[clang-tidy] bugprone-infinite-loop: Fix false positives with volatile addresses.

Low-level code may occasionally deal with direct access by concrete addresses
such as 0x1234. Values at these addresses act like globals: they can change
at any time. They typically wear volatile qualifiers.

Suppress all warnings on loops with conditions that involve casting anything to
a pointer-to-...-pointer-to-volatile type.

The closely related bugprone-redundant-branch-condition check
doesn't seem to be affected. Add a test just in case.

Differential Revision: https://reviews.llvm.org/D108808

clang-tools-extra/clang-tidy/bugprone/InfiniteLoopCheck.cpp

@@ -65,6 +65,17 @@ static bool isVarThatIsPossiblyChanged(const Decl *Func, const Stmt *LoopStmt,
                  ObjCIvarRefExpr, ObjCPropertyRefExpr, ObjCMessageExpr>(Cond)) {
     // FIXME: Handle MemberExpr.
     return true;
+  } else if (const auto *CE = dyn_cast<CastExpr>(Cond)) {
+    QualType T = CE->getType();
+    while (true) {
+      if (T.isVolatileQualified())
+        return true;
+
+      if (!T->isAnyPointerType() && !T->isReferenceType())
+        break;
+
+      T = T->getPointeeType();
+    }
   }
 
   return false;

clang-tools-extra/test/clang-tidy/checkers/bugprone-infinite-loop.cpp

@@ -591,3 +591,34 @@ void test_structured_bindings_bad() {
       // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: this loop is infinite; none of its condition variables (x) are updated in the loop body [bugprone-infinite-loop]
   }
 }
+
+void test_volatile_cast() {
+  // This is a no-op cast. Clang ignores the qualifier, we should too.
+  for (int i = 0; (volatile int)i < 10;) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: this loop is infinite; none of its condition variables (i) are updated in the loop body [bugprone-infinite-loop]
+  }
+}
+
+void test_volatile_concrete_address(int i, int size) {
+  // No warning. The value behind the volatile concrete address
+  // is beyond our control. It may change at any time.
+  for (; *((volatile int *)0x1234) < size;) {
+  }
+
+  for (; *((volatile int *)(0x1234 + i)) < size;) {
+  }
+
+  for (; **((volatile int **)0x1234) < size;) {
+  }
+
+  volatile int *x = (volatile int *)0x1234;
+  for (; *x < 10;) {
+  }
+
+  // FIXME: This one should probably also be suppressed.
+  // Whatever the developer is doing here, they can do that again anywhere else
+  // which basically makes it a global.
+  for (; *(int *)0x1234 < size;) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: this loop is infinite; none of its condition variables (size) are updated in the loop body [bugprone-infinite-loop]
+  }
+}

clang-tools-extra/test/clang-tidy/checkers/bugprone-redundant-branch-condition.cpp

@@ -1387,3 +1387,13 @@ void relational_operator_reversed() {
     }
   }
 }
+
+void volatile_concrete_address() {
+  // No warning. The value behind the volatile concrete address
+  // is beyond our control. It may change at any time.
+  if (*(volatile int *)0x1234) {
+    if (*(volatile int *)0x1234) {
+      doSomething();
+    }
+  }
+}