RE: [PATCH v2] DSE: Bugfix ICE after allow vector type in get_stored_val

["Li, Pan2" <pan2.li@intel.com>]

Kindly ping^ for this ice fix.

Pan

-----Original Message-----
From: Li, Pan2 
Sent: Saturday, April 6, 2024 8:02 PM
To: Li, Pan2 <pan2.li@intel.com>; Jeff Law <jeffreyalaw@gmail.com>; Robin Dapp <rdapp.gcc@gmail.com>; gcc-patches@gcc.gnu.org
Cc: juzhe.zhong@rivai.ai; kito.cheng@gmail.com; richard.guenther@gmail.com; Wang, Yanzhang <yanzhang.wang@intel.com>; Liu, Hongtao <Hongtao.Liu@intel.com>
Subject: RE: [PATCH v2] DSE: Bugfix ICE after allow vector type in get_stored_val

Kindly ping for this ice.

Pan

-----Original Message-----
From: Li, Pan2 <pan2.li@intel.com> 
Sent: Saturday, March 23, 2024 1:45 PM
To: Jeff Law <jeffreyalaw@gmail.com>; Robin Dapp <rdapp.gcc@gmail.com>; gcc-patches@gcc.gnu.org
Cc: juzhe.zhong@rivai.ai; kito.cheng@gmail.com; richard.guenther@gmail.com; Wang, Yanzhang <yanzhang.wang@intel.com>; Liu, Hongtao <hongtao.liu@intel.com>
Subject: RE: [PATCH v2] DSE: Bugfix ICE after allow vector type in get_stored_val

Thanks Jeff for comments.

> As Richi noted using validate_subreg here isn't great.  Does it work to 
> factor out this code from extract_low_bits
>
>>   if (!int_mode_for_mode (src_mode).exists (&src_int_mode)
>>       || !int_mode_for_mode (mode).exists (&int_mode))
>>     return NULL_RTX;
>> 
>>   if (!targetm.modes_tieable_p (src_int_mode, src_mode))
>>     return NULL_RTX;
>>   if (!targetm.modes_tieable_p (int_mode, mode))
>>     return NULL_RTX;

> And use that in the condition (and in extract_low_bits rather than 
> duplicating the code)?

It can solve the ICE but will forbid all vector modes goes gen_lowpart.
Actually only the vector mode size is less than reg nature size will trigger the ICE.
Thus, how about just add one more condition before goes to gen_lowpart as below?

Feel free to correct me if any misunderstandings. 😉!

diff --git a/gcc/dse.cc b/gcc/dse.cc
index edc7a1dfecf..258d2ccc299 100644
--- a/gcc/dse.cc
+++ b/gcc/dse.cc
@@ -1946,7 +1946,9 @@ get_stored_val (store_info *store_info, machine_mode read_mode,
                                 copy_rtx (store_info->const_rhs));
   else if (VECTOR_MODE_P (read_mode) && VECTOR_MODE_P (store_mode)
     && known_le (GET_MODE_BITSIZE (read_mode), GET_MODE_BITSIZE (store_mode))
-    && targetm.modes_tieable_p (read_mode, store_mode))
+    && targetm.modes_tieable_p (read_mode, store_mode)
+    /* It's invalid in validate_subreg if read_mode size is < reg natural.  */
+    && known_ge (GET_MODE_SIZE (read_mode), REGMODE_NATURAL_SIZE (read_mode)))
     read_reg = gen_lowpart (read_mode, copy_rtx (store_info->rhs));
   else
     read_reg = extract_low_bits (read_mode, store_mode,

Pan

-----Original Message-----
From: Jeff Law <jeffreyalaw@gmail.com> 
Sent: Saturday, March 23, 2024 2:54 AM
To: Li, Pan2 <pan2.li@intel.com>; Robin Dapp <rdapp.gcc@gmail.com>; gcc-patches@gcc.gnu.org
Cc: juzhe.zhong@rivai.ai; kito.cheng@gmail.com; richard.guenther@gmail.com; Wang, Yanzhang <yanzhang.wang@intel.com>; Liu, Hongtao <hongtao.liu@intel.com>
Subject: Re: [PATCH v2] DSE: Bugfix ICE after allow vector type in get_stored_val



On 3/4/24 11:22 PM, Li, Pan2 wrote:
> Thanks Jeff for comments.
> 
>> But in the case of a vector modes, we can usually reinterpret the
>> underlying bits in whatever mode we want and do any of the usual
>> operations on those bits.
> 
> Yes, I think that is why we can allow vector mode in get_stored_val if my understanding is correct.
> And then the different modes will return by gen_low_part. Unfortunately, there are some modes
>   (less than a vector bit size like V2SF, V2QI for vlen=128) are considered as invalid by validate_subreg,
> and return NULL_RTX result in the final ICE.
That doesn't make a lot of sense to me.  Even for vlen=128 I would have 
expected that we can still use a subreg to access low bits.  After all 
we might have had a V16QI vector and done a reduction of some sort 
storing the result in the first element and we have to be able to 
extract that result and move it around.

I'm not real keen on a target workaround.  While extremely safe, I 
wouldn't be surprised if other ports could trigger the ICE and we'd end 
up patching up multiple targets for what is, IMHO, a more generic issue.

As Richi noted using validate_subreg here isn't great.  Does it work to 
factor out this code from extract_low_bits:


>   if (!int_mode_for_mode (src_mode).exists (&src_int_mode)
>       || !int_mode_for_mode (mode).exists (&int_mode))
>     return NULL_RTX;
> 
>   if (!targetm.modes_tieable_p (src_int_mode, src_mode))
>     return NULL_RTX;
>   if (!targetm.modes_tieable_p (int_mode, mode))
>     return NULL_RTX;

And use that in the condition (and in extract_low_bits rather than 
duplicating the code)?

jeff

ps.  No need to apologize for the pings.  This completely fell off my radar.