diff --git a/flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp b/flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp
index c57f5140c29e..fdf24028c5f9 100644
--- a/flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp
+++ b/flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp
@@ -18,8 +18,8 @@
 #include "flang/Optimizer/Dialect/FIROps.h"
 #include "flang/Optimizer/Dialect/FIRType.h"
 #include "flang/Optimizer/Dialect/Support/FIRContext.h"
+#include "mlir/IR/Iterators.h"
 #include "mlir/Transforms/DialectConversion.h"
-#include "mlir/Transforms/RegionUtils.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Debug.h"
 
@@ -325,6 +325,25 @@ public:
   }
 };
 
+/// Simple DCE to erase fir.shape/shift/slice/unused shape operands after this
+/// pass (fir.shape and like have no codegen).
+/// mlir::RegionDCE is expensive and requires running
+/// mlir::eraseUnreachableBlocks. It does things that are not needed here, like
+/// removing unused block arguments. fir.shape/shift/slice cannot be block
+/// arguments.
+/// This helper does a naive backward walk of the IR. It is not even guaranteed
+/// to walk blocks according to backward dominance, but that is good enough for
+/// what is done here, fir.shape/shift/slice have no usages anymore. The
+/// backward walk allows getting rid of most of the unused operands, it is not a
+/// problem to leave some in the weird cases.
+static void simpleDCE(mlir::RewriterBase &rewriter, mlir::Operation *op) {
+  op->walk<mlir::WalkOrder::PostOrder, mlir::ReverseIterator>(
+      [&](mlir::Operation *subOp) {
+        if (mlir::isOpTriviallyDead(subOp))
+          rewriter.eraseOp(subOp);
+      });
+}
+
 class CodeGenRewrite : public fir::impl::CodeGenRewriteBase<CodeGenRewrite> {
 public:
   using CodeGenRewriteBase<CodeGenRewrite>::CodeGenRewriteBase;
@@ -356,7 +375,7 @@ public:
     }
     // Erase any residual (fir.shape, fir.slice...).
     mlir::IRRewriter rewriter(&context);
-    (void)mlir::runRegionDCE(rewriter, mod->getRegions());
+    simpleDCE(rewriter, mod.getOperation());
   }
 };
 
diff --git a/flang/test/Fir/declare-codegen.fir b/flang/test/Fir/declare-codegen.fir
index c5879facb157..fe8d84ef2d19 100644
--- a/flang/test/Fir/declare-codegen.fir
+++ b/flang/test/Fir/declare-codegen.fir
@@ -38,3 +38,17 @@ func.func @useless_shape_with_duplicate_extent_operand(%arg0: !fir.ref<!fir.arra
 
 // DECL-LABEL: func.func @useless_shape_with_duplicate_extent_operand(
 // DECL: fircg.ext_declare
+
+// Test DCE does not crash because of unreachable code.
+func.func @unreachable_code(%arg0: !fir.ref<!fir.char<1,10>>) {
+  %c10 = arith.constant 10 : index
+  %2 = fir.declare %arg0 typeparams %c10 {uniq_name = "live_code"} : (!fir.ref<!fir.char<1,10>>, index) -> (!fir.ref<!fir.char<1,10>>)
+  return
+^bb2:  // no predecessors
+  %3 = fir.declare %arg0 typeparams %c10 {uniq_name = "dead_code"} : (!fir.ref<!fir.char<1,10>>, index) -> (!fir.ref<!fir.char<1,10>>)
+  fir.unreachable
+}
+// NODECL-LABEL:   func.func @unreachable_code(
+// NODECL-NOT:       uniq_name = "live_code"
+// DECL-LABEL:    func.func @unreachable_code(
+// DECL:             uniq_name = "live_code"