[clang] introduce constexpr step limit opt-out (llvm#160440)

To address @AaronBallman's feedback from
llvm#143785 this patch implements
an explicit opt-out for `-fconstexpr-steps` by setting
`-fconstexpr-steps=0`.

This does not change any defaults, but gives users an easy way to opt
out of this limit altogether (and instead let the compiler reach the
system's resource limits).

Currently users set `constexpr-steps` to some arbitrary high number (and
I mean _arbitrary_ - see the tables in the previous PR). This isn't
actually opting out of the limit though - you're still bound by the
upper bound of the counter's type. If you have enough resources to
evaluate more than 18446744073709551615 steps that's bad news.

In any case, `=0` conveys the intent clearer. This is in line with how
we handle other flags, ie `-ftemplate-backtrace-limit` or
`-ferror-limit`.

Author: Tsche

clang/docs/ReleaseNotes.rst

@@ -286,6 +286,7 @@ 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

@@ -4036,7 +4036,7 @@ 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

@@ -2047,7 +2047,7 @@ 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

@@ -3699,7 +3699,7 @@ inline bool CheckDestruction(InterpState &S, CodePtr OpPC) {
 
 inline bool CheckArraySize(InterpState &S, CodePtr OpPC, uint64_t NumElems) {
   uint64_t Limit = S.getLangOpts().ConstexprStepLimit;
-  if (NumElems > Limit) {
+  if (Limit != 0 && NumElems > Limit) {
     S.FFDiag(S.Current->getSource(OpPC),
              diag::note_constexpr_new_exceeds_limits)
         << NumElems << Limit;

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;
@@ -1186,7 +1189,8 @@ namespace {
     /// Should we continue evaluation as much as possible after encountering a
     /// construct which can't be reduced to a value?
     bool keepEvaluatingAfterFailure() const override {
-      if (!StepsLeft)
+      uint64_t Limit = Ctx.getLangOpts().ConstexprStepLimit;
+      if (Limit != 0 && !StepsLeft)
         return false;
 
       switch (EvalMode) {