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

Hi!

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

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().

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 :-)

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?

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).

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

That is strange, that should never happen.

./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!  :-)

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.

The patch looks perfect of course :-)

Reviewed-by: Segher Boessenkool <redacted>


Segher