[PATCH v2] xtensa: Eliminate unnecessary general-purpose reg-reg moves

[Takayuki 'January June' Suwa via Gcc-patches]

Register-register move instructions that can be easily seen as
unnecessary by the human eye may remain in the compiled result.
For example:

/* example */
double test(double a, double b) {
  return __builtin_copysign(a, b);
}

test:
        add.n   a3, a3, a3
        extui   a5, a5, 31, 1
        ssai    1
                                ;; be in the same BB
        src     a7, a5, a3      ;; No '0' in the source constraints
                                ;; No CALL insns in this span
                                ;; Both A3 and A7 are irrelevant to
                                ;;   insns in this span
        mov.n   a3, a7          ;; An unnecessary reg-reg move
                                ;; A7 is not used after this
        ret.n

The last two instructions above, excluding the return instruction,
could be done like this:

        src     a3, a5, a3

This symptom often occurs when handling DI/DFmode values with SImode
instructions.  This patch solves the above problem using peephole2
pattern.

gcc/ChangeLog:

        * config/xtensa/xtensa.md: New peephole2 pattern that eliminates
        the occurrence of genral-purpose register used only once and for
        transferring intermediate value.
---
 gcc/config/xtensa/xtensa.md | 43 +++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/gcc/config/xtensa/xtensa.md b/gcc/config/xtensa/xtensa.md
index 3694d95ad..0a477e711 100644
--- a/gcc/config/xtensa/xtensa.md
+++ b/gcc/config/xtensa/xtensa.md
@@ -3091,3 +3091,46 @@ FALLTHRU:;
   df_insn_rescan (insnR);
   set_insn_deleted (insnP);
 })
+
+(define_peephole2
+  [(set (match_operand 0 "register_operand")
+       (match_operand 1 "register_operand"))]
+  "GET_MODE_SIZE (GET_MODE (operands[0])) == 4
+   && GET_MODE_SIZE (GET_MODE (operands[1])) == 4
+   && GP_REG_P (REGNO (operands[0])) && GP_REG_P (REGNO (operands[1]))
+   && peep2_reg_dead_p (1, operands[1])"
+  [(const_int 0)]
+{
+  basic_block bb = BLOCK_FOR_INSN (curr_insn);
+  rtx_insn *head = BB_HEAD (bb), *insn;
+  rtx dest = operands[0], src = operands[1], pattern, t_dest;
+  int i;
+  for (insn = PREV_INSN (curr_insn);
+       insn && insn != head;
+       insn = PREV_INSN (insn))
+    if (CALL_P (insn))
+      break;
+    else if (INSN_P (insn))
+      {
+       if (GET_CODE (pattern = PATTERN (insn)) == SET
+           && REG_P (t_dest = SET_DEST (pattern))
+           && GET_MODE_SIZE (GET_MODE (t_dest)) == 4
+           && REGNO (t_dest) == REGNO (src))
+       {
+         extract_constrain_insn (insn);
+         for (i = 1; i < recog_data.n_operands; ++i)
+           if (strchr (recog_data.constraints[i], '0'))
+             goto ABORT;
+         SET_REGNO (t_dest, REGNO (dest));
+         goto FALLTHRU;
+       }
+       if (reg_overlap_mentioned_p (dest, pattern)
+           || reg_overlap_mentioned_p (src, pattern)
+           || set_of (dest, insn)
+           || set_of (src, insn))
+         break;
+      }
+ABORT:
+  FAIL;
+FALLTHRU:;
+})
-- 
2.30.2