[Bug ld/12248] Linker plugin failed to preserve linking order

[hjl.tools at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12248

--- Comment #17 from H.J. Lu <hjl.tools at gmail dot com> 2011-01-24 01:24:01 
UTC ---
(In reply to comment #16)
> It looks counterintuitive to me.
> 
> > LTO in x.o doesn't reference __udivdi3
> 
> But a reference to __udivdi3 from bar is generated on LTO step:
> /tmp/cczLpgd5.ltrans0.ltrans.o: reference to __udivdi3

This is internal implementation of LTO, which calls __udivdi3
in libgcc. LTO may also choose to inline it.

> BTW, behavior of `gcc -B./ -o foo -O2 -fwhole-program -flto=jobserver
> -fuse-linker-plugin y.o x.o libdiv.a' depends on the presence / absence of IR
> sections in libdiv.a (links with libgcc / libdiv.a). I think it's a bug.

This is intentional. Since libdiv.a with IR isn't used by LTO in stage 1
linking, it can't be used in stage 2 either. Otherwise, stage 2 linking
may lead to undefined symbols if libdiv uses some external variables,
which are defined in some IR inputs and optimized out by LTO since LTO
didn't see their references in libdiv.a. I checked some LTO testcases
into hjl/lto-mixed branch to verify it.

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