Merge #1087

1087: Use loop to initialize repeat arrays r=philberty a=dafaust

This PR changes how we compile initializers for arrays of repeating elements. I use the same approach outlined in the comments of the linked issue, with some tweaks. It is very similar to how the D language front-end compiles, the new function `Gcc_backend::array_initializer` is heavily inspired by the D front-end's `build_array_set`

This fixes the issue where the compiler tries to allocate a vec containing all elements of the array to be constructed, and therefore explodes on huge constructions (e.g. `let x = [0u8; 4 * 1024 * 1024 * 1024 * 1024]`)

However, we can only initialize non-const arrays in this way. For arrays in const contexts we must initialize them at compile time, and therefore continue using the old method.

Fixes: #1068 


Co-authored-by: David Faust <david.faust@oracle.com>

3 files changed, 63 insertions, 11 deletions

diff --git a/gcc/rust/backend/rust-compile-context.h b/gcc/rust/backend/rust-compile-context.h
index 4bade5c..de9d03f 100644
--- a/gcc/rust/backend/rust-compile-context.h
+++ b/gcc/rust/backend/rust-compile-context.h
@@ -250,6 +250,10 @@ public:
     fn_stack.push_back (fncontext{fn, ret_addr});
   }
   void pop_fn () { fn_stack.pop_back (); }
+
+  bool in_fn () { return fn_stack.size () != 0; }
+
+  // Note: it is undefined behavior to call peek_fn () if fn_stack is empty.
   fncontext peek_fn () { return fn_stack.back (); }
 
   void push_type (tree t) { type_decls.push_back (t); }
@@ -301,6 +305,14 @@ public:
     return pop;
   }
 
+  void push_const_context (void) { const_context++; }
+  void pop_const_context (void)
+  {
+    if (const_context > 0)
+      const_context--;
+  }
+  bool const_context_p (void) { return (const_context > 0); }
+
   std::string mangle_item (const TyTy::BaseType *ty,
 			   const Resolver::CanonicalPath &path) const
   {
@@ -341,6 +353,9 @@ private:
   std::vector<::Bvariable *> var_decls;
   std::vector<tree> const_decls;
   std::vector<tree> func_decls;
+
+  // Nonzero iff we are currently compiling something inside a constant context.
+  unsigned int const_context = 0;
 };
 
 } // namespace Compile
diff --git a/gcc/rust/backend/rust-compile-expr.cc b/gcc/rust/backend/rust-compile-expr.cc
index 74ab6a4..49a6f2f 100644
--- a/gcc/rust/backend/rust-compile-expr.cc
+++ b/gcc/rust/backend/rust-compile-expr.cc
@@ -1163,19 +1163,54 @@ CompileExpr::array_copied_expr (Location expr_locus,
   unsigned HOST_WIDE_INT len
     = wi::ext (max - min + 1, precision, sign).to_uhwi ();
 
-  // create the constructor
-  size_t idx = 0;
-  std::vector<unsigned long> indexes;
-  std::vector<tree> constructor;
-  for (unsigned HOST_WIDE_INT i = 0; i < len; i++)
-    {
-      constructor.push_back (translated_expr);
-      indexes.push_back (idx++);
+  // In a const context we must initialize the entire array, which entails
+  // allocating for each element. If the user wants a huge array, we will OOM
+  // and die horribly.
+  if (ctx->const_context_p ())
+    {
+      size_t idx = 0;
+      std::vector<unsigned long> indexes;
+      std::vector<tree> constructor;
+      for (unsigned HOST_WIDE_INT i = 0; i < len; i++)
+	{
+	  constructor.push_back (translated_expr);
+	  indexes.push_back (idx++);
+	}
+
+      return ctx->get_backend ()->array_constructor_expression (array_type,
+								indexes,
+								constructor,
+								expr_locus);
     }
 
-  return ctx->get_backend ()->array_constructor_expression (array_type, indexes,
-							    constructor,
-							    expr_locus);
+  else
+    {
+      // Create a new block scope in which to initialize the array
+      tree fndecl = NULL_TREE;
+      if (ctx->in_fn ())
+	fndecl = ctx->peek_fn ().fndecl;
+
+      std::vector<Bvariable *> locals;
+      tree enclosing_scope = ctx->peek_enclosing_scope ();
+      tree init_block
+	= ctx->get_backend ()->block (fndecl, enclosing_scope, locals,
+				      expr_locus, expr_locus);
+      ctx->push_block (init_block);
+
+      tree tmp;
+      tree stmts
+	= ctx->get_backend ()->array_initializer (fndecl, init_block,
+						  array_type, capacity_expr,
+						  translated_expr, &tmp,
+						  expr_locus);
+      ctx->add_statement (stmts);
+
+      tree block = ctx->pop_block ();
+
+      // The result is a compound expression which creates a temporary array,
+      // initializes all the elements in a loop, and then yeilds the array.
+      return ctx->get_backend ()->compound_expression (block, tmp, expr_locus);
+    }
 }
 
 tree
diff --git a/gcc/rust/backend/rust-compile-item.cc b/gcc/rust/backend/rust-compile-item.cc
index 21cbb1c..80b7ceb 100644
--- a/gcc/rust/backend/rust-compile-item.cc
+++ b/gcc/rust/backend/rust-compile-item.cc
@@ -92,9 +92,11 @@ CompileItem::visit (HIR::ConstantItem &constant)
   rust_assert (ok);
 
   HIR::Expr *const_value_expr = constant.get_expr ();
+  ctx->push_const_context ();
   tree const_expr
     = compile_constant_item (ctx, resolved_type, canonical_path,
 			     const_value_expr, constant.get_locus ());
+  ctx->pop_const_context ();
 
   ctx->push_const (const_expr);
   ctx->insert_const_decl (constant.get_mappings ().get_hirid (), const_expr);