[gcc r14-1791] Avoid duplicate vector initializations during RTL expansion.

[Roger Sayle <sayle@gcc.gnu.org>]

https://gcc.gnu.org/g:40eafcd83d6ee644ec48985fd5a9696921ea10e7

commit r14-1791-g40eafcd83d6ee644ec48985fd5a9696921ea10e7
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Jun 13 17:20:31 2023 +0100

    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.

Diff:
---
 gcc/expr.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 868fa6e4cca..62cd8facf75 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);