[PATCH] hppa: PR middle-end/87256: Improved hppa_rtx_costs avoids synth_mult madness.

[Jeff Law]

On Wed, 2020-08-26 at 12:19 +0100, Roger Sayle wrote:
> Hi Dave,
> 
> > This change is also fine.
> > 
> > The gcc.target/hppa/shadd-2.c test now fails because there are two
> additional sh1add instructions.
> > However, the total number of instructions is the same as before.
> 
> Just to be on the safe side, I took a deeper look into this.
> We're now generating a slightly different sequence for multiply by 87.
> 
> Before:
>         zdep %r28,28,29,%r20            ; r20 = r28*8
>         ldo RR'default_target_regs-$global$(%r1),%r19
>         sub %r20,%r28,%r20              ; r20 = r28*7
>         sh2addl %r20,%r28,%r20          ; r20 = r28*29
>         sh2addl %r20,%r19,%r19          ; r20 = addr+r28*116
>         sub %r19,%r20,%r19              ; r20 = addr+r28*87
> 
> After:
>         sh2addl %r28,%r28,%r19          ; r19 = r28*5
>         ldo RR'default_target_regs-$global$(%r1),%r1
>         sh2addl %r19,%r28,%r19          ; r19 = r28*21
>         sh1addl %r19,%r28,%r19          ; r19 = r28*43
>         sh1addl %r19,%r28,%r19          ; r19 = r28*87
>         addl %r1,%r19,%r1               ; r1 = addr+r28*87
> 
> As you point out, both sequences have exactly the same length and rtx_costs.
> I suspect the middle-end has a minor preference for simple shifts and adds.
> If shadd-2.c is there to check we make use of sh?add instructions, then
> we're
> making even more use of them now.
> 
> I agree it's reasonable to increase the scan-assembler-times count for this
> test.
Yes, those tests were there to catch failure to create shadd insns after some
changes in the combiner.  If we're generating more, then that's good enough for
those tests :-)

jeff