Re: [PATCH] powerpc: Force inlining of csum_add()

Hi,

Le 11/05/2021 à 12:51, Segher Boessenkool a écrit :

Hi!

On Tue, May 11, 2021 at 06:08:06AM +0000, Christophe Leroy wrote:

quoted

Commit 328e7e487a46 ("powerpc: force inlining of csum_partial() to
avoid multiple csum_partial() with GCC10") inlined csum_partial().

Now that csum_partial() is inlined, GCC outlines csum_add() when
called by csum_partial().

quoted

c064fb28 <csum_add>:
c064fb28:	7c 63 20 14 	addc    r3,r3,r4
c064fb2c:	7c 63 01 94 	addze   r3,r3
c064fb30:	4e 80 00 20 	blr

Could you build this with -fdump-tree-einline-all and send me the
results?  Or open a GCC PR yourself :-)

Ok, I'll forward it to you in a minute.

Something seems to have decided this asm is more expensive than it is.
That isn't always avoidable -- the compiler cannot look inside asms --
but it seems it could be improved here.

Do you have (or can make) a self-contained testcase?

I have not tried, and I fear it might be difficult, because on a kernel build with dozens of calls 
to csum_add(), only ip6_tunnel.o exhibits such an issue.

quoted

The sum with 0 is useless, should have been skipped.

That isn't something the compiler can do anything about (not sure if you
were suggesting that); it has to be done in the user code (and it tries
to already, see below).

I was not suggesting that, only that when properly inlined the sum with 0 is skipped (because we put 
the necessary stuff in csum_add() of course).

quoted

And there is even one completely unused instance of csum_add().

That is strange, that should never happen.

It seems that several .o include unused versions of csum_add. After the final link, one remains (in 
addition to the used one) in vmlinux.

quoted

./arch/powerpc/include/asm/checksum.h: In function '__ip6_tnl_rcv':
./arch/powerpc/include/asm/checksum.h:94:22: warning: inlining failed in call to 'csum_add': call is unlikely and code size would grow [-Winline]
    94 | static inline __wsum csum_add(__wsum csum, __wsum addend)
       |                      ^~~~~~~~
./arch/powerpc/include/asm/checksum.h:172:31: note: called from here
   172 |                         sum = csum_add(sum, (__force __wsum)*(const u32 *)buff);
       |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

At least we say what happened.  Progress!  :-)

Lol. I've seen this warning for long, that's not something new I guess.

quoted

In the non-inlined version, the first sum with 0 was performed.
Here it is skipped.

That is because of how __builtin_constant_p works, most likely.  As we
discussed elsewhere it is evaluated before all forms of loop unrolling.

But we are not talking about loop unrolling here, are we ?

It seems that the reason here is that __builtin_constant_p() is evaluated long after GCC decided to 
not inline that call to csum_add().

Christophe