On Wed, 17 Mar 2021, Tiezhu Yang wrote:
> > > The code produced by GCC remains the same between (1), (3) and (4),
> > > the last changes looks like better (with less changes based on commit
> > > 198688edbf77), so I will send v3 later.
> > Aren't those all the same - apart from register selection.
I did a quick size check with my MIPS64 config between (1) and (2):
text data bss total filename
- 6129704 2997475 215280 9342459 vmlinux
+ 6129752 2997475 215280 9342507 vmlinux
so obviously there's more to that and the code snippets quoted did not
show the full picture. The size difference is in `tcp_v4_send_reset'
AFAICT, maybe elsewhere as well.
FAOD the former binary is as at 198688edbf77^ and the latter is as at
198688edbf77, and the command used was `size --format=gnu' to prevent
`.rodata' from interfering with the text size.
> > Not that I grok the mips opcodes.
> > But that code has horridness on its side.
It's a 32-bit one's-complement addition. The use of 64-bit operations
reduces the number of calculations as any 32-bit carries accumulate in the
high 32-bit word allowing one instruction to be saved total compared to
the 32-bit variant. Nothing particularly unusual for me here; I've seen
worse stuff with x86.
> Is this patch necessary? If no, we can ignore it.
> If yes, I will send v3 with the above (4) changes.
I have also verified this patch directly on top of 198688edbf77 and it
does not bring the old `vmlinux' size back (there's no difference in code
produced at all here actually), so you need to investigate this further.
This is with an older checkout of GCC 11.
I've attached a stripped version of my .config that you can use for
reference.
Maciej
