[gcc r13-577] compiler: load LHS subexpressions of op= assignment only once

[gcc r13-577] compiler: load LHS subexpressions of op= assignment only once

[Ian Lance Taylor]

https://gcc.gnu.org/g:1bfb823e2a7346ef55bd53a5354770599f7a550b

commit r13-577-g1bfb823e2a7346ef55bd53a5354770599f7a550b
Author: Ian Lance Taylor <iant@golang.org>
Date:   Tue May 10 17:05:08 2022 -0700

    compiler: load LHS subexpressions of op= assignment only once
    
    Fixes golang/go#52811
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/405617

Diff:
---
 gcc/go/gofrontend/MERGE         |  2 +-
 gcc/go/gofrontend/statements.cc | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index daa725f9de9..5fa8becde3e 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-f5bc28a30b7503015bbef38afb5812313184e822
+9d07072e58ca4f9f05343dfd3475b9f49dae5ec5
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/gcc/go/gofrontend/statements.cc b/gcc/go/gofrontend/statements.cc
index 95fa3c48709..b3db843365e 100644
--- a/gcc/go/gofrontend/statements.cc
+++ b/gcc/go/gofrontend/statements.cc
@@ -1260,6 +1260,16 @@ Assignment_operation_statement::do_lower(Gogo*, Named_object*,
   Move_ordered_evals moe(b);
   this->lhs_->traverse_subexpressions(&moe);
 
+  // We can still be left with subexpressions that have to be loaded
+  // even if they don't have side effects themselves, in case the RHS
+  // changes variables named on the LHS.
+  int i;
+  if (this->lhs_->must_eval_subexpressions_in_order(&i))
+    {
+      Move_subexpressions ms(i, b);
+      this->lhs_->traverse_subexpressions(&ms);
+    }
+
   Expression* lval = this->lhs_->copy();
 
   Operator op;