d: Fix accesses of immutable arrays using constant index still bounds checked

Starts setting TREE_READONLY against specific kinds of VAR_DECLs, so
that the middle-end/optimization passes can more aggressively constant
fold D code that makes use of `immutable' or `const'.

	PR d/110514

gcc/d/ChangeLog:

	* decl.cc (get_symbol_decl): Set TREE_READONLY on certain kinds of
	const and immutable variables.
	* expr.cc (ExprVisitor::visit (ArrayLiteralExp *)): Set TREE_READONLY
	on immutable dynamic array literals.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr110514a.d: New test.
	* gdc.dg/pr110514b.d: New test.
	* gdc.dg/pr110514c.d: New test.
	* gdc.dg/pr110514d.d: New test.

(cherry picked from commit 61b1c562f8c703bff045e91257120e42b7fae523)

gcc/d/decl.cc

@@ -1277,6 +1277,20 @@ get_symbol_decl (Declaration *decl)
 		DECL_INITIAL (decl->csym) = build_expr (ie, true);
 	    }
 	}
+
+      /* [type-qualifiers/const-and-immutable]
+
+	 `immutable` applies to data that cannot change. Immutable data values,
+	 once constructed, remain the same for the duration of the program's
+	 execution.  */
+      if (vd->isImmutable () && !vd->setInCtorOnly ())
+	TREE_READONLY (decl->csym) = 1;
+
+      /* `const` applies to data that cannot be changed by the const reference
+	 to that data. It may, however, be changed by another reference to that
+	 same data.  */
+      if (vd->isConst () && !vd->isDataseg ())
+	TREE_READONLY (decl->csym) = 1;
     }
 
   /* Set the declaration mangled identifier if static.  */

gcc/d/expr.cc

@@ -2701,6 +2701,10 @@ public:
 	    if (tb->ty == TY::Tarray)
 	      ctor = d_array_value (type, size_int (e->elements->length), ctor);
 
+	    /* Immutable literals can be placed in rodata.  */
+	    if (tb->isImmutable ())
+	      TREE_READONLY (decl) = 1;
+
 	    d_pushdecl (decl);
 	    rest_of_decl_compilation (decl, 1, 0);
 	  }

gcc/testsuite/gdc.dg/pr110514a.d

@@ -0,0 +1,9 @@
+// { dg-do "compile" }
+// { dg-options "-O -fdump-tree-optimized" }
+immutable uint[] imm_arr = [1,2,3];
+int test_imm(immutable uint[] ptr)
+{
+    return imm_arr[2] == 3 ? 123 : 456;
+}
+// { dg-final { scan-assembler-not "_d_arraybounds_indexp" } }
+// { dg-final { scan-tree-dump "return 123;" optimized } }

gcc/testsuite/gdc.dg/pr110514b.d

@@ -0,0 +1,8 @@
+// { dg-do "compile" }
+// { dg-options "-O" }
+immutable uint[] imm_ctor_arr;
+int test_imm_ctor(immutable uint[] ptr)
+{
+    return imm_ctor_arr[2] == 3;
+}
+// { dg-final { scan-assembler "_d_arraybounds_indexp" } }

gcc/testsuite/gdc.dg/pr110514c.d

@@ -0,0 +1,8 @@
+// { dg-do "compile" }
+// { dg-options "-O" }
+const uint[] cst_arr = [1,2,3];
+int test_cst(const uint[] ptr)
+{
+    return cst_arr[2] == 3;
+}
+// { dg-final { scan-assembler "_d_arraybounds_indexp" } }

gcc/testsuite/gdc.dg/pr110514d.d

@@ -0,0 +1,8 @@
+// { dg-do "compile" }
+// { dg-options "-O" }
+const uint[] cst_ctor_arr;
+int test_cst_ctor(const uint[] ptr)
+{
+    return cst_ctor_arr[2] == 3;
+}
+// { dg-final { scan-assembler "_d_arraybounds_indexp" } }