PR middle-end/105853: Call store_constructor directly from calls.cc.

This patch fixes both ICE regressions PR middle-end/105853 and
PR target/105856 caused by my recent patch to expand small const structs
as immediate constants.  That patch updated code generation in three
places: two in expr.cc that call store_constructor directly, and the
third in calls.cc's load_register_parameters that expands its CONSTRUCTOR
via expand_expr, as store_constructor is local/static to expr.cc, and
the "public" API, should usually simply forward the constructor to the
appropriate store_constructor function.

Alas, despite the clean regression testing on multiple targets, the above
ICEs show that expand_expr isn't a suitable proxy for store_constructor,
and things that (I'd assumed) shouldn't affect how/whether a struct is
placed in a register [such as whether the struct is considered packed/
aligned or not] actually interfere with the optimization that is being
attempted.

The (proposed) solution is to export store_constructor (and it's helper
function int_expr_size) from expr.cc, by removing their static qualifier
and prototyping both functions in expr.h, so they can be called directly
from load_register_parameters in calls.cc.  This cures both ICEs, but
almost as importantly improves code generation over GCC 12.

For PR 105853, GCC 12 generates:

compose_nd_na_ipv6_src:
	movzx eax, WORD PTR eth_addr_zero[rip+2]
	movzx edx, WORD PTR eth_addr_zero[rip]
	movzx edi, WORD PTR eth_addr_zero[rip+4]
	sal rax, 16
	or rax, rdx
	sal rdi, 32
	or rdi, rax
	xor eax, eax
	jmp packet_set_nd
eth_addr_zero:	.zero 6

where now (with this fix) GCC 13 generates:
compose_nd_na_ipv6_src:
        xorl    %edi, %edi
        xorl    %eax, %eax
        jmp     packet_set_nd

Likewise, for PR 105856 on ARM, we'd previously generate:
g_329_3:
	movw r3, #:lower16:.LANCHOR0
	movt r3, #:upper16:.LANCHOR0
	ldr r0, [r3]
	b func_19

but with this optimization we now generate:
g_329_3:
        mov     r0, gcc-mirror#6
        b       func_19

2022-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/105853
	PR target/105856
	* calls.cc (load_register_parameters): Call store_constructor
	and int_expr_size directly instead of expanding via expand_expr.
	* expr.cc (static void store_constructor): Don't prototype here.
	(static HOST_WIDE_INT int_expr_size): Likewise.
	(store_constructor): No longer static.
	(int_expr_size): Likewise, no longer static.
	* expr.h (store_constructor): Prototype here.
	(int_expr_size): Prototype here.

gcc/testsuite/ChangeLog
	PR middle-end/105853
	PR target/105856
	* gcc.dg/pr105853.c: New test case.
	* gcc.dg/pr105856.c: New test case.

gcc/calls.cc

@@ -2186,10 +2186,11 @@ load_register_parameters (struct arg_data *args, int num_actuals,
 		   && immediate_const_ctor_p (DECL_INITIAL (tree_value)))
 	    {
 	      rtx target = gen_reg_rtx (word_mode);
-	      rtx x = expand_expr (DECL_INITIAL (tree_value),
-				   target, word_mode, EXPAND_NORMAL);
+	      store_constructor (DECL_INITIAL (tree_value), target, 0,
+				 int_expr_size (DECL_INITIAL (tree_value)),
+				 false);
 	      reg = gen_rtx_REG (word_mode, REGNO (reg));
-	      emit_move_insn (reg, x);
+	      emit_move_insn (reg, target);
 	    }
 	  else if (partial == 0 || args[i].pass_on_stack)
 	    {

gcc/expr.cc

@@ -84,7 +84,6 @@ static void emit_block_move_via_loop (rtx, rtx, rtx, unsigned);
 static void clear_by_pieces (rtx, unsigned HOST_WIDE_INT, unsigned int);
 static rtx_insn *compress_float_constant (rtx, rtx);
 static rtx get_subtarget (rtx);
-static void store_constructor (tree, rtx, int, poly_int64, bool);
 static rtx store_field (rtx, poly_int64, poly_int64, poly_uint64, poly_uint64,
 			machine_mode, tree, alias_set_type, bool, bool);
 
@@ -100,7 +99,6 @@ static void do_tablejump (rtx, machine_mode, rtx, rtx, rtx,
 			  profile_probability);
 static rtx const_vector_from_tree (tree);
 static tree tree_expr_size (const_tree);
-static HOST_WIDE_INT int_expr_size (const_tree);
 static void convert_mode_scalar (rtx, rtx, int);
 
 
@@ -6757,7 +6755,7 @@ fields_length (const_tree type)
    which has been packed to exclude padding bits.
    If REVERSE is true, the store is to be done in reverse order.  */
 
-static void
+void
 store_constructor (tree exp, rtx target, int cleared, poly_int64 size,
 		   bool reverse)
 {
@@ -13209,7 +13207,7 @@ expr_size (tree exp)
 /* Return a wide integer for the size in bytes of the value of EXP, or -1
    if the size can vary or is larger than an integer.  */
 
-static HOST_WIDE_INT
+HOST_WIDE_INT
 int_expr_size (const_tree exp)
 {
   tree size;

gcc/expr.h

@@ -339,6 +339,8 @@ extern bool categorize_ctor_elements (const_tree, HOST_WIDE_INT *,
 				      HOST_WIDE_INT *, HOST_WIDE_INT *,
 				      bool *);
 extern bool immediate_const_ctor_p (const_tree, unsigned int words = 1);
+extern void store_constructor (tree, rtx, int, poly_int64, bool);
+extern HOST_WIDE_INT int_expr_size (const_tree exp);
 
 extern void expand_operands (tree, tree, rtx, rtx*, rtx*,
 			     enum expand_modifier);

gcc/testsuite/gcc.dg/pr105853.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+struct {
+  struct {
+    short e16[3];
+  }
+} const eth_addr_zero = {{}};  /* { dg-warning "no semicolon at" } */
+void compose_nd_na_ipv6_src() {
+  packet_set_nd(eth_addr_zero); /* { dg-warning "implicit declaration" } */
+}

gcc/testsuite/gcc.dg/pr105856.c

@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+#pragma pack(1)
+struct {
+  unsigned f0;
+} static g_251 = {6};
+void g_329_3() {
+  func_19(g_251);  /* { dg-warning "implicit declaration" } */
+}
+