compiler: eliminate bound checks in append expression

    
    The compiler generates two array index expressions when lowering
    an append expression. Currently they generate bound checks.
    Bound checks are not necessary in this case, as we know the slice
    has, or will grow to, enough length and capacity. Eliminate them.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/166817

From-SVN: r269699

1 files changed, 18 insertions, 10 deletions

diff --git a/gcc/go/gofrontend/expressions.h b/gcc/go/gofrontend/expressions.h
index 5d61b69..c521d9b 100644
--- a/gcc/go/gofrontend/expressions.h
+++ b/gcc/go/gofrontend/expressions.h
@@ -2854,7 +2854,7 @@ class Array_index_expression : public Expression
 			 Expression* end, Expression* cap, Location location)
     : Expression(EXPRESSION_ARRAY_INDEX, location),
       array_(array), start_(start), end_(end), cap_(cap), type_(NULL),
-      is_lvalue_(false)
+      is_lvalue_(false), needs_bounds_check_(true)
   { }
 
   // Return the array.
@@ -2898,6 +2898,10 @@ class Array_index_expression : public Expression
   set_is_lvalue()
   { this->is_lvalue_ = true; }
 
+  void
+  set_needs_bounds_check(bool b)
+  { this->needs_bounds_check_ = b; }
+
  protected:
   int
   do_traverse(Traverse*);
@@ -2917,15 +2921,17 @@ class Array_index_expression : public Expression
   Expression*
   do_copy()
   {
-    return Expression::make_array_index(this->array_->copy(),
-					this->start_->copy(),
-					(this->end_ == NULL
-					 ? NULL
-					 : this->end_->copy()),
-					(this->cap_ == NULL
-					 ? NULL
-					 : this->cap_->copy()),
-					this->location());
+    Expression* ret = Expression::make_array_index(this->array_->copy(),
+                                                   this->start_->copy(),
+                                                   (this->end_ == NULL
+                                                    ? NULL
+                                                    : this->end_->copy()),
+                                                   (this->cap_ == NULL
+                                                    ? NULL
+                                                    : this->cap_->copy()),
+                                                   this->location());
+    ret->array_index_expression()->set_needs_bounds_check(this->needs_bounds_check_);
+    return ret;
   }
 
   bool
@@ -2962,6 +2968,8 @@ class Array_index_expression : public Expression
   Type* type_;
   // Whether expr appears in an lvalue context.
   bool is_lvalue_;
+  // Whether bounds check is needed.
+  bool needs_bounds_check_;
 };
 
 // A string index.  This is used for both indexing and slicing.