PR middle-end/95126: Expand small const structs as immediate constants.

PR middle-end/95126: Expand small const structs as immediate constants.

This patch resolves PR middle-end/95126 which is a code quality regression,
by teaching the RTL expander to emit small const structs/unions as integer
immediate constants.

The motivating example from the bugzilla PR is:

struct small{ short a,b; signed char c; };
extern int func(struct small X);
void call_func(void)
{
    static struct small const s = { 1, 2, 0 };
    func(s);
}

which on x86_64 is currently compiled to:

call_func:
        movzwl  s.0+2(%rip), %eax
        movzwl  s.0(%rip), %edx
        movzwl  s.0+4(%rip), %edi
        salq    $16, %rax
        orq     %rdx, %rax
        salq    $32, %rdi
        orq     %rax, %rdi
        jmp     func

but with this patch is now optimized to:

call_func:
        movl    $131073, %edi
        jmp     func

2022-06-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/95126
	* calls.cc (load_register_parameters): When loading a suitable
	immediate_const_ctor_p VAR_DECL into a single word_mode register,
	construct it directly in a pseudo rather than read it (by parts)
	from memory.
	* expr.cc (int_expr_size): Make tree argument a const_tree.
	(immediate_const_ctor_p): Helper predicate.  Return true for
	simple constructors that may be materialized in a register.
	(expand_expr_real_1) [VAR_DECL]: When expanding a constant
	VAR_DECL with a suitable immediate_const_ctor_p constructor
	use store_constructor to materialize it directly in a pseudo.
	* expr.h (immediate_const_ctor_p): Prototype here.
	* varasm.cc (initializer_constant_valid_for_bitfield_p): Change
	VALUE argument from tree to const_tree.
	* varasm.h (initializer_constant_valid_for_bitfield_p): Update
	prototype.

gcc/testsuite/ChangeLog
	PR middle-end/95126
	* gcc.target/i386/pr95126-m32-1.c: New test case.
	* gcc.target/i386/pr95126-m32-2.c: New test case.
	* gcc.target/i386/pr95126-m32-3.c: New test case.
	* gcc.target/i386/pr95126-m32-4.c: New test case.
	* gcc.target/i386/pr95126-m64-1.c: New test case.
	* gcc.target/i386/pr95126-m64-2.c: New test case.
	* gcc.target/i386/pr95126-m64-3.c: New test case.
	* gcc.target/i386/pr95126-m64-4.c: New test case.