[-Wunsafe-buffer-usage] Fix false warnings when const sized array is safely accessed (array [idx %const]) (llvm#140113)

The -Wunsafe-buffer-usage analysis currently warns when a const sized
array is safely accessed, with an index that is bound by the remainder
operator. The false alarm is now suppressed.

Example:

int circular_buffer(int array[10], uint idx) {
   return array[idx %10]; // Safe operation
}

rdar://148443453

---------

Co-authored-by: MalavikaSamak <[email protected]>

Author: malavikasamak

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -600,12 +600,27 @@ static bool isSafeArraySubscript(const ArraySubscriptExpr &Node,
   } else if (const auto *BE = dyn_cast<BinaryOperator>(IndexExpr)) {
     // For an integer expression `e` and an integer constant `n`, `e & n` and
     // `n & e` are bounded by `n`:
-    if (BE->getOpcode() != BO_And)
+    if (BE->getOpcode() != BO_And && BE->getOpcode() != BO_Rem)
       return false;
 
     const Expr *LHS = BE->getLHS();
     const Expr *RHS = BE->getRHS();
 
+    if (BE->getOpcode() == BO_Rem) {
+      // If n is a negative number, then n % const can be greater than const
+      if (!LHS->getType()->isUnsignedIntegerType()) {
+        return false;
+      }
+
+      if (!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
+        llvm::APSInt result = EVResult.Val.getInt();
+        if (result.isNonNegative() && result.getLimitedValue() <= limit)
+          return true;
+      }
+
+      return false;
+    }
+
     if ((!LHS->isValueDependent() &&
          LHS->EvaluateAsInt(EVResult, Ctx)) || // case: `n & e`
         (!RHS->isValueDependent() &&

clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp

@@ -35,7 +35,21 @@ void constant_idx_safe0(unsigned idx) {
   buffer[0] = 0;
 }
 
-int array[10]; // expected-warning {{'array' is an unsafe buffer that does not perform bounds checks}}
+int array[10]; // expected-warning 3{{'array' is an unsafe buffer that does not perform bounds checks}}
+
+void circular_access_unsigned(unsigned idx) {
+  array[idx % 10];
+  array[idx % 11]; // expected-note {{used in buffer access here}}
+  array[(idx + 3) % 10];
+  array[(--idx) % 8];
+  array[idx & 9 % 10];
+  array[9 & idx % 11];
+  array [12 % 10];
+}
+
+void circular_access_signed(int idx) {
+  array[idx % 10]; // expected-note {{used in buffer access here}}
+}
 
 void masked_idx1(unsigned long long idx, Foo f) {
   // Bitwise and operation