compiler: avoid copy for string([]byte) conversion used in string concatenation

    
    If a string([]byte) conversion is used immediately in a string
    concatenation, we don't need to copy the backing store of the
    byte slice, as the runtime function doesn't hold any reference
    to it.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182437

	* go.dg/concatstring.go: New test.

From-SVN: r272460

gcc/go/gofrontend/MERGE

-b1ae35965cadac235d7d218e689944286cccdd90
+62d1b667f3e85f72a186b04aad36d701160a4611
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -7408,6 +7408,26 @@ String_concat_expression::do_flatten(Gogo*, Named_object*,
     return this;
   Location loc = this->location();
   Type* type = this->type();
+
+  // Mark string([]byte) operands to reuse the backing store.
+  // runtime.concatstrings does not keep the reference.
+  //
+  // Note: in the gc runtime, if all but one inputs are empty,
+  // concatstrings returns the only nonempty input without copy.
+  // So it is not safe to reuse the backing store if it is a
+  // string([]byte) conversion. So the gc compiler does the
+  // no-copy optimization only when there is at least one
+  // constant nonempty input. Currently the gccgo runtime
+  // doesn't do this, so we don't do the check.
+  for (Expression_list::iterator p = this->exprs_->begin();
+       p != this->exprs_->end();
+       ++p)
+    {
+      Type_conversion_expression* tce = (*p)->conversion_expression();
+      if (tce != NULL)
+        tce->set_no_copy(true);
+    }
+
   Expression* nil_arg = Expression::make_nil(loc);
   Expression* call;
   switch (this->exprs_->size())

gcc/testsuite/ChangeLog

+2019-06-18  Cherry Zhang  <cherryyz@google.com>
+
+	* go.dg/concatstring.go: New test.
+
 2019-06-18  Thomas Schwinge  <thomas@codesourcery.com>
 
 	PR fortran/90921

gcc/testsuite/go.dg/concatstring.go

+// { dg-do compile }
+// { dg-options "-fgo-debug-optimization" }
+
+package p
+
+func F(b []byte, x string) string {
+	return "hello " + string(b) + x // { dg-error "no copy string\\(\\\[\\\]byte\\)" }
+}