Re: [PATCH] Fix PR84427, PRE ANTIC iteration (again)

[Jeff Law <law at redhat dot com>]

On 03/01/2018 06:57 AM, Richard Biener wrote:
> 
> This fixes another testcase that shows that our ANTIC iteration can
> fail to converge.  The fix continues what we did with previous fixes,
> avoid spuriously removing stuff from the expression side of the sets.
> This time going full-in and allowing multiple expressions for the
> same value in the sets.  I've added verification that the value-sets
> never grow during iteration which barfed on some of the related
> existing testcases and thus helped finishing this and fixing all
> places where we end up somewhat randomly choosing one or another
> expression to drop.
> 
> I've verified with a GIMPLE testcase that if at insertion time
> we have multiple partially redundant expressions for the same value
> we insert/hoist it only once.  So the only "bad" effect of the
> patch is that the expression part of the ANTIC sets grows -- by
> not throwing away random stuff we might also arrive at larger
> (value) solutions for ANTIC which means we may catch previously
> missed optimizations (or pessimizations as you know PRE...).
> 
> I'm not entirely happy with the timing but I'm quite confident in
> the approach also (again) having heavily discussed this with Micha.
> 
> Re-boostrap and regtest running on x86_64-unknown-linux-gnu.
> 
> I managed to go into a different solution at the beginning asking
> for that pred/phiblock->edge cleanup and decided to leave that in...
> 
> The new verification is guarded with flag_checking so if it would
> trigger but wouldn't result in not converging a release build
> should be not affected.
> 
> Richard.
> 
> 2018-01-03  Richard Biener  <rguenther@suse.de>
> 
> 	PR tree-optimization/84427
> 	* tree-ssa-pre.c (bitmap_remove_expr_from_set): Remove.
> 	(bitmap_set_subtract_values): Rewrite to handle multiple
> 	exprs per value.
> 	(clean): Likewise.
> 	(prune_clobbered_mems): Likewise.
> 	(phi_translate): Take edge instead of pred/phiblock.
> 	(phi_translate_1): Likewise.
> 	(phi_translate_set): Likewise.  Insert all translated
> 	exprs for a value into the set, keeping possibly multiple
> 	expressions per value.
> 	(compute_antic_aux): Adjust for phi_translate changes.
> 	When intersecting union the expressions and prune those
> 	not in the final value set, keeping possibly multiple
> 	expressions per value.  Do not use value-insertion
> 	for unioning ANTIC_OUT U EXP_GEN - TMP_GEN but merge
> 	all expressions.  Add verification that the value-sets
> 	only shrink during iteration.
> 	(compute_partial_antic_aux): Adjust for the phi_translate changes.
> 	(do_pre_regular_insertion): Likewise.
> 	(do_pre_partial_partial_insertion): Likewise.
> 
> 	* gcc.dg/torture/pr84427.c: New testcase.
It looks like Zdenek has already filed a report against this change.
I'll wait for resolution on that before reducing the various linux
kernel build failures.

jeff