Re: Calculating instruction costs

[Chung-Ju Wu <jasonwucj at gmail dot com>]

2013/7/9, David Given <dg@cowlark.com>:
> I'm working on a gcc backend for an architecture. The architecture has
> instructions for indexed array access; so, ld r0, (r1, r2) is equivalent
> to r0 = r1[r2] where r1 is a int32_t*.
>
> I'm representing this in the .md file with the following pattern:
>
> (define_insn "*si_load_indexed"
>   [
>     (set
>       (match_operand:SI 0 "register_operand" "=r")
>       (mem:SI
>         (plus:SI
>           (mult:SI
>             (match_operand:SI 1 "register_operand" "%r")
>             (const_int 4))
>           (match_operand:SI 2 "register_operand" "r"))))
>   ]
>   ""
>   "ld %0, (%2, %1)"
>   [(set_attr "length" "4")]
> )
>
> However, the instruction is never actually being emitted. Looking at the
> debug output from the instruction combining stage, I see this:
>
> Trying 8, 9 -> 10:
> Successfully matched this instruction:
> (set (reg:SI 47 [ *_5 ])
>     (mem:SI (plus:SI (mult:SI (reg/v:SI 43 [ b ])
>                 (const_int 4 [0x4]))
>             (reg:SI 0 r0 [ a ])) [2 *_5+0 S4 A32]))
> rejecting combination of insns 8, 9 and 10
> original costs 8 + 4 + 4 = 16
> replacement cost 32
>
> Instructions 8, 9 and 10 are:
>
> (insn 8 5 9 2 (set (reg:SI 45)
>         (ashift:SI (reg/v:SI 43 [ b ])
>             (const_int 2 [0x2]))) test.c:5 15 {ashlsi3}
>      (expr_list:REG_DEAD (reg/v:SI 43 [ b ])
>         (nil)))
> (insn 9 8 10 2 (set (reg/f:SI 46)
>         (plus:SI (reg/v/f:SI 42 [ a ])
>             (reg:SI 45))) test.c:5 13 {addsi3}
>      (expr_list:REG_DEAD (reg:SI 45)
>         (expr_list:REG_DEAD (reg/v/f:SI 42 [ a ])
>             (nil))))
> (insn 10 9 15 2 (set (reg:SI 47 [ *_5 ])
>         (mem:SI (reg/f:SI 46) [2 *_5+0 S4 A32])) test.c:5 6 {*si_load}
>      (expr_list:REG_DEAD (reg/f:SI 46)
>         (nil)))
>
> If I've read this correctly, it indicates that the instruction pattern
> has been matched, but the instruction has been rejected due to being
> more expensive than the original instructions.
>
> So, how is it calculating the cost of my instruction? Where's it getting
> that 32 from (which seems weirdly high)?
>
> Right now all the cost macros are left as the default, which is probably
> the root of the problem; but I'm having a lot of trouble getting my head
> around them. In the interest of actually getting something to work, are
> there any ways of using a simplified cost model where the cost of each
> instruction is specified manually in the instruction pattern alongside
> the length? (Or even just *using* the length as the cost...)
>

Hi, David,

I suggest setting a break point on rtx_costs and
see how it calculates the overall costs when combining
insns 8, 9, and 10.

Best regards,
jasonwucj