Prune unused constant expressions recursively. (#678)

Author: maleadt

src/irgen.jl

@@ -633,16 +633,8 @@ function add_kernel_state!(mod::LLVM.Module)
 
     # ensure the old (stateless) functions don't have uses anymore, and remove them
     for f in keys(workmap)
-        for use in uses(f)
-            val = user(use)
-            if val isa ConstantExpr
-                # XXX: shouldn't clone_into! remove unused CEs?
-                isempty(uses(val)) || error("old function still has uses (via a constant expr)")
-                LLVM.unsafe_destroy!(val)
-            else
-                error("old function still has uses")
-            end
-        end
+        prune_constexpr_uses!(f)
+        @assert isempty(uses(f))
         replace_metadata_uses!(f, workmap[f])
         erase!(f)
     end

src/metal.jl

@@ -406,14 +406,7 @@ function add_global_address_spaces!(@nospecialize(job::CompilerJob), mod::LLVM.M
 
     # delete old globals
     for (old, new) in global_map
-        for use in uses(old)
-            val = user(use)
-            if val isa ConstantExpr
-                # XXX: shouldn't clone_into! remove unused CEs?
-                isempty(uses(val)) || error("old function still has uses (via a constant expr)")
-                LLVM.unsafe_destroy!(val)
-            end
-        end
+        prune_constexpr_uses!(old)
         @assert isempty(uses(old))
         replace_metadata_uses!(old, new)
         erase!(old)
@@ -637,12 +630,7 @@ function add_input_arguments!(@nospecialize(job::CompilerJob), mod::LLVM.Module,
     # drop unused constants that may be referring to the old functions
     # XXX: can we do this differently?
     for f in worklist
-        for use in uses(f)
-            val = user(use)
-            if val isa LLVM.ConstantExpr && isempty(uses(val))
-                LLVM.unsafe_destroy!(val)
-            end
-        end
+        prune_constexpr_uses!(f)
     end
 
     # update other uses of the old function, modifying call sites to pass the arguments

src/utils.jl

@@ -132,3 +132,20 @@ macro unlocked(ex)
     end
     esc(combinedef(def))
 end
+
+
+## constant expression pruning
+
+# for some reason, after cloning the LLVM IR can contain unused constant expressions.
+# these result in false positives when checking that values are unused and can be deleted.
+# this helper function removes such unused constant expression uses of a value.
+# the process needs to be recursive, as constant expressions can refer to one another.
+function prune_constexpr_uses!(root::LLVM.Value)
+    for use in uses(root)
+        val = user(use)
+        if val isa ConstantExpr
+            prune_constexpr_uses!(val)
+            isempty(uses(val)) && LLVM.unsafe_destroy!(val)
+        end
+    end
+end