[Bug middle-end/71509] Bitfield causes load hit store with larger store than load

[luoxhu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71509

luoxhu at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |luoxhu at gcc dot gnu.org

--- Comment #11 from luoxhu at gcc dot gnu.org ---
(In reply to Anton Blanchard from comment #4)
> Created attachment 39683 [details]
> Another bitop LHS test case
> 
> Here's another issue found in the Linux kernel. Seems like this should be a
> single lwz/stw since the union of counter and the bitops completely overlap.
> 
> The half word store followed by word load is going to prevent it from store
> forwarding.
> 
> 0000000000000000 <set_page_slub_counters>:
>    0:	00 00 03 81 	lwz     r8,0(r3)
>    4:	20 00 89 78 	clrldi  r9,r4,32
>    8:	c2 0f 2a 79 	rldicl  r10,r9,33,31
>    c:	00 f8 48 51 	rlwimi  r8,r10,31,0,0
>   10:	5e 00 2a 55 	rlwinm  r10,r9,0,1,15
>   14:	00 00 03 91 	stw     r8,0(r3)
>   18:	00 00 83 b0 	sth     r4,0(r3)
>   1c:	00 00 42 60 	ori     r2,r2,0
>   20:	00 00 23 81 	lwz     r9,0(r3)
>   24:	00 04 29 55 	rlwinm  r9,r9,0,16,0
>   28:	78 53 29 7d 	or      r9,r9,r10
>   2c:	00 00 23 91 	stw     r9,0(r3)
>   30:	20 00 80 4e 	blr

This case only is fixed on latest gcc 10 already (issues in case
__skb_decr_checksum_unnecessary from Anton Blanchard and test2 from Nicholas
Piggin  still exist).

gcc version 10.0.1 20200210 

objdump -d set_page_slub_counters.o

set_page_slub_counters.o:     file format elf64-powerpcle


Disassembly of section .text:

0000000000000000 <set_page_slub_counters>:
   0:   22 84 89 78     rldicl  r9,r4,48,48
   4:   00 00 83 b0     sth     r4,0(r3)
   8:   02 00 23 b1     sth     r9,2(r3)
   c:   20 00 80 4e     blr