[PATCH][RFC] final-value replacement from DCE

[Jeff Law]

On 5/29/19 7:36 AM, Richard Biener wrote:
> 
> The following tries to address PR90648 by performing final
> value replacement from DCE when DCE knows the final value
> computation is not used during loop iteration.  This fits
> neatly enough into existing tricks performed by DCE like
> removing unused malloc/free pairs.
DO you have the right BZ #?  90648 is a ICE in tree checking and doesn't
have a loop :-)



> 
> There's a few complications, one is it fails to bootstrap
> because it exposes a few uninit warning false positives,
> another is that -fno-tree-sccp is no longer effective.
> As written this turns gcc.dg/pr34027-1.c into a division
> again (I did not copy the expression_expensive checking).
> It seems to also need -ftrapv adjustements (gcc.dg/pr81661.c).
> 
> The goal of this patch is to remove the SCCP pass, or rather
> us unconditionally replacing loop-closed PHIs with final
> value computations which we've got complaints in the past
> already that it duplicates computation that is readily
> available.  I've not yet figured testsuite fallout from that
> change.
> 
> For the -fno-tree-sccp I consider to simply honor that
> flag in the DCE path, for the gcc.dg/pr34027-1.c I'll
> re-install the expression_expensive checking.  I'll
> also fix the -ftrapv issue.
> 
> Does this otherwise look a sensible way forward?

> 
> Thanks,
> Richard.
> 
> FAIL: gcc.dg/builtin-object-size-1.c execution test
> FAIL: gcc.dg/builtin-object-size-5.c scan-assembler-not abort
> FAIL: gcc.dg/pr34027-1.c scan-tree-dump-times optimized " / " 0
> FAIL: gcc.dg/pr81661.c (internal compiler error)
> FAIL: gcc.dg/pr81661.c (test for excess errors)
> XPASS: gcc.dg/tree-ssa/loop-15.c scan-tree-dump-times optimized " \\\\+ " 0
> FAIL: gcc.dg/tree-ssa/loop-15.c scan-tree-dump-times optimized "if " 1
> FAIL: gcc.dg/tree-ssa/loop-26.c scan-tree-dump-times optimized "if" 2
> FAIL: gcc.dg/tree-ssa/pr32044.c scan-tree-dump-times optimized " / " 0
> FAIL: gcc.dg/tree-ssa/pr32044.c scan-tree-dump-times optimized "if" 6
> FAIL: gcc.dg/tree-ssa/pr64183.c scan-tree-dump cunroll "Loop 2 iterates at most 3 times"
> FAIL: gcc.dg/tree-ssa/ssa-pre-3.c scan-tree-dump-times pre "Eliminated: 2" 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-3.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-4.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-5.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-11.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-13.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-14.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-15.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-16.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-17.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-18.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-19.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-2.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED" 1
> FAIL: gcc.dg/vect/no-scevccp-outer-20.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-21.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-3.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-5.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-6-global.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-6.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-7.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-8.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-1.c scan-tree-dump-times vect "vectorized 1 loops" 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-3.c scan-tree-dump-times vect "vect_recog_widen_sum_pattern: detected" 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-3.c scan-tree-dump-times vect "vectorized 1 loops" 1
> 
> Running target unix//-m32
> FAIL: gcc.dg/builtin-object-size-1.c execution test
> FAIL: gcc.dg/builtin-object-size-5.c scan-assembler-not abort
> FAIL: gcc.dg/pr34027-1.c scan-tree-dump-times optimized " / " 0
> FAIL: gcc.dg/pr81661.c (internal compiler error)
> FAIL: gcc.dg/pr81661.c (test for excess errors)
> XPASS: gcc.dg/tree-ssa/loop-15.c scan-tree-dump-times optimized " \\\\+ " 0
> FAIL: gcc.dg/tree-ssa/loop-15.c scan-tree-dump-times optimized "if " 1
> FAIL: gcc.dg/tree-ssa/loop-26.c scan-tree-dump-times optimized "if" 2
> FAIL: gcc.dg/tree-ssa/pr32044.c scan-tree-dump-times optimized " / " 0
> FAIL: gcc.dg/tree-ssa/pr32044.c scan-tree-dump-times optimized "if" 6
> FAIL: gcc.dg/tree-ssa/pr64183.c scan-tree-dump cunroll "Loop 2 iterates at most 3 times"
> FAIL: gcc.dg/tree-ssa/ssa-pre-3.c scan-tree-dump-times pre "Eliminated: 2" 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-3.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-4.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-noreassoc-outer-5.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-11.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-13.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-14.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-15.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-16.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-17.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-18.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-19.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-2.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED" 1
> FAIL: gcc.dg/vect/no-scevccp-outer-20.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-21.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-3.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-5.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-6-global.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-6.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-7.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-outer-8.c scan-tree-dump-times vect "OUTER LOOP VECTORIZED." 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-1.c scan-tree-dump-times vect "vectorized 1 loops" 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-3.c scan-tree-dump-times vect "vect_recog_widen_sum_pattern: detected" 1
> FAIL: gcc.dg/vect/no-scevccp-vect-iv-3.c scan-tree-dump-times vect "vectorized 1 loops" 1
> FAIL: gcc.target/i386/pr85073.c scan-assembler-times test 1
> 
> 2019-05-29  Richard Biener  <rguenther@suse.de>
> 
> 	PR tree-optimization/90594
> 	* tree-scalar-evolution.h (final_value_replacement): Declare.
> 	* tree-scalar-evolution.c (final_value_replacement): Split out
> 	worker on one PHI from ...
> 	(final_value_replacement_loop): ... here.
> 	* gimple-fold.h (rewrite_seq_to_defined_overflow): Declare.
> 	* gimple-fold.c (rewrite_seq_to_defined_overflow): Wrap
> 	around rewrite_to_defined_overflow to handle a whole sequence.
> 	* tree-ssa-dce.c (lcphi_map): New.
> 	(mark_operands_necessary): New walk_tree worker.
> 	(propagate_necessity): Elide defs of LC PHIs we can perform value
> 	replacement on.
> 	(remove_dead_phis): Perform value replacement on LC PHIs that
> 	had their def removed.
> 	(perform_tree_ssa_dce): Go into LC SSA in aggressive mode.
> 	Precompute final value replacements.
> 
> 	* gcc.dg/tree-ssa/loop-14.c: Adjust for new expected place
> 	for transform.
The approach here looks quite reasonable to me, so it's just a matter of
the fallout, as are so many things these days...

jeff