[clang] [Clang] Be less strict about diagnosing null pointer dereference. (PR #149648)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Corentin Jabot (cor3ntin)

<details>
<summary>Changes</summary>

In #<!-- -->143667, we made constant evaluation fail on `*null_ptr`, as this is UB. However, `&(*(foo*)0)` seems to be a common pattern, which made #<!-- -->143667 too disruptive.

So instead of failing the evaluation, we note the UB, which let clang recovers when checking for constant initialization.

Fixes #<!-- -->149500

---
Full diff: https://github.com/llvm/llvm-project/pull/149648.diff


4 Files Affected:

- (modified) clang/lib/AST/ExprConstant.cpp (+6-3) 
- (modified) clang/test/AST/ByteCode/const-eval.c (-2) 
- (modified) clang/test/Sema/const-eval.c (+4-3) 
- (modified) clang/test/SemaCXX/constant-expression-cxx14.cpp (+5) 


``````````diff
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index cfc4729be4184..e07dd317cfb3b 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9346,9 +9346,12 @@ bool LValueExprEvaluator::VisitUnaryDeref(const UnaryOperator *E) {
   // [C++26][expr.unary.op]
   // If the operand points to an object or function, the result
   // denotes that object or function; otherwise, the behavior is undefined.
-  return Success &&
-         (!E->getType().getNonReferenceType()->isObjectType() ||
-          findCompleteObject(Info, E, AK_Dereference, Result, E->getType()));
+  // Because &(*(type*)0) is a common pattern, we do not fail the evaluation
+  // immediately.
+  if (!Success || !E->getType().getNonReferenceType()->isObjectType())
+    return Success;
+  return !!findCompleteObject(Info, E, AK_Dereference, Result, E->getType()) ||
+         Info.noteUndefinedBehavior();
 }
 
 bool LValueExprEvaluator::VisitUnaryReal(const UnaryOperator *E) {
diff --git a/clang/test/AST/ByteCode/const-eval.c b/clang/test/AST/ByteCode/const-eval.c
index c8651a744f969..eab14c08ec809 100644
--- a/clang/test/AST/ByteCode/const-eval.c
+++ b/clang/test/AST/ByteCode/const-eval.c
@@ -51,8 +51,6 @@ struct s {
 };
 
 EVAL_EXPR(19, ((int)&*(char*)10 == 10 ? 1 : -1));
-// ref-error at -1 {{expression is not an integer constant expression}} \
-// ref-note at -1 {{dereferencing a null pointer}}
 
 #ifndef NEW_INTERP
 EVAL_EXPR(20, __builtin_constant_p(*((int*) 10)));
diff --git a/clang/test/Sema/const-eval.c b/clang/test/Sema/const-eval.c
index 87c21120e7c5d..11cc7fbc0feb3 100644
--- a/clang/test/Sema/const-eval.c
+++ b/clang/test/Sema/const-eval.c
@@ -41,9 +41,6 @@ struct s {
 };
 
 EVAL_EXPR(19, ((int)&*(char*)10 == 10 ? 1 : -1));
-// expected-error at -1 {{not an integer constant expression}} \
-// expected-note at -1 {{dereferencing a null pointer is not allowed in a constant expression}}
-
 
 EVAL_EXPR(20, __builtin_constant_p(*((int*) 10)));
 
@@ -153,3 +150,7 @@ struct PR35214_X {
 int PR35214_x;
 int PR35214_y = ((struct PR35214_X *)&PR35214_x)->arr[1]; // expected-error {{not a compile-time constant}}
 int *PR35214_z = &((struct PR35214_X *)&PR35214_x)->arr[1]; // ok, &PR35214_x + 2
+
+
+int * GH149500_p = &(*(int *)0x400);
+static const void *GH149500_q = &(*(const struct sysrq_key_op *)0);
diff --git a/clang/test/SemaCXX/constant-expression-cxx14.cpp b/clang/test/SemaCXX/constant-expression-cxx14.cpp
index 182c0d01141ff..1743e0e3ac4b5 100644
--- a/clang/test/SemaCXX/constant-expression-cxx14.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx14.cpp
@@ -1445,3 +1445,8 @@ static_assert(test_member_null(), "");
 
 }
 }
+
+namespace GH149500 {
+  unsigned int * p = &(*(unsigned int *)0x400);
+  static const void *q = &(*(const struct sysrq_key_op *)0);
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/149648