Avoid duplicate vector initializations during RTL expansion.

This middle-end patch avoids some redundant RTL for vector initialization
during RTL expansion.  For the simple test case:

typedef __int128 v1ti __attribute__ ((__vector_size__ (16)));
__int128 key;

v1ti foo() {
    return (v1ti){key};
}

the middle-end currently expands:

(set (reg:V1TI 85) (const_vector:V1TI [ (const_int 0) ]))

(set (reg:V1TI 85) (mem/c:V1TI (symbol_ref:DI ("key"))))

where we create a dead instruction that initializes the vector to zero,
immediately followed by a set of the entire vector.  This patch skips
this zeroing instruction when the vector has only a single element.
It also updates the code to indicate when we've cleared the vector,
so that we don't need to initialize zero elements.

2023-06-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expr.cc (store_constructor) <case VECTOR_TYPE>: Don't bother
	clearing vectors with only a single element.  Set CLEARED if the
	vector was initialized to zero.

1 files changed, 5 insertions, 2 deletions

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 868fa6e..62cd8fa 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -7531,8 +7531,11 @@ store_constructor (tree exp, rtx target, int cleared, poly_int64 size,
 	  }
 
 	/* Inform later passes that the old value is dead.  */
-	if (!cleared && !vector && REG_P (target))
-	  emit_move_insn (target, CONST0_RTX (mode));
+	if (!cleared && !vector && REG_P (target) && maybe_gt (n_elts, 1u))
+	  {
+	    emit_move_insn (target, CONST0_RTX (mode));
+	    cleared = 1;
+	  }
 
         if (MEM_P (target))
 	  alias = MEM_ALIAS_SET (target);