-"], "falloc-token-max=">,
   Group<f_Group>, Visibility<[ClangOption, CC1Option]>,
   MetaVarName<"<N>">,
-  HelpText<"Limit to maximum N allocation tokens (0 = no max)">;
+  HelpText<"Limit to maximum N allocation tokens (0 = target SIZE_MAX)">;
 
 def falloc_token_mode_EQ : Joined<["-"], "falloc-token-mode=">,
   Group<f_Group>, Visibility<[CC1Option]>,

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1317,8 +1317,9 @@ static bool interp__builtin_infer_alloc_token(InterpState &S, CodePtr OpPC,
   uint64_t BitWidth = ASTCtx.getTypeSize(ASTCtx.getSizeType());
   auto Mode =
       ASTCtx.getLangOpts().AllocTokenMode.value_or(llvm::DefaultAllocTokenMode);
+  auto MaxTokensOpt = ASTCtx.getLangOpts().AllocTokenMax;
   uint64_t MaxTokens =
-      ASTCtx.getLangOpts().AllocTokenMax.value_or(~0ULL >> (64 - BitWidth));
+      MaxTokensOpt.value_or(0) ? *MaxTokensOpt : (~0ULL >> (64 - BitWidth));
 
   // We do not read any of the arguments; discard them.
   for (int I = Call->getNumArgs() - 1; I >= 0; --I)

clang/lib/AST/ExprConstant.cpp

@@ -15559,8 +15559,9 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     auto Mode =
         Info.getLangOpts().AllocTokenMode.value_or(llvm::DefaultAllocTokenMode);
     uint64_t BitWidth = Info.Ctx.getTypeSize(Info.Ctx.getSizeType());
+    auto MaxTokensOpt = Info.getLangOpts().AllocTokenMax;
     uint64_t MaxTokens =
-        Info.getLangOpts().AllocTokenMax.value_or(~0ULL >> (64 - BitWidth));
+        MaxTokensOpt.value_or(0) ? *MaxTokensOpt : (~0ULL >> (64 - BitWidth));
     auto MaybeToken = llvm::getAllocToken(Mode, *ATMD, MaxTokens);
     if (!MaybeToken)
       return Error(E, diag::note_constexpr_infer_alloc_token_stateful_mode);

clang/test/SemaCXX/alloc-token.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -falloc-token-max=0
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -fexperimental-new-constant-interpreter
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -falloc-token-mode=typehash -DMODE_TYPEHASH
 // RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -falloc-token-max=2 -DTOKEN_MAX=2

llvm/lib/Transforms/Instrumentation/AllocToken.cpp

@@ -67,9 +67,10 @@ cl::opt<std::string> ClFuncPrefix("alloc-token-prefix",
                                   cl::desc("The allocation function prefix"),
                                   cl::Hidden, cl::init("__alloc_token_"));
 
-cl::opt<uint64_t> ClMaxTokens("alloc-token-max",
-                              cl::desc("Maximum number of tokens (0 = no max)"),
-                              cl::Hidden, cl::init(0));
+cl::opt<uint64_t>
+    ClMaxTokens("alloc-token-max",
+                cl::desc("Maximum number of tokens (0 = target SIZE_MAX)"),
+                cl::Hidden, cl::init(0));
 
 cl::opt<bool>
     ClFastABI("alloc-token-fast-abi",