[clang] introduce constexpr step limit opt-out

[Tsche]

To address @AaronBallman's feedback from #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.

Please advise if a similar opt-out would be desirable for -fconstexpr-depth (and possibly -ftemplate-depth?).

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[llvmbot]

@llvm/pr-subscribers-clang

Author: None (Tsche)

Changes

To address @AaronBallman's feedback from #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.

Please advise if a similar opt-out would be desirable for -fconstexpr-depth (and possibly -ftemplate-depth?).

---

Full diff: https://github.com/llvm/llvm-project/pull/160440.diff

3 Files Affected:

• (modified) clang/docs/UsersManual.rst (+1-1)
• (modified) clang/lib/AST/ByteCode/Interp.h (+3)
• (modified) clang/lib/AST/ExprConstant.cpp (+4-1)

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;

[cor3ntin]

I think the idea makes sense
Can you update the docs in Options.td, UserManual.rst?
We should say somewhere that =0 will consume all available resources and somewhat discourage it.

[Tsche]

Thanks. I've added the (0 = no limit) note to the driver's help text. The changes to UserManual.rst had already been made.

We should say somewhere that =0 will consume all available resources and somewhat discourage it.

Is this not implied? What else is it supposed to do if I disable self-imposed limits?

[Sirraide]

This is still missing a release note

[Tsche]

This is still missing a release note

Damn, forgot about it again. I've added it now :)

[Sirraide]

I think keepEvaluatingAfterFailure() also needs to be updated.

[AaronBallman]

LGTM!

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