Richard Biener <richard.guent...@gmail.com> writes:
> On Tue, Aug 22, 2023 at 12:42 PM Szabolcs Nagy via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> From: Richard Sandiford <richard.sandif...@arm.com>
>>
>> The prologue/epilogue pass allows the prologue sequence
>> to contain jumps. The sequence is then partitioned into
>> basic blocks using find_many_sub_basic_blocks.
>>
>> This patch treats epilogues in the same way. It's needed for
>> a follow-on aarch64 patch that adds conditional code to both
>> the prologue and the epilogue.
>>
>> Tested on aarch64-linux-gnu (including with a follow-on patch)
>> and x86_64-linux-gnu. OK to install?
>>
>> Richard
>>
>> gcc/
>> * function.cc (thread_prologue_and_epilogue_insns): Handle
>> epilogues that contain jumps.
>> ---
>>
>> This is a previously approved patch that was not committed
>> because it was not needed at the time, but i'd like to commit
>> it as it is needed for the followup aarch64 eh_return changes:
>>
>> https://gcc.gnu.org/pipermail/gcc-patches/2022-November/605769.html
>>
>> ---
>> gcc/function.cc | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/gcc/function.cc b/gcc/function.cc
>> index dd2c1136e07..70d1cd65303 100644
>> --- a/gcc/function.cc
>> +++ b/gcc/function.cc
>> @@ -6120,6 +6120,11 @@ thread_prologue_and_epilogue_insns (void)
>> && returnjump_p (BB_END (e->src)))
>> e->flags &= ~EDGE_FALLTHRU;
>> }
>> +
>> + auto_sbitmap blocks (last_basic_block_for_fn (cfun));
>> + bitmap_clear (blocks);
>> + bitmap_set_bit (blocks, BLOCK_FOR_INSN (epilogue_seq)->index);
>> + find_many_sub_basic_blocks (blocks);
>> }
>> else if (next_active_insn (BB_END (exit_fallthru_edge->src)))
>> {
>> @@ -6218,6 +6223,11 @@ thread_prologue_and_epilogue_insns (void)
>> set_insn_locations (seq, epilogue_location);
>>
>> emit_insn_before (seq, insn);
>> +
>> + auto_sbitmap blocks (last_basic_block_for_fn (cfun));
>> + bitmap_clear (blocks);
>> + bitmap_set_bit (blocks, BLOCK_FOR_INSN (insn)->index);
>> + find_many_sub_basic_blocks (blocks);
>
> I'll note that clearing a full sbitmap to pass down a single basic block
> to find_many_sub_basic_blocks is a quite expensive operation. May I suggest
> to add an overload operating on a single basic block? It's only
>
> FOR_EACH_BB_FN (bb, cfun)
> SET_STATE (bb,
> bitmap_bit_p (blocks, bb->index) ? BLOCK_TO_SPLIT :
> BLOCK_ORIGINAL);
>
> using the bitmap, so factoring the rest of the function and customizing this
> walk would do the trick. Note that the whole function could be refactored to
> handle single blocks more efficiently.
Sorry for the late reply, but does this look OK? Tested on
aarch64-linux-gnu and x86_64-linux-gnu.
Thanks,
Richard
---
The prologue/epilogue pass allows the prologue sequence to contain
jumps. The sequence is then partitioned into basic blocks using
find_many_sub_basic_blocks.
This patch treats epilogues in a similar way. Since only one block
might need to be split, the patch (re)introduces a find_sub_basic_blocks
routine to handle a single block.
The new routine hard-codes the assumption that split_block will chain
the new block immediately after the original block. The routine doesn't
try to replicate the fix for PR81030, since that was specific to
gimple->rtl expansion.
The patch is needed for follow-on aarch64 patches that add conditional
code to the epilogue. The tests are part of those patches.
gcc/
* cfgbuild.h (find_sub_basic_blocks): Declare.
* cfgbuild.cc (update_profile_for_new_sub_basic_block): New function,
split out from...
(find_many_sub_basic_blocks): ...here.
(find_sub_basic_blocks): New function.
* function.cc (thread_prologue_and_epilogue_insns): Handle
epilogues that contain jumps.
---
gcc/cfgbuild.cc | 95 +++++++++++++++++++++++++++++++++----------------
gcc/cfgbuild.h | 1 +
gcc/function.cc | 4 +++
3 files changed, 70 insertions(+), 30 deletions(-)
diff --git a/gcc/cfgbuild.cc b/gcc/cfgbuild.cc
index 15ed4deb5f7..9a6b34fb4b1 100644
--- a/gcc/cfgbuild.cc
+++ b/gcc/cfgbuild.cc
@@ -693,6 +693,43 @@ compute_outgoing_frequencies (basic_block b)
}
}
+/* Update the profile information for BB, which was created by splitting
+ an RTL block that had a non-final jump. */
+
+static void
+update_profile_for_new_sub_basic_block (basic_block bb)
+{
+ edge e;
+ edge_iterator ei;
+
+ bool initialized_src = false, uninitialized_src = false;
+ bb->count = profile_count::zero ();
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ if (e->count ().initialized_p ())
+ {
+ bb->count += e->count ();
+ initialized_src = true;
+ }
+ else
+ uninitialized_src = true;
+ }
+ /* When some edges are missing with read profile, this is
+ most likely because RTL expansion introduced loop.
+ When profile is guessed we may have BB that is reachable
+ from unlikely path as well as from normal path.
+
+ TODO: We should handle loops created during BB expansion
+ correctly here. For now we assume all those loop to cycle
+ precisely once. */
+ if (!initialized_src
+ || (uninitialized_src
+ && profile_status_for_fn (cfun) < PROFILE_GUESSED))
+ bb->count = profile_count::uninitialized ();
+
+ compute_outgoing_frequencies (bb);
+}
+
/* Assume that some pass has inserted labels or control flow
instructions within a basic block. Split basic blocks as needed
and create edges. */
@@ -744,40 +781,15 @@ find_many_sub_basic_blocks (sbitmap blocks)
if (profile_status_for_fn (cfun) != PROFILE_ABSENT)
FOR_BB_BETWEEN (bb, min, max->next_bb, next_bb)
{
- edge e;
- edge_iterator ei;
-
if (STATE (bb) == BLOCK_ORIGINAL)
continue;
if (STATE (bb) == BLOCK_NEW)
{
- bool initialized_src = false, uninitialized_src = false;
- bb->count = profile_count::zero ();
- FOR_EACH_EDGE (e, ei, bb->preds)
- {
- if (e->count ().initialized_p ())
- {
- bb->count += e->count ();
- initialized_src = true;
- }
- else
- uninitialized_src = true;
- }
- /* When some edges are missing with read profile, this is
- most likely because RTL expansion introduced loop.
- When profile is guessed we may have BB that is reachable
- from unlikely path as well as from normal path.
-
- TODO: We should handle loops created during BB expansion
- correctly here. For now we assume all those loop to cycle
- precisely once. */
- if (!initialized_src
- || (uninitialized_src
- && profile_status_for_fn (cfun) < PROFILE_GUESSED))
- bb->count = profile_count::uninitialized ();
+ update_profile_for_new_sub_basic_block (bb);
+ continue;
}
- /* If nothing changed, there is no need to create new BBs. */
- else if (EDGE_COUNT (bb->succs) == n_succs[bb->index])
+ /* If nothing changed, there is no need to create new BBs. */
+ if (EDGE_COUNT (bb->succs) == n_succs[bb->index])
{
/* In rare occassions RTL expansion might have mistakely assigned
a probabilities different from what is in CFG. This happens
@@ -788,10 +800,33 @@ find_many_sub_basic_blocks (sbitmap blocks)
update_br_prob_note (bb);
continue;
}
-
compute_outgoing_frequencies (bb);
}
FOR_EACH_BB_FN (bb, cfun)
SET_STATE (bb, 0);
}
+
+/* Like find_many_sub_basic_blocks, but look only within BB. */
+
+void
+find_sub_basic_blocks (basic_block bb)
+{
+ basic_block end_bb = bb->next_bb;
+ find_bb_boundaries (bb);
+ if (bb->next_bb == end_bb)
+ return;
+
+ /* Re-scan and wire in all edges. This expects simple (conditional)
+ jumps at the end of each new basic blocks. */
+ make_edges (bb, end_bb->prev_bb, 1);
+
+ /* Update branch probabilities. Expect only (un)conditional jumps
+ to be created with only the forward edges. */
+ if (profile_status_for_fn (cfun) != PROFILE_ABSENT)
+ {
+ compute_outgoing_frequencies (bb);
+ for (bb = bb->next_bb; bb != end_bb; bb = bb->next_bb)
+ update_profile_for_new_sub_basic_block (bb);
+ }
+}
diff --git a/gcc/cfgbuild.h b/gcc/cfgbuild.h
index 51d3eccb1ae..4191fb3fcba 100644
--- a/gcc/cfgbuild.h
+++ b/gcc/cfgbuild.h
@@ -24,5 +24,6 @@ extern bool inside_basic_block_p (const rtx_insn *);
extern bool control_flow_insn_p (const rtx_insn *);
extern void rtl_make_eh_edge (sbitmap, basic_block, rtx);
extern void find_many_sub_basic_blocks (sbitmap);
+extern void find_sub_basic_blocks (basic_block);
#endif /* GCC_CFGBUILD_H */
diff --git a/gcc/function.cc b/gcc/function.cc
index 336af28fb22..afb0b33da9e 100644
--- a/gcc/function.cc
+++ b/gcc/function.cc
@@ -6112,6 +6112,8 @@ thread_prologue_and_epilogue_insns (void)
&& returnjump_p (BB_END (e->src)))
e->flags &= ~EDGE_FALLTHRU;
}
+
+ find_sub_basic_blocks (BLOCK_FOR_INSN (epilogue_seq));
}
else if (next_active_insn (BB_END (exit_fallthru_edge->src)))
{
@@ -6210,6 +6212,8 @@ thread_prologue_and_epilogue_insns (void)
set_insn_locations (seq, epilogue_location);
emit_insn_before (seq, insn);
+
+ find_sub_basic_blocks (BLOCK_FOR_INSN (insn));
}
}
--
2.25.1
