[lto] Fix symlookup in archives vs shared

[Jeff Law]

On 3/31/25 9:16 AM, Michael Matz wrote:
> when a shared library defines 'foo@@FOO' (default version),
> a static archive defines 'foo', the shared lib comes in front
> of the archive and under effect of --as-needed, and the requesting
> object file uses LTO, then the link editor was wrongly including
> the definition from the static archive.  It must use the one
> from the shared lib, like in the non-LTO or the --no-as-needed case.
> See the added testcase that would wrongly print "FAIL" before
> this patch.
> 
> The problem stems from several connected problems:
> (1) only the decorated symbol was entered into first_hash (the hash
>      table designed to handle definition order in the pre-LTO-plugin
>      phase of the symbol table walks)
> (2) in the archive symbol walk only the undecorated name would be
>      looked up in first_hash (and hence not found due to (1))
> (3) in the archive symbol walk first_hash would only be consulted
>      when the linker hash table had a defined symbol.  In pre-LTO
>      phase shared lib symbols aren't entered into the linker symbol
>      table.
> 
> So: add also the undecorated name into first_hash when it stems from
> a default version and consult first_hash in the archive walker also
> for currently undefined symbols.  If it has an entry which doesn't
> point to the archive, then it comes from an earlier library (shared or
> static), and so _this_ archive won't provide the definition.
This caused a number of the embedded targets to regress in their 
testsuites, typically due to an undefined symbol such as __muldf3 that 
should have been provided by libgcc.

Just picking one, ft32-elf:

> Tests that now fail, but worked before (110 tests):
> 
> ft32-sim: gcc: c-c++-common/torture/vector-compare-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: c-c++-common/torture/vector-compare-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: gcc.c-torture/execute/20020406-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: gcc.c-torture/execute/20021120-3.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: gcc.c-torture/execute/20070201-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: gcc.c-torture/execute/20121108-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
> ft32-sim: gcc: gcc.c-torture/execute/920501-8.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)

[ ... ]

But this was pretty pervasive across the crosses.

Jeff