diff --git a/mlir/lib/IR/PatternMatch.cpp b/mlir/lib/IR/PatternMatch.cpp
--- a/mlir/lib/IR/PatternMatch.cpp
+++ b/mlir/lib/IR/PatternMatch.cpp
@@ -10,9 +10,12 @@
 #include "mlir/IR/BlockAndValueMapping.h"
 #include "mlir/IR/Operation.h"
 #include "mlir/IR/Value.h"
+#include "llvm/Support/Debug.h"
 
 using namespace mlir;
 
+#define DEBUG_TYPE "pattern-match"
+
 PatternBenefit::PatternBenefit(unsigned benefit) : representation(benefit) {
   assert(representation == benefit && benefit != ImpossibleToMatchSentinel &&
          "This pattern match benefit is too large to represent");
@@ -206,9 +209,19 @@
   patterns.clear();
   anyOpPatterns.clear();
   for (const auto &pat : owningPatternList) {
+
     // If the pattern is always impossible to match, just ignore it.
-    if (pat->getBenefit().isImpossibleToMatch())
+    if (pat->getBenefit().isImpossibleToMatch()) {
+
+      LLVM_DEBUG({
+        llvm::dbgs()
+            << "Ignoring pattern '" << pat->getRootKind()
+            << "' because it is impossible to match (by pattern benefit)\n";
+      });
+
       continue;
+    }
+
     if (Optional<OperationName> opName = pat->getRootKind())
       patterns[*opName].push_back(pat.get());
     else
@@ -223,8 +236,16 @@
   auto processPatternList = [&](SmallVectorImpl<RewritePattern *> &list) {
     // Special case for one pattern in the list, which is the most common case.
     if (list.size() == 1) {
-      if (model(*list.front()).isImpossibleToMatch())
+      if (model(*list.front()).isImpossibleToMatch()) {
+
+        LLVM_DEBUG({
+          llvm::dbgs() << "Ignoring pattern '" << list.front()->getRootKind()
+                       << "' because it is impossible to match or cannot lead "
+                          "to legal IR (by cost model)\n";
+        });
+
         list.clear();
+      }
       return;
     }
 
@@ -236,8 +257,16 @@
     // Sort patterns with highest benefit first, and remove those that are
     // impossible to match.
     std::stable_sort(list.begin(), list.end(), cmp);
-    while (!list.empty() && benefits[list.back()].isImpossibleToMatch())
+    while (!list.empty() && benefits[list.back()].isImpossibleToMatch()) {
+
+      LLVM_DEBUG({
+        llvm::dbgs() << "Ignoring pattern '" << list.back()->getRootKind()
+                     << "' because it is impossible to match or cannot lead to "
+                        "legal IR (by cost model)\n";
+      });
+
       list.pop_back();
+    }
   };
   for (auto &it : patterns)
     processPatternList(it.second);