diff --git a/flang/include/flang/Optimizer/Dialect/FIROps.td b/flang/include/flang/Optimizer/Dialect/FIROps.td
--- a/flang/include/flang/Optimizer/Dialect/FIROps.td
+++ b/flang/include/flang/Optimizer/Dialect/FIROps.td
@@ -2919,7 +2919,13 @@
   let results = (outs BoolLike);
 }
 
+// fir.declare leads to no codegen so the side effects implementation should be
+// Pure. However, this would allow dead code elimination to remove these
+// operations if the values are unused. fir.declare may be used to generate
+// debug information so we would like to keep this around even if the value
+// is not used. I've given it the MemAlloc effect to model this.
 def fir_DeclareOp : fir_Op<"declare", [AttrSizedOperandSegments,
+    MemoryEffects<[MemAlloc<AutomaticAllocationScopeResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable";
 
diff --git a/flang/include/flang/Optimizer/HLFIR/HLFIROps.td b/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
--- a/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
+++ b/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
@@ -28,8 +28,14 @@
   : Op<hlfir_Dialect, mnemonic, traits>;
 
 
-
+// DeclareOp can be lowered to EmboxOp, EmboxCharOp, ReboxOp, etc and so could
+// generate code. All of the operations it can generate are modelled with
+// NoMemoryEffect. However, if hlfir.declare is given NoMemoryEffect, it can be
+// removed by dead code elimination if the value result is unused. Information
+// from the delclare operation can be used to generate debug information so we
+// don't want to remove it as dead code. This is modelled as a MemAlloc effect.
 def hlfir_DeclareOp : hlfir_Op<"declare", [AttrSizedOperandSegments,
+    MemoryEffects<[MemAlloc<AutomaticAllocationScopeResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable and produce an SSA value that can be used as a variable in HLFIR operations";