[clang] introduce constexpr step limit opt-out (#160440)
To address @AaronBallman's feedback from
https://github.com/llvm/llvm-project/pull/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`.
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index e387d3a..1a4ec40 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -286,6 +286,7 @@
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
-------------------------
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 5745e4b..2d10489 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -4036,7 +4036,7 @@
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/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index a55a523..611b68e 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2047,7 +2047,7 @@
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">,
diff --git a/clang/lib/AST/ByteCode/Interp.h b/clang/lib/AST/ByteCode/Interp.h
index a9c71c7..57cc705 100644
--- a/clang/lib/AST/ByteCode/Interp.h
+++ b/clang/lib/AST/ByteCode/Interp.h
@@ -3699,7 +3699,7 @@
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;
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index a3037c3f..dfdfef2 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -990,7 +990,7 @@
// 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 @@
}
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 @@
/// 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) {
