From 826f35d8124731d0f4f5415c84fb59d2b6adf58b Mon Sep 17 00:00:00 2001 From: Segher Boessenkool Date: Wed, 9 May 2018 14:48:43 +0200 Subject: [PATCH] shrink-wrap: Improve spread_components (PR85645) In the testcase for PR85645 we do a pretty dumb placement of the prologue/epilogue for the LR component: we place an epilogue for LR before a control flow split where one of the branches clobbers LR eventually, and the other does not. The branch that does clobber it will need a prologue again some time later. Because saving and restoring LR is a two step process---it needs to be moved via a GPR--- the backend emits CFI directives so that we get correct unwind information. But both regcprop and regrename do not properly handle such CFI directives leading to ICEs. Now, neither of the two branches needs to have LR restored at all, because both of the branches end up in an infinite loop. This patch makes spread_component return a boolean saying if anything was changed, and if so, it is called again. This obviously is finite (there is a finite number of basic blocks, each with a finite number of components, and spread_components can only assign more components to a block, never less). I also instrumented the code, and on a bootstrap+regtest spread_components made changes a maximum of two times. Interestingly though it made changes on two iterations in a third of the cases it did anything at all! PR rtl-optimization/85645 * shrink-wrap.c (spread_components): Return a boolean saying if anything was changed. (try_shrink_wrapping_separate): Iterate spread_components until nothing changes anymore. From-SVN: r260076 --- gcc/ChangeLog | 8 ++++++++ gcc/shrink-wrap.c | 25 ++++++++++++++++++++++--- 2 files changed, 30 insertions(+), 3 deletions(-) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 72e6768a71f..1c81e4f5552 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,11 @@ +2018-05-09 Segher Boessenkool + + PR rtl-optimization/85645 + * shrink-wrap.c (spread_components): Return a boolean saying if + anything was changed. + (try_shrink_wrapping_separate): Iterate spread_components until + nothing changes anymore. + 2018-05-09 Segher Boessenkool PR rtl-optimization/85645 diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c index 6b47d4ea8cd..1ad73798747 100644 --- a/gcc/shrink-wrap.c +++ b/gcc/shrink-wrap.c @@ -1253,8 +1253,9 @@ place_prologue_for_one_component (unsigned int which, basic_block head) /* Set HAS_COMPONENTS in every block to the maximum it can be set to without setting it on any path from entry to exit where it was not already set somewhere (or, for blocks that have no path to the exit, consider only - paths from the entry to the block itself). */ -static void + paths from the entry to the block itself). Return whether any changes + were made to some HAS_COMPONENTS. */ +static bool spread_components (sbitmap components) { basic_block entry_block = ENTRY_BLOCK_PTR_FOR_FN (cfun); @@ -1377,12 +1378,19 @@ spread_components (sbitmap components) /* Finally, mark everything not not needed both forwards and backwards. */ + bool did_changes = false; + FOR_EACH_BB_FN (bb, cfun) { + bitmap_copy (old, SW (bb)->has_components); + bitmap_and (SW (bb)->head_components, SW (bb)->head_components, SW (bb)->tail_components); bitmap_and_compl (SW (bb)->has_components, components, SW (bb)->head_components); + + if (!did_changes && !bitmap_equal_p (old, SW (bb)->has_components)) + did_changes = true; } FOR_ALL_BB_FN (bb, cfun) @@ -1394,6 +1402,8 @@ spread_components (sbitmap components) fprintf (dump_file, "\n"); } } + + return did_changes; } /* If we cannot handle placing some component's prologues or epilogues where @@ -1797,7 +1807,16 @@ try_shrink_wrapping_separate (basic_block first_bb) EXECUTE_IF_SET_IN_BITMAP (components, 0, j, sbi) place_prologue_for_one_component (j, first_bb); - spread_components (components); + /* Try to minimize the number of saves and restores. Do this as long as + it changes anything. This does not iterate more than a few times. */ + int spread_times = 0; + while (spread_components (components)) + { + spread_times++; + + if (dump_file) + fprintf (dump_file, "Now spread %d times.\n", spread_times); + } disqualify_problematic_components (components); -- 2.30.2