RISC-V: Folding memory for FP + constant case

Accessing local arrays element turned into load form (fp + (index << C1)) +
C2 address.

In the case when access is in the loop we got loop invariant computation.  For
some reason, moving out that part cannot be done in loop-invariant passes.  But
we can handle that in target-specific hook (legitimize_address).  That provides
an opportunity to rewrite memory access more suitable for the target
architecture.

This patch solves the mentioned case by rewriting mentioned case to ((fp +
C2) + (index << C1))

I have evaluated it on SPEC2017 and got an improvement on leela (over 7b
instructions, .39% of the dynamic count) and dwarfs the regression for gcc (14m
instructions, .0012% of the dynamic count).

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_legitimize_address): Handle folding.
	(mem_shadd_or_shadd_rtx_p): New function.

1 files changed, 42 insertions, 0 deletions

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 77892da..7f2041a 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -1805,6 +1805,22 @@ riscv_shorten_lw_offset (rtx base, HOST_WIDE_INT offset)
   return addr;
 }
 
+/* Helper for riscv_legitimize_address. Given X, return true if it
+   is a left shift by 1, 2 or 3 positions or a multiply by 2, 4 or 8.
+
+   This respectively represent canonical shift-add rtxs or scaled
+   memory addresses.  */
+static bool
+mem_shadd_or_shadd_rtx_p (rtx x)
+{
+  return ((GET_CODE (x) == ASHIFT
+	   || GET_CODE (x) == MULT)
+	  && CONST_INT_P (XEXP (x, 1))
+	  && ((GET_CODE (x) == ASHIFT && IN_RANGE (INTVAL (XEXP (x, 1)), 1, 3))
+	      || (GET_CODE (x) == MULT
+		  && IN_RANGE (exact_log2 (INTVAL (XEXP (x, 1))), 1, 3))));
+}
+
 /* This function is used to implement LEGITIMIZE_ADDRESS.  If X can
    be legitimized in a way that the generic machinery might not expect,
    return a new address, otherwise return NULL.  MODE is the mode of
@@ -1830,6 +1846,32 @@ riscv_legitimize_address (rtx x, rtx oldx ATTRIBUTE_UNUSED,
       rtx base = XEXP (x, 0);
       HOST_WIDE_INT offset = INTVAL (XEXP (x, 1));
 
+      /* Handle (plus (plus (mult (a) (mem_shadd_constant)) (fp)) (C)) case.  */
+      if (GET_CODE (base) == PLUS && mem_shadd_or_shadd_rtx_p (XEXP (base, 0))
+	  && SMALL_OPERAND (offset))
+	{
+	  rtx index = XEXP (base, 0);
+	  rtx fp = XEXP (base, 1);
+	  if (REGNO (fp) == VIRTUAL_STACK_VARS_REGNUM)
+	    {
+
+	      /* If we were given a MULT, we must fix the constant
+		 as we're going to create the ASHIFT form.  */
+	      int shift_val = INTVAL (XEXP (index, 1));
+	      if (GET_CODE (index) == MULT)
+		shift_val = exact_log2 (shift_val);
+
+	      rtx reg1 = gen_reg_rtx (Pmode);
+	      rtx reg2 = gen_reg_rtx (Pmode);
+	      rtx reg3 = gen_reg_rtx (Pmode);
+	      riscv_emit_binary (PLUS, reg1, fp, GEN_INT (offset));
+	      riscv_emit_binary (ASHIFT, reg2, XEXP (index, 0), GEN_INT (shift_val));
+	      riscv_emit_binary (PLUS, reg3, reg2, reg1);
+
+	      return reg3;
+	    }
+	}
+
       if (!riscv_valid_base_register_p (base, mode, false))
 	base = copy_to_mode_reg (Pmode, base);
       if (optimize_function_for_size_p (cfun)