On 6/5/24 8:42 PM, Fei Gao wrote:
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.
So that seems like the critical issue. Generation of the
prologue/epilogue really shouldn't be changing other instructions in the
instruction stream. I'm not immediately aware of another target that
does that, an it seems like a rather risky thing to do.
It looks like the cm.popretz's RTL exposes the assignment to a0 and
there's a DCE pass that runs after insertion of the prologue/epilogue.
So I would suggest leaving the assignment to a0 in the RTL chain and see
if the later DCE pass after prologue generation eliminates the redundant
assignment. That seems a lot cleaner.
Jeff
