[PATCH v2] Add if-chain to switch conversion pass.
Fri Nov 6 12:31:26 GMT 2020
On Fri, Oct 16, 2020 at 4:04 PM Martin Liška <email@example.com> wrote:
> There's another version of the patch that should be based on what
> I discussed with Richi and Jakub:
> - the first patch introduces a new option -fbit-tests that analogue to -fjump-tables
> and will control the new if-to-switch conversion pass
> - the second patch adds the pass
> - I share code with tree-ssa-reassoc.c (range_entry and init_range_entry)
> - a local discovery phase is run first
> - later than these local BBs are chained into a candidate list for the conversion
> I'm also sending transformed chains for 'make all-host' (620 transformations).
> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
no_side_effect_bb (basic_block bb)
exporting this with this name is dangerous I think because the function
seems to allow side-effects in the last stmt - not sure exactly what
it tries to allow - there's no comment to that :/
+ free (rpo);
+ free_dominance_info (CDI_DOMINATORS);
+ if (!all_candidates.is_empty ())
+ mark_virtual_operands_for_renaming (fun);
please avoid freeing dominance info when there was no change done
(move it to the !all_candidates.is_empty () block).
+ basic_block bb;
+ FOR_EACH_BB_FN (bb, fun)
+ find_conditions (bb, &conditions_in_bbs);
if we didn't find any conditions (or found just one?) we can elide the
rest of the function, no?
+ if_chain *chain = new if_chain ();
+ chain->m_entries.safe_push (info);
+ /* Try to find a chain starting in this BB. */
+ while (true)
+ if (!single_pred_p (gimple_bb (info->m_cond)))
+ edge e = single_pred_edge (gimple_bb (info->m_cond));
+ condition_info *info2 = conditions_in_bbs.get (e->src);
+ if (!info2 || info->m_ranges.exp != info2->m_ranges.exp)
+ chain->m_entries.safe_push (info2);
+ bitmap_set_bit (seen_bbs, e->src->index);
+ info = info2;
so while we now record conditions per BB the above doesn't really
allow matching a binary tree. What I was thinking of is to record
if_chain * per BB as well and look at successors, thus (pseudo-code)
if (block ends in cond)
if (if_chain on true edge && if_chain on false edge)
else if (if_chain on true edge && this-cond tests same var)
else if (if_chan on false edge && ...)
record if_chain for block
where merging would eventually detach the if_chains from the successors.
For now we'd just handle the true (and maybe false) edge combos to handle
linear chains. Walking reverse RPO (I'm not 100% sure reverse RPO is what
we want here, but guess it will work fine for now) will gather chains
When merging from a successor to a BB fails we push the successor chain
to the candidate list.
+/* Algorithm of the pass runs in the following steps:
+ a) We walk basic blocks in DOMINATOR order so that we first reach
+ a first condition of a future switch.
+ b) We follow false edges of a if-else-chain and we record chain
+ of GIMPLE conditions. These blocks are only used for comparison
+ of a common SSA_NAME and we do not allow any side effect.
+ c) We remove all basic blocks (except first) of such chain and
+ GIMPLE switch replaces the condition in the first basic block.
+ d) We move all GIMPLE statements in the removed blocks into the
+ first one. */
the overall comment is now a bit out-of-date?
Please remove the PHI mapping as I outlined in earlier review.
The 0001-Add-fbit-tests-option.patch is OK for trunk.
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