894829 - Avoid symbol resolution for relocations for the same symbol

Status: RESOLVED FIXED

(Core :: mozglue, defect)

Description

[Mike Hommey [:glandium]]

The static linker usefully lays out relocations ordered by symbol, which means relocations for the same symbol are following each other. Instead of resolving the same symbol over and over, just resolve it once and reuse the resolved value subsequently.

Comment 1

[Mike Hommey [:glandium]]

Attachment: Avoid symbol resolution for relocations for the same symbol in faulty.lib

Comment 2

[Mike Hommey [:glandium]]

This cuts startup by about 40 to 50ms on my nexus S, but this is without elfhack, so it might be less in reality. But it's an easy win.

Comment 3

[Mike Hommey [:glandium]]

Attachment: Avoid symbol resolution for relocations for the same symbol in faulty.lib

With a brown-paper bag fix.

Comment 4

[Nathan Froyd [:froydnj]]

Comment on attachment 777603 [details] [diff] [review]
Avoid symbol resolution for relocations for the same symbol in faulty.lib

Review of attachment 777603 [details] [diff] [review]:
-----------------------------------------------------------------

::: mozglue/linker/CustomElf.cpp
@@ +652,5 @@
>      /* Other relocation types need a symbol resolution */
> +    /* Avoid symbol resolution when it's the same symbol as last iteration */
> +    if (symtab_index != ELF_R_SYM(rel->r_info)) {
> +      symtab_index = ELF_R_SYM(rel->r_info);
> +      const Sym sym = symtab[ELF_R_SYM(rel->r_info)];

Nit: might as well use symtab_index here?

Comment 5

[Mike Hommey [:glandium]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/5435370944bf

Comment 6

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/5435370944bf