Le 19/10/2020 à 06:55, Joel Stanley a écrit :

On Sat, 17 Oct 2020 at 15:55, Christophe Leroy
[off-list ref] wrote:

quoted

When building mpc885_ads_defconfig with gcc 10.1,
the function get_order() appears 50 times in vmlinux:

[linux]# ppc-linux-objdump -x vmlinux | grep get_order | wc -l
50

[linux]# size vmlinux
    text    data     bss     dec     hex filename
3842620  675624  135160 4653404  47015c vmlinux

In the old days, marking a function 'static inline' was forcing
GCC to inline, but since commit ac7c3e4ff401 ("compiler: enable
CONFIG_OPTIMIZE_INLINING forcibly") GCC may decide to not inline
a function.

It looks like GCC 10 is taking poor decisions on this.

get_order() compiles into the following tiny function,
occupying 20 bytes of text.

0000007c <get_order>:
   7c:   38 63 ff ff     addi    r3,r3,-1
   80:   54 63 a3 3e     rlwinm  r3,r3,20,12,31
   84:   7c 63 00 34     cntlzw  r3,r3
   88:   20 63 00 20     subfic  r3,r3,32
   8c:   4e 80 00 20     blr

By forcing get_order() to be __always_inline, the size of text is
reduced by 1940 bytes, that is almost twice the space occupied by
50 times get_order()

[linux-powerpc]# size vmlinux
    text    data     bss     dec     hex filename
3840680  675588  135176 4651444  46f9b4 vmlinux

I see similar results with GCC 10.2 building for arm32. There are 143
instances of get_order with aspeed_g5_defconfig.

Before:
  9071838 2630138  186468 11888444         b5673c vmlinux
After:
  9069886 2630126  186468 11886480         b55f90 vmlinux

1952 bytes smaller with your patch applied. Did you raise this with
anyone from GCC?

Yes I did, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445

For the time being, it's at a standstill.

Christophe

Reviewed-by: Joel Stanley <joel@jms.id.au>

quoted

Signed-off-by: Christophe Leroy <redacted>
---
  include/asm-generic/getorder.h | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/getorder.h b/include/asm-generic/getorder.h
index e9f20b813a69..f2979e3a96b6 100644
--- a/include/asm-generic/getorder.h
+++ b/include/asm-generic/getorder.h

@@ -26,7 +26,7 @@
   *
   * The result is undefined if the size is 0.
   */
-static inline __attribute_const__ int get_order(unsigned long size)
+static __always_inline __attribute_const__ int get_order(unsigned long size)
  {
         if (__builtin_constant_p(size)) {
                 if (!size)
--

2.25.0