Avoid undefined behavior in shift operators during constant folding of DIExpressions. (#116466)

Bit shift operations with a shift operand greater than or equal to the bit width
of the (promoted) value type result in undefined behavior according to C++
[expr.shift]p1. This change adds checking for this situation and avoids attempts
to constant fold DIExpressions that would otherwise provoke such behavior.
An existing test that presumably intended to exercise shifts at the UB boundary
has been updated; it now checks for shifts of 64 bits instead of 65. This issue
was reported by a static analysis tool; no actual cases of shift operations that
would result in undefined behavior in practice have been identified.

Author: tahonermann

llvm/lib/IR/DIExpressionOptimizer.cpp

@@ -59,12 +59,14 @@ foldOperationIfPossible(uint64_t Const1, uint64_t Const2,
     return Const1 - Const2;
   }
   case dwarf::DW_OP_shl: {
-    if ((uint64_t)countl_zero(Const1) < Const2)
+    if (Const2 >= std::numeric_limits<uint64_t>::digits ||
+        static_cast<uint64_t>(countl_zero(Const1)) < Const2)
       return std::nullopt;
     return Const1 << Const2;
   }
   case dwarf::DW_OP_shr: {
-    if ((uint64_t)countr_zero(Const1) < Const2)
+    if (Const2 >= std::numeric_limits<uint64_t>::digits ||
+        static_cast<uint64_t>(countr_zero(Const1)) < Const2)
       return std::nullopt;
     return Const1 >> Const2;
   }

llvm/unittests/IR/MetadataTest.cpp

@@ -3541,38 +3541,38 @@ TEST_F(DIExpressionTest, Fold) {
   ResExpr = DIExpression::get(Context, ResOps);
   EXPECT_EQ(E, ResExpr);
 
-  // Test a left shift greater than 64.
+  // Test a left shift greater than 63.
   Ops.clear();
   Ops.push_back(dwarf::DW_OP_constu);
   Ops.push_back(1);
   Ops.push_back(dwarf::DW_OP_constu);
-  Ops.push_back(65);
+  Ops.push_back(64);
   Ops.push_back(dwarf::DW_OP_shl);
   Expr = DIExpression::get(Context, Ops);
   E = Expr->foldConstantMath();
   ResOps.clear();
   ResOps.push_back(dwarf::DW_OP_constu);
   ResOps.push_back(1);
   ResOps.push_back(dwarf::DW_OP_constu);
-  ResOps.push_back(65);
+  ResOps.push_back(64);
   ResOps.push_back(dwarf::DW_OP_shl);
   ResExpr = DIExpression::get(Context, ResOps);
   EXPECT_EQ(E, ResExpr);
 
-  // Test a right shift greater than 64.
+  // Test a right shift greater than 63.
   Ops.clear();
   Ops.push_back(dwarf::DW_OP_constu);
   Ops.push_back(1);
   Ops.push_back(dwarf::DW_OP_constu);
-  Ops.push_back(65);
+  Ops.push_back(64);
   Ops.push_back(dwarf::DW_OP_shr);
   Expr = DIExpression::get(Context, Ops);
   E = Expr->foldConstantMath();
   ResOps.clear();
   ResOps.push_back(dwarf::DW_OP_constu);
   ResOps.push_back(1);
   ResOps.push_back(dwarf::DW_OP_constu);
-  ResOps.push_back(65);
+  ResOps.push_back(64);
   ResOps.push_back(dwarf::DW_OP_shr);
   ResExpr = DIExpression::get(Context, ResOps);
   EXPECT_EQ(E, ResExpr);