[Bug target/100181] hot-cold partitioned code doesn't assemble

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100181

--- Comment #9 from Tobias Burnus <burnus at gcc dot gnu.org> ---
(In reply to Thomas Schwinge from comment #8)
> (In reply to Tobias Burnus from comment #7)
> > (I could not reproduce the LLVM 9 issue in PR94278 back then.)
> 
> Hmm, but didn't you say in the LLVM issue
> <https://bugs.llvm.org/show_bug.cgi?id=45887> that you _did_ reproduce this back then?

I partially mixed it up - I could not reproduce it with a self-build LLVM but
could do so with the distro LLVM ones.

Our/my LLVM bug 45887 is similar to
https://bugs.llvm.org/show_bug.cgi?id=41914, which was fixed by
https://reviews.llvm.org/D79943 and the latter adds the check which causes the
error message: "undefined label '.L33'"

However, the other bug (41914) and the the testcase in the fix (D79943) were
about a label which was truly missing as opposed to one which is just in a
differently named .section.

 * * *

Obviously, with D79943 at place (LLVM 11, 12, ...), I can reproduce the issue
with -O2 (with < -O0 there is only .text) as then 's_branch .L33' is in
.section .text but '.L33:' is in '.section .text.unlikely'.

Whether it is intended that cross-.section s_branch are no longer permitted or
whether it happened by chance by this patch or on purpose, I don't know.

Sections themselves are supported – and the  LVM testsuite for GCN has
testcases with '.section' (under llvm/test/MC/AMDGPU/*.s) – but in the
testsuite one at a time.

GCC guards the .text.unlikely generation by (→ varasm.c's
default_function_section) by

  if (!flag_reorder_functions
      || !targetm_common.have_named_sections)
    return NULL;

while the patch in PR 94278 comment 3 uses
   flag_reorder_blocks_and_partition = 0