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

[via cfe-commits]

https://github.com/Tsche updated https://github.com/llvm/llvm-project/pull/160440

>From a9d97d453d6d6587ec28aee9381020b1cbd54412 Mon Sep 17 00:00:00 2001
From: Tsche <che at pydong.org>
Date: Wed, 24 Sep 2025 07:09:29 +0200
Subject: [PATCH 1/2] [clang] introduce constexpr step limit opt-out

---
 clang/docs/UsersManual.rst      | 2 +-
 clang/lib/AST/ByteCode/Interp.h | 3 +++
 clang/lib/AST/ExprConstant.cpp  | 5 ++++-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index a8bbf146431ea..1a062475728dd 100644
--- a/clang/docs/UsersManual.rst
+++ b/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
 
diff --git a/clang/lib/AST/ByteCode/Interp.h b/clang/lib/AST/ByteCode/Interp.h
index b3b4b998439cc..deba2d294abe9 100644
--- a/clang/lib/AST/ByteCode/Interp.h
+++ b/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)
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index d10e2afeb2341..0fe3fce5b64a8 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/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;

>From 40fe30d71f3a0fc45662267112cad978e27c7df6 Mon Sep 17 00:00:00 2001
From: Tsche <che at pydong.org>
Date: Wed, 24 Sep 2025 17:21:57 +0200
Subject: [PATCH 2/2] [clang] add help text for constexpr step limit opt-out

---
 clang/include/clang/Driver/Options.td | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 77f19a240a7f9..ddf984a5ab447 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/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">,