d: Fix no RVO when returning struct literals initialized with constructor.

Backports a change from upstream dmd that moves front-end NRVO checking
from ReturnStatement semantic to the end of FuncDeclaration semantic.

In the codegen, retStyle has been partially implemented so that only
structs and static arrays return RETstack.  This isn't accurate, but
don't need to be for the purposes of semantic analysis.

If a function either has TREE_ADDRESSABLE or must return in memory, then
DECL_RESULT is set as the shidden field for the function.  This is used
in the codegen pass for ReturnStatement where it is now detected whether
a function is returning a struct literal or a constructor function, then
the DECL_RESULT is used to directly construct the return value, instead
of doing so via temporaries.

Reviewed-on: https://github.com/dlang/dmd/pull/11622

gcc/d/ChangeLog:

	PR d/96156
	* d-frontend.cc (retStyle): Only return RETstack for struct and static
	array types.
	* decl.cc (DeclVisitor::visit (FuncDeclaration *)): Use NRVO return
	for all TREE_ADDRESSABLE types.  Set shidden to the RESULT_DECL.
	* expr.cc (ExprVisitor::visit (CallExp *)): Force TARGET_EXPR if the
	'this' pointer reference is a CONSTRUCTOR.
	(ExprVisitor::visit (StructLiteralExp *)): Generate assignment to the
	symbol to initialize with literal.
	* toir.cc (IRVisitor::visit (ReturnStatement *)): Detect returning
	struct literals and write directly into the RESULT_DECL.
	* dmd/MERGE: Merge upstream dmd fe5f388d8.

gcc/testsuite/ChangeLog:

	PR d/96156
	* gdc.dg/pr96156.d: New test.

8 files changed, 131 insertions, 64 deletions

diff --git a/gcc/d/d-frontend.cc b/gcc/d/d-frontend.cc
index 3b9fc1a..da34e90 100644
--- a/gcc/d/d-frontend.cc
+++ b/gcc/d/d-frontend.cc
@@ -143,12 +143,20 @@ Loc::equals (const Loc &loc)
    hidden pointer to the caller's stack.  */
 
 RET
-retStyle (TypeFunction *)
+retStyle (TypeFunction *tf)
 {
   /* Need the backend type to determine this, but this is called from the
      frontend before semantic processing is finished.  An accurate value
      is not currently needed anyway.  */
-  return RETstack;
+  if (tf->isref)
+    return RETregs;
+
+  Type *tn = tf->next->toBasetype ();
+
+  if (tn->ty == Tstruct || tn->ty == Tsarray)
+    return RETstack;
+
+  return RETregs;
 }
 
 /* Determine if function FD is a builtin one that we can evaluate in CTFE.  */
diff --git a/gcc/d/decl.cc b/gcc/d/decl.cc
index 295f780..008a2bb 100644
--- a/gcc/d/decl.cc
+++ b/gcc/d/decl.cc
@@ -944,20 +944,11 @@ public:
        Implemented by overriding all the RETURN_EXPRs and replacing all
        occurrences of VAR with the RESULT_DECL for the function.
        This is only worth doing for functions that can return in memory.  */
-    if (d->nrvo_can)
-      {
-	tree restype = TREE_TYPE (DECL_RESULT (fndecl));
-
-	if (!AGGREGATE_TYPE_P (restype))
-	  d->nrvo_can = 0;
-	else
-	  d->nrvo_can = aggregate_value_p (restype, fndecl);
-      }
+    tree resdecl = DECL_RESULT (fndecl);
 
-    if (d->nrvo_can)
+    if (TREE_ADDRESSABLE (TREE_TYPE (resdecl))
+	|| aggregate_value_p (TREE_TYPE (resdecl), fndecl))
       {
-	tree resdecl = DECL_RESULT (fndecl);
-
 	/* Return non-trivial structs by invisible reference.  */
 	if (TREE_ADDRESSABLE (TREE_TYPE (resdecl)))
 	  {
@@ -965,9 +956,12 @@ public:
 	    DECL_BY_REFERENCE (resdecl) = 1;
 	    TREE_ADDRESSABLE (resdecl) = 0;
 	    relayout_decl (resdecl);
+	    d->shidden = build_deref (resdecl);
 	  }
+	else
+	  d->shidden = resdecl;
 
-	if (d->nrvo_var)
+	if (d->nrvo_can && d->nrvo_var)
 	  {
 	    tree var = get_symbol_decl (d->nrvo_var);
 
@@ -976,12 +970,9 @@ public:
 	    /* Don't forget that we take its address.  */
 	    TREE_ADDRESSABLE (var) = 1;
 
-	    if (DECL_BY_REFERENCE (resdecl))
-	      resdecl = build_deref (resdecl);
-
 	    SET_DECL_VALUE_EXPR (var, resdecl);
 	    DECL_HAS_VALUE_EXPR_P (var) = 1;
-	    SET_DECL_LANG_NRVO (var, resdecl);
+	    SET_DECL_LANG_NRVO (var, d->shidden);
 	  }
       }
 
diff --git a/gcc/d/dmd/MERGE b/gcc/d/dmd/MERGE
index 2dc2ef3..276406c 100644
--- a/gcc/d/dmd/MERGE
+++ b/gcc/d/dmd/MERGE
@@ -1,4 +1,4 @@
-cb4a96faecb6b521ac4749581bcfa39f61143db0
+fe5f388d8e5d97dccaa4ef1349f931c36a2cbc46
 
 The first line of this file holds the git revision number of the last
 merge done from the dlang/dmd repository.
diff --git a/gcc/d/dmd/declaration.h b/gcc/d/dmd/declaration.h
index 251c407..65ac3f7 100644
--- a/gcc/d/dmd/declaration.h
+++ b/gcc/d/dmd/declaration.h
@@ -669,6 +669,7 @@ public:
     static FuncDeclaration *genCfunc(Parameters *args, Type *treturn, const char *name, StorageClass stc=0);
     static FuncDeclaration *genCfunc(Parameters *args, Type *treturn, Identifier *id, StorageClass stc=0);
     void checkDmain();
+    bool checkNrvo();
 
     FuncDeclaration *isFuncDeclaration() { return this; }
 
diff --git a/gcc/d/dmd/func.c b/gcc/d/dmd/func.c
index 30ba8dd..f8e9601 100644
--- a/gcc/d/dmd/func.c
+++ b/gcc/d/dmd/func.c
@@ -1704,9 +1704,6 @@ void FuncDeclaration::semantic3(Scope *sc)
                 }
             }
 
-            if (!inferRetType && retStyle(f) != RETstack)
-                nrvo_can = 0;
-
             bool inferRef = (f->isref && (storage_class & STCauto));
 
             fbody = ::semantic(fbody, sc2);
@@ -1761,7 +1758,7 @@ void FuncDeclaration::semantic3(Scope *sc)
                 if (storage_class & STCauto)
                     storage_class &= ~STCauto;
             }
-            if (retStyle(f) != RETstack)
+            if (retStyle(f) != RETstack || checkNrvo())
                 nrvo_can = 0;
 
             if (fbody->isErrorStatement())
@@ -4293,6 +4290,53 @@ void FuncDeclaration::checkDmain()
         error("parameters must be main() or main(string[] args)");
 }
 
+/***********************************************
+ * Check all return statements for a function to verify that returning
+ * using NRVO is possible.
+ *
+ * Returns:
+ *      true if the result cannot be returned by hidden reference.
+ */
+bool FuncDeclaration::checkNrvo()
+{
+    if (!nrvo_can)
+        return true;
+
+    if (returns == NULL)
+        return true;
+
+    TypeFunction *tf = type->toTypeFunction();
+    if (tf->isref)
+        return true;
+
+    for (size_t i = 0; i < returns->length; i++)
+    {
+        ReturnStatement *rs = (*returns)[i];
+
+        if (VarExp *ve = rs->exp->isVarExp())
+        {
+            VarDeclaration *v = ve->var->isVarDeclaration();
+            if (!v || v->isOut() || v->isRef())
+                return true;
+            else if (nrvo_var == NULL)
+            {
+                if (!v->isDataseg() && !v->isParameter() && v->toParent2() == this)
+                {
+                    //printf("Setting nrvo to %s\n", v->toChars());
+                    nrvo_var = v;
+                }
+                else
+                    return true;
+            }
+            else if (nrvo_var != v)
+                return true;
+        }
+        else //if (!exp->isLvalue())    // keep NRVO-ability
+            return true;
+    }
+    return false;
+}
+
 const char *FuncDeclaration::kind() const
 {
     return generated ? "generated function" : "function";
diff --git a/gcc/d/dmd/statementsem.c b/gcc/d/dmd/statementsem.c
index 599c52e..9049347 100644
--- a/gcc/d/dmd/statementsem.c
+++ b/gcc/d/dmd/statementsem.c
@@ -2861,42 +2861,9 @@ public:
                  *    return x; return 3;  // ok, x can be a value
                  */
             }
-
-            // handle NRVO
-            if (fd->nrvo_can && rs->exp->op == TOKvar)
-            {
-                VarExp *ve = (VarExp *)rs->exp;
-                VarDeclaration *v = ve->var->isVarDeclaration();
-
-                if (tf->isref)
-                {
-                    // Function returns a reference
-                    if (!inferRef)
-                        fd->nrvo_can = 0;
-                }
-                else if (!v || v->isOut() || v->isRef())
-                    fd->nrvo_can = 0;
-                else if (fd->nrvo_var == NULL)
-                {
-                    if (!v->isDataseg() && !v->isParameter() && v->toParent2() == fd)
-                    {
-                        //printf("Setting nrvo to %s\n", v->toChars());
-                        fd->nrvo_var = v;
-                    }
-                    else
-                        fd->nrvo_can = 0;
-                }
-                else if (fd->nrvo_var != v)
-                    fd->nrvo_can = 0;
-            }
-            else //if (!exp->isLvalue())    // keep NRVO-ability
-                fd->nrvo_can = 0;
         }
         else
         {
-            // handle NRVO
-            fd->nrvo_can = 0;
-
             // infer return type
             if (fd->inferRetType)
             {
diff --git a/gcc/d/expr.cc b/gcc/d/expr.cc
index 85407ac..2b13615 100644
--- a/gcc/d/expr.cc
+++ b/gcc/d/expr.cc
@@ -1782,6 +1782,9 @@ public:
 		    thisexp = TREE_OPERAND (thisexp, 1);
 		  }
 
+		if (TREE_CODE (thisexp) == CONSTRUCTOR)
+		  thisexp = force_target_expr (thisexp);
+
 		/* Want reference to `this' object.  */
 		if (!POINTER_TYPE_P (TREE_TYPE (thisexp)))
 		  thisexp = build_address (thisexp);
@@ -2886,7 +2889,16 @@ public:
        processing has complete.  Build the static initializer now.  */
     if (e->useStaticInit && !this->constp_)
       {
-	this->result_ = aggregate_initializer_decl (e->sd);
+	tree init = aggregate_initializer_decl (e->sd);
+
+	/* If initializing a symbol, don't forget to set it.  */
+	if (e->sym != NULL)
+	  {
+	    tree var = build_deref (e->sym);
+	    init = compound_expr (modify_expr (var, init), var);
+	  }
+
+	this->result_ = init;
 	return;
       }
 
diff --git a/gcc/d/toir.cc b/gcc/d/toir.cc
index fd18b9e..f9bc095 100644
--- a/gcc/d/toir.cc
+++ b/gcc/d/toir.cc
@@ -1015,19 +1015,63 @@ public:
 	&& type->toBasetype ()->ty == Tvoid)
       type = Type::tint32;
 
-    if (this->func_->nrvo_can && this->func_->nrvo_var)
+    if (this->func_->shidden)
       {
-	/* Just refer to the DECL_RESULT; this differs from using
-	   NULL_TREE in that it indicates that we care about the value
-	   of the DECL_RESULT.  */
+	/* Returning by hidden reference, store the result into the retval decl.
+	   The result returned then becomes the retval reference itself.  */
 	tree decl = DECL_RESULT (get_symbol_decl (this->func_));
+	gcc_assert (!tf->isref);
+
+	/* If returning via NRVO, just refer to the DECL_RESULT; this differs
+	   from using NULL_TREE in that it indicates that we care about the
+	   value of the DECL_RESULT.  */
+	if (this->func_->nrvo_can && this->func_->nrvo_var)
+	  {
+	    add_stmt (return_expr (decl));
+	    return;
+	  }
+
+	/* Detect a call to a constructor function, or if returning a struct
+	   literal, write result directly into the return value.  */
+	StructLiteralExp *sle = NULL;
+
+	if (DotVarExp *dve = (s->exp->op == TOKcall
+			      && s->exp->isCallExp ()->e1->op == TOKdotvar
+			      ? s->exp->isCallExp ()->e1->isDotVarExp ()
+			      : NULL))
+	  {
+	    sle = (dve->var->isCtorDeclaration ()
+		   ? dve->e1->isStructLiteralExp () : NULL);
+	  }
+	else
+	  sle = s->exp->isStructLiteralExp ();
+
+	if (sle != NULL)
+	  {
+	    StructDeclaration *sd = type->baseElemOf ()->isTypeStruct ()->sym;
+	    sle->sym = build_address (this->func_->shidden);
+
+	    /* Fill any alignment holes in the return slot using memset.  */
+	    if (!identity_compare_p (sd) || sd->isUnionDeclaration ())
+	      add_stmt (build_memset_call (this->func_->shidden));
+
+	    add_stmt (build_expr_dtor (s->exp));
+	  }
+	else
+	  {
+	    /* Generate: (<retval> = expr, return <retval>);  */
+	    tree expr = build_expr_dtor (s->exp);
+	    tree init = stabilize_expr (&expr);
+	    expr = build_assign (INIT_EXPR, this->func_->shidden, expr);
+	    add_stmt (compound_expr (init, expr));
+	  }
+
 	add_stmt (return_expr (decl));
       }
     else
       {
 	/* Convert for initializing the DECL_RESULT.  */
-	tree expr = build_return_dtor (s->exp, type, tf);
-	add_stmt (expr);
+	add_stmt (build_return_dtor (s->exp, type, tf));
       }
   }