Give fast DCE a separate dirty flag

Thomas pointed out that we sometimes failed to eliminate some dead code
(specifically clobbers of otherwise unused registers) on nvptx when
late-combine is enabled.  This happens because:

- combine is able to optimise the function in a way that exposes dead code.
  This leaves the df information in a "dirty" state.

- late_combine calls df_analyze without DF_LR_RUN_DCE run set.
  This updates the df information and clears the "dirty" state.

- late_combine doesn't find any extra optimisations, and so leaves
  the df information up-to-date.

- if_after_combine (ce2) calls df_analyze with DF_LR_RUN_DCE set.
  Because the df information is already up-to-date, fast DCE is
  not run.

The upshot is that running late-combine has the effect of suppressing
a DCE opportunity that would have been noticed without late_combine.

I think this shows that we should track the state of the DCE separately
from the LR problem.  Every pass updates the latter, but not all passes
update the former.

gcc/
	* df.h (DF_LR_DCE): New df_problem_id.
	(df_lr_dce): New macro.
	* df-core.cc (rest_of_handle_df_finish): Check for a null free_fun.
	* df-problems.cc (df_lr_finalize): Split out fast DCE handling to...
	(df_lr_dce_finalize): ...this new function.
	(problem_LR_DCE): New df_problem.
	(df_lr_add_problem): Register LR_DCE rather than LR itself.
	* dce.cc (fast_dce): Clear df_lr_dce->solutions_dirty.

1 files changed, 2 insertions, 0 deletions

diff --git a/gcc/df.h b/gcc/df.h
index c4e690b..bbb4f29 100644
--- a/gcc/df.h
+++ b/gcc/df.h
@@ -47,6 +47,7 @@ enum df_problem_id
   {
     DF_SCAN,
     DF_LR,                /* Live Registers backward. */
+    DF_LR_DCE,            /* Dead code elimination post-pass for LR.  */
     DF_LIVE,              /* Live Registers & Uninitialized Registers */
     DF_RD,                /* Reaching Defs. */
     DF_CHAIN,             /* Def-Use and/or Use-Def Chains. */
@@ -940,6 +941,7 @@ extern class df_d *df;
 #define df_scan    (df->problems_by_index[DF_SCAN])
 #define df_rd      (df->problems_by_index[DF_RD])
 #define df_lr      (df->problems_by_index[DF_LR])
+#define df_lr_dce  (df->problems_by_index[DF_LR_DCE])
 #define df_live    (df->problems_by_index[DF_LIVE])
 #define df_chain   (df->problems_by_index[DF_CHAIN])
 #define df_word_lr (df->problems_by_index[DF_WORD_LR])