[flang] add memory effects interface to (hl)fir.declare

These operations should have no memory effect, but doing so causes the
declare to be removed by dead code elimination if the result value is
unused. fir.declare is intended to be used to generate debug information
about variables. Debug information may still be desirable even about
unused variables, so we don't want to remove the declare operations when
performing dead code elimination.

Differential Revision: https://reviews.llvm.org/D157626

Author: tblah

flang/include/flang/Optimizer/Dialect/FIRAttr.td

@@ -57,4 +57,7 @@ def fir_FortranVariableFlagsAttr : fir_Attr<"FortranVariableFlags"> {
           "::fir::FortranVariableFlagsAttr::get($_builder.getContext(), $0)";
 }
 
+// mlir::SideEffects::Resource for modelling operations which add debugging information
+def DebuggingResource : Resource<"::fir::DebuggingResource">;
+
 #endif // FIR_DIALECT_FIR_ATTRS

flang/include/flang/Optimizer/Dialect/FIROps.h

@@ -45,6 +45,12 @@ static constexpr llvm::StringRef getNormalizedLowerBoundAttrName() {
   return "normalized.lb";
 }
 
+/// Model operations which affect global debugging information
+struct DebuggingResource
+    : public mlir::SideEffects::Resource::Base<DebuggingResource> {
+  mlir::StringRef getName() final { return "DebuggingResource"; }
+};
+
 } // namespace fir
 
 #define GET_OP_CLASSES

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -2919,7 +2919,13 @@ def fir_IsPresentOp : fir_SimpleOp<"is_present", [NoMemoryEffect]> {
   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.
 def fir_DeclareOp : fir_Op<"declare", [AttrSizedOperandSegments,
+    MemoryEffects<[MemWrite<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable";
 

flang/include/flang/Optimizer/HLFIR/HLFIROps.td

@@ -28,8 +28,14 @@ class hlfir_Op<string mnemonic, list<Trait> traits>
   : 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
 def hlfir_DeclareOp : hlfir_Op<"declare", [AttrSizedOperandSegments,
+    MemoryEffects<[MemWrite<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable and produce an SSA value that can be used as a variable in HLFIR operations";