[gcc r17-2589] Simplify (vec_select:<scalar> (vec_concat ..))

https://gcc.gnu.org/g:10f6e51b8250ce4f553c055ad737607332311a5d

commit r17-2589-g10f6e51b8250ce4f553c055ad737607332311a5d
Author: Richard Biener <[email protected]>
Date:   Mon Jul 20 13:57:36 2026 +0200

    Simplify (vec_select:<scalar> (vec_concat ..))
    
    The following adds simplification of a scalar mode vec_select wrapping
    a vec_concat.  combine sees
    
    (insn 7 4 8 2 (set (reg:V2DF 106)
            (vec_concat:V2DF (plus:DF (vec_select:DF (reg/v:V2DF 101 [ p ])
                        (parallel [
                                (const_int 0 [0])
                            ]))
                    (vec_select:DF (reg/v:V2DF 101 [ p ])
                        (parallel [
                                (const_int 1 [0x1])
                            ])))
                (plus:DF (vec_select:DF (reg/v:V2DF 101 [ p ])
                        (parallel [
                                (const_int 0 [0])
                            ]))
                    (vec_select:DF (reg/v:V2DF 101 [ p ])
                        (parallel [
                                (const_int 1 [0x1])
                            ]))))) "t.c":5:11 3234 {*sse3_haddv2df3}
         (expr_list:REG_DEAD (reg/v:V2DF 101 [ p ])
            (nil)))
    (insn 8 7 9 2 (set (reg:DF 105)
            (vec_select:DF (reg:V2DF 106)
                (parallel [
                        (const_int 0 [0])
                    ]))) "t.c":5:11 7297 {sse2_storelpd}
         (expr_list:REG_DEAD (reg:V2DF 106)
            (nil)))
    
    and this should simplify and combine to
    
    (set (reg:DF 105)
        (plus:DF (vec_select:DF (reg:V2DF 109 [ p ])
                (parallel [
                        (const_int 0 [0])
                    ]))
            (vec_select:DF (reg:V2DF 109 [ p ])
                (parallel [
                        (const_int 1 [0x1])
                    ]))))
    
    which x86 can recognize (but rejects due to consting, see PR126328).
    There'll be test coverage in gcc.target/i386/pr54400.c
    
            * simplify-rtx.cc (simplify_context::simplify_binary_operation_1):
            Simplify scalar vec_select of vec_concat.

Diff:
---
 gcc/simplify-rtx.cc | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index 882a11c5760d..6f8ee53f209f 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -5210,6 +5210,20 @@ simplify_ashift:
 				    tmp_op, gen_rtx_PARALLEL (VOIDmode, vec));
 	      return tmp;
 	    }
+	  /* If we select one half of a vec_concat, return that.  */
+	  else if (GET_CODE (trueop0) == VEC_CONCAT)
+	    {
+	      rtx subop0 = XEXP (trueop0, 0);
+	      rtx subop1 = XEXP (trueop0, 1);
+	      machine_mode mode0 = GET_MODE (subop0);
+	      machine_mode mode1 = GET_MODE (subop1);
+	      int i0 = INTVAL (XVECEXP (trueop1, 0, 0));
+	      if (i0 == 0 && mode == mode0 && !side_effects_p (subop1))
+		return subop0;
+	      if (known_eq (i0, GET_MODE_NUNITS (mode0))
+		  && mode == mode1 && !side_effects_p (subop0))
+		return subop1;
+	    }
 	}
       else
 	{