xtensa: Eliminate [DS]Cmode hard register clobber that is immediately followed by whole overwrite the register

RTL expansion of substitution to [DS]Cmode hard register includes obstructive
register clobber.

A simplest example:

    double _Complex test(double _Complex c) {
      return c;
    }

will be converted to:

    (set (reg:DF 42 [ c ]) (reg:DF 2 a2))
    (set (reg:DF 43 [ c+8 ]) (reg:DF 4 a4))
    (clobber (reg:DC 2 a2))
    (set (reg:DF 2 a2) (reg:DF 42 [ c ]))
    (set (reg:DF 4 a4) (reg:DF 43 [ c+8 ]))
    (use (reg:DC 2 a2))
    (return)

and then finally:

    test:
	mov	a8, a2
	mov	a9, a3
	mov	a6, a4
	mov	a7, a5
	mov	a2, a8
	mov	a3, a9
	mov	a4, a6
	mov	a5, a7
	ret

As you see, it is so ridiculous.

This patch eliminates such clobber in order to prune away the wasted move
instructions by the optimizer:

    test:
	ret

gcc/ChangeLog:

	* config/xtensa/xtensa.md (DSC): New split pattern and mode iterator.

1 files changed, 28 insertions, 0 deletions

diff --git a/gcc/config/xtensa/xtensa.md b/gcc/config/xtensa/xtensa.md
index 8ed0ff5..ef6bbc4 100644
--- a/gcc/config/xtensa/xtensa.md
+++ b/gcc/config/xtensa/xtensa.md
@@ -87,6 +87,10 @@
 ;; This code iterator is for *shlrd and its variants.
 (define_code_iterator ior_op [ior plus])
 
+;; This mode iterator allows the DC and SC patterns to be defined from
+;; the same template.
+(define_mode_iterator DSC [DC SC])
+
 
 ;; Attributes.
 
@@ -2780,3 +2784,27 @@
   operands[6] = gen_rtx_MEM (SFmode, XEXP (operands[6], 0));
   operands[7] = gen_rtx_MEM (SFmode, XEXP (operands[7], 0));
 })
+
+(define_split
+  [(clobber (match_operand:DSC 0 "register_operand"))]
+  "GP_REG_P (REGNO (operands[0]))"
+  [(const_int 0)]
+{
+  unsigned int regno = REGNO (operands[0]);
+  machine_mode inner_mode = GET_MODE_INNER (<MODE>mode);
+  rtx_insn *insn;
+  rtx x;
+  if (! ((insn = next_nonnote_nondebug_insn (curr_insn))
+	 && NONJUMP_INSN_P (insn)
+	 && GET_CODE (x = PATTERN (insn)) == SET
+	 && REG_P (x = XEXP (x, 0))
+	 && GET_MODE (x) == inner_mode
+	 && REGNO (x) == regno
+	 && (insn = next_nonnote_nondebug_insn (insn))
+	 && NONJUMP_INSN_P (insn)
+	 && GET_CODE (x = PATTERN (insn)) == SET
+	 && REG_P (x = XEXP (x, 0))
+	 && GET_MODE (x) == inner_mode
+	 && REGNO (x) == regno + REG_NREGS (operands[0]) / 2))
+    FAIL;
+})