Re: [PATCH] tree-optimization/109237 - last_stmt is possibly slow

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Mar 22, 2023 at 12:29:52PM +0000, Richard Biener wrote:
> Most uses of last_stmt are interested in control transfer stmts
> and for the testcase gimple_purge_dead_eh_edges shows up in
> the profile.  But last_stmt looks past trailing debug stmts but
> those would be rejected by GIMPLEs verify_flow_info.  The following
> adds possible_ctrl_stmt besides last_stmt which does not look
> past trailing debug stmts and adjusts gimple_purge_dead_eh_edges.
> 
> I've put checking code into possible_ctrl_stmt that it will not
> miss a control statement if the real last stmt is a debug stmt.
> 
> The alternative would be to change last_stmt, explicitely introducing
> last_nondebug_stmt.  I remember we chickened out and made last_stmt
> conservative here but not anticipating the compile-time issues this
> creates.  I count 227 last_stmt and 12 last_and_only_stmt uses.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> 
> Any opinions?  I probably lean towards s/last_stmt/last_nondebug_stmt/
> in next stage1 and then adding last_stmt and changing some
> uses back - through for maintainance that's going to be a
> nightmare (or maybe not, a "wrong" last_stmt should be safe to
> backport and a last_nondebug_stmt will fail to build).
> 
> Richard.
> 
> 	PR tree-optimization/109237
> 	* tree-cfg.h (possible_ctrl_stmt): New function returning
> 	the last stmt not skipping debug stmts.
> 	(gimple_purge_dead_eh_edges): Use it.

LGTM.  But finding out which of those 227+12 calls want to skip
debug stmts and which don't will be a nightmare...

	Jakub