Go patch committed: Omit write barrier for assignment to *(convert(&local))

Ian Lance Taylor iant@golang.org
Fri Jun 21 14:34:00 GMT 2019


This Go frontend patch by Cherry Zhang omits write barriers for
assignments to *(convert(&local)).  Assignments to local variables
don't need a write barrier.  But currently the compiler inserts a
write barrier if the LHS is a local variable with type converted, as
*(convert(&local)).  Let the compiler recognize this pattern and omit
the write barrier.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian
-------------- next part --------------
Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE	(revision 272549)
+++ gcc/go/gofrontend/MERGE	(working copy)
@@ -1,4 +1,4 @@
-62e3a8cc0a862b0abd3d0b1ef6cf4b228992a137
+593f94f008c24f5abfe7f917a717cf2b0a2585e2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/wb.cc
===================================================================
--- gcc/go/gofrontend/wb.cc	(revision 272130)
+++ gcc/go/gofrontend/wb.cc	(working copy)
@@ -735,6 +735,26 @@ Gogo::assign_needs_write_barrier(Express
 	}
     }
 
+  // Nothing to do for an assignment to *(convert(&x)) where
+  // x is local variable or a temporary variable.
+  Unary_expression* ue = lhs->unary_expression();
+  if (ue != NULL && ue->op() == OPERATOR_MULT)
+    {
+      Expression* expr = ue->operand();
+      while (true)
+        {
+          if (expr->conversion_expression() != NULL)
+            expr = expr->conversion_expression()->expr();
+          else if (expr->unsafe_conversion_expression() != NULL)
+            expr = expr->unsafe_conversion_expression()->expr();
+          else
+            break;
+        }
+      ue = expr->unary_expression();
+      if (ue != NULL && ue->op() == OPERATOR_AND)
+        return this->assign_needs_write_barrier(ue->operand());
+    }
+
   // For a struct assignment, we don't need a write barrier if all the
   // pointer types can not be in the heap.
   Struct_type* st = lhs->type()->struct_type();


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