On 2024-06-05 21:58 Jeff Law <jeffreya...@gmail.com> wrote:
>
>
>
>On 6/5/24 1:47 AM, Fei Gao wrote:
>>
>> On 2024-06-05 14:36 Kito Cheng <kito.ch...@gmail.com> wrote:
>>>
>>> Thanks for fixing this issue, and I am wondering doest it possible to
>>> fix that without introduce target hook? I ask that because...GCC 14
>>> also has this bug, but I am not sure it's OK to introduce new target
>>> hook for release branch? or would you suggest we just revert patch to
>>> fix that on GCC 14?
>>
>> If hook is unacceptable in GCC14, I suggest to revert on GCC 14 the
>> following commit.
>> https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b27d323a368033f0b37e93c57a57a35fd9997864
>>
>> I started fixing this issue by adding changes in mach pass but abandoned it
>> due to the following reasons:
>> 1. more codes to detect location of epilogue in the whole insn list.
>> 2. due to impact by scheduling pass, clear a0 and use a0 insns get
>> reordered, resulting in more
>> codes.
>> 3. data flow analysis is needed, but insn does't have bb info any more, so
>> rescan actually does
>> nothing, which I guess there's some hidden issue in
>>riscv_remove_unneeded_save_restore_calls
>> using dfa.
>>
>> So I came up this hook based patch in prologue and epilogue pass to make the
>> optimization
>> happen as earlier as possible. It ends up with simplicity and clear logic.
>But let's back up and get a good explanation of what the problem is.
>Based on patch 2/2 it looks like we have lost an assignment to the
>return register.
>
>To someone not familiar with this code, it sounds to me like we've made
>a mistake earlier and we're now defining a hook that lets us go back and
>fix that earlier mistake. I'm probably wrong, but so far that's what
>it sounds like.
Hi Jeff
You're right. Let me rephrase patch 2/2 with more details. Search /* feigao to
location the point I'm
tring to explain.
code snippets from gcc/function.cc
void
thread_prologue_and_epilogue_insns (void)
{
...
/*feigao:
targetm.gen_epilogue () is called here to generate epilogue sequence.
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b27d323a368033f0b37e93c57a57a35fd9997864
Commit above tries in targetm.gen_epilogue () to detect if
there's li a0,0 insn at the end of insn chain, if so, cm.popret
is replaced by cm.popretz and li a0,0 insn is deleted.
issue of the commit above:
Insertion of the generated epilogue sequence
into the insn chain doesn't happen at this moment.
If later shrink-wrap decides NOT to insert the epilogue sequence at the
end
of insn chain, then the li a0,0 insn has already been mistakeny
removed. */
rtx_insn *epilogue_seq = make_epilogue_seq ();
/* Try to perform a kind of shrink-wrapping, making sure the
prologue/epilogue is emitted only around those parts of the
function that require it. */
/* feigao:
Make a simple_return for those exits that run without prologue and
force nonfallthru by e->flags &= ~EDGE_FALLTHRU */
try_shrink_wrapping (&entry_edge, prologue_seq);
...
edge exit_fallthru_edge = find_fallthru_edge (EXIT_BLOCK_PTR_FOR_FN
(cfun)->preds);
/* feigao: fasle here in simple_return case, so no insertion of epilogue,
but the li a0,0 insn has already been mistakeny removed. */
if (exit_fallthru_edge)
{
if (epilogue_seq)
{
insert_insn_on_edge (epilogue_seq, exit_fallthru_edge);
commit_edge_insertions ();
/* The epilogue insns we inserted may cause the exit edge to no longer
be fallthru. */
FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR_FOR_FN (cfun)->preds)
{
if (((e->flags & EDGE_FALLTHRU) != 0)
&& returnjump_p (BB_END (e->src)))
e->flags &= ~EDGE_FALLTHRU;
}
find_sub_basic_blocks (BLOCK_FOR_INSN (epilogue_seq));
}
...
if (epilogue_seq)
{
rtx_insn *insn, *next;
/* Similarly, move any line notes that appear after the epilogue.
There is no need, however, to be quite so anal about the existence
of such a note. Also possibly move
NOTE_INSN_FUNCTION_BEG notes, as those can be relevant for debug
info generation. */
for (insn = epilogue_seq; insn; insn = next)
{
next = NEXT_INSN (insn);
if (NOTE_P (insn)
&& (NOTE_KIND (insn) == NOTE_INSN_FUNCTION_BEG))
reorder_insns (insn, insn, PREV_INSN (epilogue_seq));
}
}
/* feigao: new hook */
targetm.post_epilogue_proc (epilogue_seq);
...
BR,
Fei
>
>So let's get a good explanation of the problem and perhaps we'll find a
>better way to solve it.
>
>jeff
>
