[clang] introduce constexpr step limit opt-out

clang/docs/ReleaseNotes.rst

@@ -258,6 +258,7 @@ Deprecated Compiler Flags
 Modified Compiler Flags
 -----------------------
 - The `-gkey-instructions` compiler flag is now enabled by default when DWARF is emitted for plain C/C++ and optimizations are enabled. (#GH149509)
+- The `-fconstexpr-steps` compiler flag now accepts value `0` to opt out of this limit. (#GH160440)
 
 Removed Compiler Flags
 -------------------------

clang/docs/UsersManual.rst

@@ -4028,7 +4028,7 @@ Controlling implementation limits
   Sets the limit for the number of full-expressions evaluated in a single
   constant expression evaluation. This also controls the maximum size
   of array and dynamic array allocation that can be constant evaluated.
-  The default is 1048576.
+  The default is 1048576, and the limit can be disabled with `-fconstexpr-steps=0`..
 
 .. option:: -ftemplate-depth=N
 

clang/include/clang/Driver/Options.td

@@ -2045,7 +2045,7 @@ def fconstexpr_depth_EQ : Joined<["-"], "fconstexpr-depth=">, Group<f_Group>,
   MarshallingInfoInt<LangOpts<"ConstexprCallDepth">, "512">;
 def fconstexpr_steps_EQ : Joined<["-"], "fconstexpr-steps=">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option]>,
-  HelpText<"Set the maximum number of steps in constexpr function evaluation">,
+  HelpText<"Set the maximum number of steps in constexpr function evaluation (0 = no limit)">,
   MarshallingInfoInt<LangOpts<"ConstexprStepLimit">, "1048576">;
 def fexperimental_new_constant_interpreter : Flag<["-"], "fexperimental-new-constant-interpreter">, Group<f_Group>,
   HelpText<"Enable the experimental new constant interpreter">,

clang/lib/AST/ByteCode/Interp.h

@@ -3686,6 +3686,9 @@ inline bool CheckDestruction(InterpState &S, CodePtr OpPC) {
 
 inline bool CheckArraySize(InterpState &S, CodePtr OpPC, uint64_t NumElems) {
   uint64_t Limit = S.getLangOpts().ConstexprStepLimit;
+  if (Limit == 0)
+    return true;
+
   if (NumElems > Limit) {
     S.FFDiag(S.Current->getSource(OpPC),
              diag::note_constexpr_new_exceeds_limits)

clang/lib/AST/ExprConstant.cpp

@@ -990,7 +990,7 @@ namespace {
       // of arrays to avoid exhausting the system resources, as initialization
       // of each element is likely to take some number of steps anyway.
       uint64_t Limit = Ctx.getLangOpts().ConstexprStepLimit;
-      if (ElemCount > Limit) {
+      if (Limit != 0 && ElemCount > Limit) {
         if (Diag)
           FFDiag(Loc, diag::note_constexpr_new_exceeds_limits)
               << ElemCount << Limit;
@@ -1016,6 +1016,9 @@ namespace {
     }
 
     bool nextStep(const Stmt *S) {
+      if (Ctx.getLangOpts().ConstexprStepLimit == 0)
+        return true;
+
       if (!StepsLeft) {
         FFDiag(S->getBeginLoc(), diag::note_constexpr_step_limit_exceeded);
         return false;