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* [gcc r14-3111] RISC-V: Folding memory for FP + constant case
@ 2023-08-09 19:29 Jeff Law
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From: Jeff Law @ 2023-08-09 19:29 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:a16dc729fda9fabd6472d50cce45791cb3b6ada8
commit r14-3111-ga16dc729fda9fabd6472d50cce45791cb3b6ada8
Author: Jivan Hakobyan <jivanhakobyan9@gmail.com>
Date: Wed Aug 9 13:26:58 2023 -0600
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.
Diff:
---
gcc/config/riscv/riscv.cc | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 77892da29201..7f2041a54ba6 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)
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