Re: [PATCH v3] modpost: check for static EXPORT_SYMBOL* functions

Masahiro Yamada
2019-07-30T17:22:37Z

On Wed, Jul 31, 2019 at 1:44 AM Denis Efremov <efremov@linux.com> wrote:
>
> On 30.07.2019 19:29, Masahiro Yamada wrote:
> > I prefer this, but why do you need to check type?
> >
> > Doesn't this work?
> >
> > for (sym = info.symtab_start; sym < info.symtab_stop; sym++) {
> > unsigned char bind = ELF_ST_BIND(sym->st_info);
> >
> > struct symbol *s = find_symbol(remove_dot(info.strtab +
> > sym->st_name));
> >
> > if (s && (bind == STB_GLOBAL || bind == STB_WEAK))
> > s->is_static = 0;
> > }
>
> This works. However, I thought it will be too costly to call find_symbol
> on each symbol. Hence, 'type == STT_OBJECT || type == STT_FUNC || type
> == STT_NOTYPE' is a small performance optimization because we need to
> check only variables and functions. Is it worth to remove it in v4?
>
> Denis

I checked the symbol table for ppc64_defconfig.

The following is the number of entries
for each combination of type and bind.

[1] type: STT_NOTYPE, bind: STB_LOCAL -> 39502 entries
[2] type: STT_NOTYPE, bind: STB_GLOBAL -> 30161 entries
[3] type: STT_NOTYPE, bind: STB_WEAK -> 5 entries
[4] type: STT_OBJECT, bind: STB_LOCAL -> 60326 entries
[5] type: STT_OBJECT, bind: STB_GLOBAL -> 4126 entries
[6] type: STT_OBJECT, bind: STB_WEAK -> 11 entries
[7] type: STT_FUNC, bind: STB_LOCAL -> 38816 entries
[8] type: STT_FUNC, bind: STB_GLOBAL -> 56196 entries
[9] type: STT_FUNC, bind: STB_WEAK -> 350 entries
[10] type: STT_SECTION, bind: STB_LOCAL -> 9027 entries
[11] type: STT_FILE, bind: STB_LOCAL -> 2918 entries

Checking 'type' beforehand
saves only 11945 look-ups ( [10] + [11]).

You can check 'bind' before the look-up, not after.
If bind == STB_LOCAL, you do not need to lookup the hash-table,
since you do not do anything.
This saves [1], [4], [7], [10], [11].

I think the following is simpler, and works more efficiently.

for (sym = info.symtab_start; sym < info.symtab_stop; sym++) {
unsigned char bind = ELF_ST_BIND(sym->st_info);

if (bind == STB_GLOBAL || bind == STB_WEAK) {
struct symbol *s =
find_symbol(remove_dot(info.strtab +
sym->st_name));

if (s)
s->is_static = 0;
}
}

--
Best Regards
Masahiro Yamada