[DebugInfo] Limit the number of values that may be referenced by a dbg.value

Following the addition of salvaging dbg.values using DIArgLists to
reference multiple values, a case has been found where excessively large
DIArgLists are produced as a result of this salvaging, resulting in
large enough performance costs to effectively freeze the compiler.

This patch introduces an upper bound of 16 to the number of values that
may be salvaged into a dbg.value, to limit the impact of these extreme
cases to performance.

Differential Revision: https://reviews.llvm.org/D103162

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 38ecb75..eeaf43a 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1724,6 +1724,9 @@ void llvm::salvageDebugInfo(Instruction &I) {
 
 void llvm::salvageDebugInfoForDbgValues(
     Instruction &I, ArrayRef<DbgVariableIntrinsic *> DbgUsers) {
+  // This is an arbitrary chosen limit on the maximum number of values we can
+  // salvage up to in a DIArgList, used for performance reasons.
+  const unsigned MaxDebugArgs = 16;
   bool Salvaged = false;
 
   for (auto *DII : DbgUsers) {
@@ -1748,11 +1751,15 @@ void llvm::salvageDebugInfoForDbgValues(
     DII->replaceVariableLocationOp(&I, I.getOperand(0));
     if (AdditionalValues.empty()) {
       DII->setExpression(SalvagedExpr);
-    } else if (isa<DbgValueInst>(DII)) {
+    } else if (isa<DbgValueInst>(DII) &&
+               DII->getNumVariableLocationOps() + AdditionalValues.size() <=
+                   MaxDebugArgs) {
       DII->addVariableLocationOps(AdditionalValues, SalvagedExpr);
     } else {
       // Do not salvage using DIArgList for dbg.addr/dbg.declare, as it is
       // currently only valid for stack value expressions.
+      // Also do not salvage if the resulting DIArgList would contain an
+      // unreasonably large number of values.
       Value *Undef = UndefValue::get(I.getOperand(0)->getType());
       DII->replaceVariableLocationOp(I.getOperand(0), Undef);
     }