middle-end/102285 - refine .DEFERRED_INIT expansion

This refines the way we figure whether we are facing a register
that cannot be initialized by emitting a memset away from inspecting
expanded RTL of the LHS to using the predicates expand_assignment
is using to detect decls or MEM_REFs with non-memory DECL_RTL.

2021-10-04  Richard Biener  <rguenther@suse.de>

	* expr.h (non_mem_decl_p): Declare.
	(mem_ref_refers_to_non_mem_p): Likewise.
	* expr.c (non_mem_decl_p): Export.
	(mem_ref_refers_to_non_mem_p): Likewise.
	* internal-fn.c (expand_DEFERRED_INIT): Do not expand the LHS
	but check the base with mem_ref_refers_to_non_mem_p
	and non_mem_decl_p.

	* c-c++-common/pr102285.c: New testcase.

4 files changed, 20 insertions, 4 deletions

diff --git a/gcc/expr.c b/gcc/expr.c
index e0bcbcc..eb33643 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -5305,7 +5305,7 @@ get_bit_range (poly_uint64_pod *bitstart, poly_uint64_pod *bitend, tree exp,
    has non-BLKmode.  DECL_RTL must not be a MEM; if
    DECL_RTL was not set yet, return false.  */
 
-static inline bool
+bool
 non_mem_decl_p (tree base)
 {
   if (!DECL_P (base)
@@ -5322,7 +5322,7 @@ non_mem_decl_p (tree base)
 /* Returns true if REF refers to an object that does not
    reside in memory and has non-BLKmode.  */
 
-static inline bool
+bool
 mem_ref_refers_to_non_mem_p (tree ref)
 {
   tree base;
diff --git a/gcc/expr.h b/gcc/expr.h
index 94a85b4..890ec19 100644
--- a/gcc/expr.h
+++ b/gcc/expr.h
@@ -346,4 +346,7 @@ extern void expand_operands (tree, tree, rtx, rtx*, rtx*,
 /* Return an rtx for the size in bytes of the value of an expr.  */
 extern rtx expr_size (tree);
 
+extern bool mem_ref_refers_to_non_mem_p (tree);
+extern bool non_mem_decl_p (tree);
+
 #endif /* GCC_EXPR_H */
diff --git a/gcc/internal-fn.c b/gcc/internal-fn.c
index ef5dc90..1101452 100644
--- a/gcc/internal-fn.c
+++ b/gcc/internal-fn.c
@@ -3015,8 +3015,11 @@ expand_DEFERRED_INIT (internal_fn, gcall *stmt)
     reg_lhs = true;
   else
     {
-      rtx tem = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
-      reg_lhs = !MEM_P (tem);
+      tree lhs_base = lhs;
+      while (handled_component_p (lhs_base))
+	lhs_base = TREE_OPERAND (lhs_base, 0);
+      reg_lhs = (mem_ref_refers_to_non_mem_p (lhs_base)
+		 || non_mem_decl_p (lhs_base));
     }
 
   if (!reg_lhs)
diff --git a/gcc/testsuite/c-c++-common/pr102285.c b/gcc/testsuite/c-c++-common/pr102285.c
new file mode 100644
index 0000000..644054b
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr102285.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O -ftrivial-auto-var-init=zero -Wuninitialized" } */
+
+int
+qy (void)
+{
+  int tw = 4;
+  int fb[tw];
+  return fb[2]; /* { dg-warning "uninitialized" } */
+}