Re: [PATCH v3] RISC-V: elide unnecessary sign extend when expanding cmp_and_jump

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/31/23 18:05, Vineet Gupta wrote:
> On 10/30/23 13:33, Jeff Law wrote:
>>
>>> +/* Helper function for riscv_extend_comparands to Sign-extend the OP.
>>> +   However if the OP is SI subreg promoted with an inner DI, such as
>>> +       (subreg/s/v:SI (reg/v:DI) 0
>>> +   just peel off the SUBREG to get DI, avoiding extraneous 
>>> extension.  */
>>> +
>>> +static void
>>> +riscv_sign_extend_if_not_subreg_prom (rtx *op)
>>> +{
>>> +  if (GET_MODE (*op) == SImode
> 
> So I may have been partially wrong about v2 patch being wrong and 
> needing this fixup ;-) [1]
> 
> It seems we don't have to limit this to SImode. I re-read the calling 
> convention doc [2] and it says this
> 
> "When passed in registers or on the stack, integer scalars narrower than 
> XLEN
>   bits are widened according to the sign of their type up to 32 bits, then
>   sign-extended to XLEN bits."
> 
> This essentially means signed short, and signed char will be already 
> sign-extended at caller site and need not be done in callee: Palmer 
> mention in internal slack that unadorned char is unsigned on RISC-V 
> hence we don't see the compiler extra work for say 
> gcc.dg/torture/pr75964.c. If the test is however tweaked to use signed 
> chars (or short), it seems caller is doing the work (adjusting the 
> constant being passed to be a sign-extended variant).
> 
> This further validates Jeff's comment about checking for 
> SUBREG_PROMOTED_SIGNED_P (it was anyhow the right thing to begin with 
> anyways).
> 
> At this point I feel like I'm into splitting hairs (in vain) territory, 
> as fixing this might not matter much in practice ...
> 
> I'd suppose we go ahead with the v3 with changes Jeff asked for and 
> maybe do a later fixup to relax SI.
Consider any such fixup pre-approved.  I was thinking that the 8/16 bit 
sub-objects should probably be extended one way or another.  It wouldn't 
make much sense not to.

Not as much of a win as I'd hoped.  But I'll take it

jeff