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Re: About hardfloat in ppc

From: Richard Henderson
Subject: Re: About hardfloat in ppc
Date: Fri, 1 May 2020 07:18:42 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0

On 5/1/20 6:10 AM, Alex Bennée wrote:
> 罗勇刚(Yonggang Luo) <address@hidden> writes:
>> On Fri, May 1, 2020 at 7:58 PM BALATON Zoltan <address@hidden> wrote:
>>> On Fri, 1 May 2020, 罗勇刚(Yonggang Luo) wrote:
>>>> That's what I suggested,
>>>> We preserve a  float computing cache
>>>> typedef struct FpRecord {
>>>>  uint8_t op;
>>>>  float32 A;
>>>>  float32 B;
>>>> }  FpRecord;
>>>> FpRecord fp_cache[1024];
>>>> int fp_cache_length;
>>>> uint32_t fp_exceptions;
>>>> 1. For each new fp operation we push it to the  fp_cache,
>>>> 2. Once we read the fp_exceptions , then we re-compute
>>>> the fp_exceptions by re-running the fp FpRecord sequence.
>>>> and clear  fp_cache_length.
>>> Why do you need to store more than the last fp op? The cumulative bits can
>>> be tracked like it's done for other targets by not clearing fp_status then
>>> you can read it from there. Only the non-sticky FI bit needs to be
>>> computed but that's only determined by the last op so it's enough to
>>> remember that and run that with softfloat (or even hardfloat after
>>> clearing status but softfloat may be faster for this) to get the bits for
>>> last op when status is read.
>> Yeap, store only the last fp op is also an option. Do you means that store
>> the last fp op,
>> and calculate it when necessary?  I am thinking about a general fp
>> optmize method that suite
>> for all target.
> I think that's getting a little ahead of yourself. Let's prove the
> technique is valuable for PPC (given it has the most to gain). We can
> always generalise later if it's worthwhile.


> Rather than creating a new structure I would suggest creating 3 new tcg
> globals (op, inA, inB) and re-factor the front-end code so each FP op
> loaded the TCG globals. The TCG optimizer should pick up aliased loads
> and automatically eliminate the dead ones. We might need some new
> machinery for the TCG to avoid spilling the values over potentially
> faulting loads/stores but that is likely a phase 2 problem. 

There's no point in new tcg globals.

Every fp operation can raise an exception, and therefore every fp operation
will flush tcg globals to memory.  Therefore there is no optimization to be
done at the tcg opcode level.

However, every fp operation calls a helper function, and the quickest thing to
do is store the inputs to env->(op, inA, inB, inC) in the helper before
performing the operation.

> Next you will want to find places that care about the per-op bits of
> cpu_fpscr and call a helper with the new globals to re-run the
> computation and feed the values in.

Before we even get to this deferred fp operation thing, there are several giant
improvements to ppc emulation that can be made:

Step 1 is to rearrange the fp helpers to eliminate helper_reset_fpstatus().
I've mentioned this before, that it's possible to leave the steady-state of
env->fp_status.exception_flags == 0, so there's no need for a separate function
call.  I suspect this is worth a decent speedup by itself.

Step 2 is to notice when all fp exceptions are masked, so that no exception can
be raised, and set a tb_flags bit.  This is the default fp environment that
libc enables and therefore extremely common.

Currently, ppc has 3 helpers called per fp operation.  If step 1 is handled
correctly, then we're down to 2 fp helpers per fp operation.  If no exceptions
need raising, then we can perform the entire operation with a single function 

We would require a parallel set of fp helpers that (1) performs the operation
and (2) does any post-processing of the exception bits straight away, but (3)
without raising any exceptions.  Sort of like helper_fadd +
do_float_check_status, but less.  IIRC the only real extra work is categorizing
invalid exceptions.  We could even plausibly extend softfloat to do that while
it is recording the invalid exception.

Step 3 is to improve softfloat.c with Yonggang Luo's idea to compute inexact
from the inverse hardfloat operation.  This would let us relax the restriction
of only using hardfloat when we have already have an accrued inexact exception.

Only after all of these are done is it worth experimenting with caching the
last fp operation.


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