On Wed, 26 Apr 2017, Peter Bergner wrote:
> On 4/20/17 8:26 AM, Peter Bergner wrote:
> > On 4/20/17 2:37 AM, Richard Biener wrote:
> >> Ok, so I think we should ensure that we remove the regular cases
> >> with unreachable destination, like in
> >>
> >> switch (i)
> >> {
> >> case 0:
> >> __builtin_unreachable ();
> >> default:;
> >> }
> >>
> >> and handle default with __builtin_unreachable () from switch
> >> expansion by omitting the range check for the jump table
> >> indexing (and choose a non-default case label for gaps).
> >
> > Ok, I'll modify optimize_unreachable() to remove the unreachable
> > case statements, and leave the unreachable default case statement
> > for switch expansion so we can eliminate the range check. Thanks.
>
> Ok, I lied. :-) It ended up being a fair amount of code to remove
> the unreachable case statements within optimize_unreachable().
> Instead, I hooked into group_case_labels_stmt() which is much
> simpler! That code eliminates all case statements that have the
> same destination as the default case statement. With the patch,
> we also eliminate case statements that are unreachable, thus
> treating them like default cases. This has the added benefit of
> being able to reduce the size of the dispatch table, if those cases
> were at the end of the dispatch table entries.
>
> One difference from the last patch is that I am no longer setting
> default_label to NULL when we emit a decision tree. I noticed that
> the decision tree code seemed to generate slightly better code for
> some of my unit tests if I left it alone. This simplified the
> patch somewhat by removing the changes to emit_case_nodes().
>
> This passed bootstrap and regtesting on powerpc64le-linux and
> x86_64-linux. Is this ok for trunk now? If so, I can hold off
> committing it until GCC 7 has been released if that helps.
Can you do the gimple_unreachable_bb_p check earlier in
expand_case so it covers the emit_case_decision_tree path as well
(and verify that works, of course)? So basically right at
/* Find the default case target label. */
default_label = jump_target_rtx
(CASE_LABEL (gimple_switch_default_label (stmt)));
edge default_edge = EDGE_SUCC (bb, 0);
int default_prob = default_edge->probability;
handle this case.
As for Bernhards concern I share this -- please intead make the
interface take either a gimple_seq or a gimple_stmt_iterator
instead of a basic-block. That makes it more obvious you
can't use things like gsi_after_labels. Also I think it's more
natural to work backwards as the last stmt in the sequence
_has_ to be __builtin_unreachable () and thus checking that first
is the cheapest thing to do given that in most cases it will
not be __builtin_unreachable () (but a noreturn call or an
inifinite loop).
Thus, name it gimple_seq_unreachable_p.
Thanks,
Richard.
> Peter
>
>
> gcc/
> * tree-cfg.c (gimple_unreachable_bb_p): New function.
> (assert_unreachable_fallthru_edge_p): Use it.
> (group_case_labels_stmt): Likewise.
> * tree-cfg.h: Prototype it.
> * stmt.c: Include cfghooks.h and tree-cfg.h.
> (emit_case_dispatch_table) <gap_label>: New local variable.
> Use it to fill dispatch table gaps.
> Test for default_label before updating probabilities.
> (expand_case) <default_label>: Remove unneeded initialization.
> Test for unreachable default case statement and remove its edge.
> Set default_label accordingly.
> * gcc/tree-ssa-ccp.c (optimize_unreachable): Update comment.
>
> gcc/testsuite/
> * gcc.target/powerpc/pr51513.c: New test.
> * gcc.dg/predict-13.c: Replace __builtin_unreachable() with
> __builtin_abort().
> * gcc.dg/predict-14.c: Likewise.
>
> Index: gcc/tree-cfg.c
> ===================================================================
> --- gcc/tree-cfg.c (revision 247291)
> +++ gcc/tree-cfg.c (working copy)
> @@ -452,6 +452,37 @@ computed_goto_p (gimple *t)
> && TREE_CODE (gimple_goto_dest (t)) != LABEL_DECL);
> }
>
> +/* Returns true if the basic block BB has no successors and only contains
> + a call to __builtin_unreachable (). */
> +
> +bool
> +gimple_unreachable_bb_p (basic_block bb)
> +{
> + gimple_stmt_iterator gsi;
> + gimple_seq stmts = bb_seq (bb);
> + gimple *stmt;
> +
> + if (EDGE_COUNT (bb->succs) != 0)
> + return false;
> +
> + gsi = gsi_start (stmts);
> + while (!gsi_end_p (gsi) && gimple_code (gsi_stmt (gsi)) == GIMPLE_LABEL)
> + gsi_next (&gsi);
> +
> + if (gsi_end_p (gsi))
> + return false;
> +
> + stmt = gsi_stmt (gsi);
> + while (is_gimple_debug (stmt) || gimple_clobber_p (stmt))
> + {
> + gsi_next (&gsi);
> + if (gsi_end_p (gsi))
> + return false;
> + stmt = gsi_stmt (gsi);
> + }
> + return gimple_call_builtin_p (stmt, BUILT_IN_UNREACHABLE);
> +}
> +
> /* Returns true for edge E where e->src ends with a GIMPLE_COND and
> the other edge points to a bb with just __builtin_unreachable ().
> I.e. return true for C->M edge in:
> @@ -475,23 +506,7 @@ assert_unreachable_fallthru_edge_p (edge
> basic_block other_bb = EDGE_SUCC (pred_bb, 0)->dest;
> if (other_bb == e->dest)
> other_bb = EDGE_SUCC (pred_bb, 1)->dest;
> - if (EDGE_COUNT (other_bb->succs) == 0)
> - {
> - gimple_stmt_iterator gsi = gsi_after_labels (other_bb);
> - gimple *stmt;
> -
> - if (gsi_end_p (gsi))
> - return false;
> - stmt = gsi_stmt (gsi);
> - while (is_gimple_debug (stmt) || gimple_clobber_p (stmt))
> - {
> - gsi_next (&gsi);
> - if (gsi_end_p (gsi))
> - return false;
> - stmt = gsi_stmt (gsi);
> - }
> - return gimple_call_builtin_p (stmt, BUILT_IN_UNREACHABLE);
> - }
> + return gimple_unreachable_bb_p (other_bb);
> }
> return false;
> }
> @@ -1668,9 +1683,10 @@ group_case_labels_stmt (gswitch *stmt)
> gcc_assert (base_case);
> base_bb = label_to_block (CASE_LABEL (base_case));
>
> - /* Discard cases that have the same destination as the
> - default case. */
> - if (base_bb == default_bb)
> + /* Discard cases that have the same destination as the default case
> + or if their destination block is unreachable. */
> + if (base_bb == default_bb
> + || gimple_unreachable_bb_p (base_bb))
> {
> gimple_switch_set_label (stmt, i, NULL_TREE);
> i++;
> Index: gcc/tree-cfg.h
> ===================================================================
> --- gcc/tree-cfg.h (revision 247291)
> +++ gcc/tree-cfg.h (working copy)
> @@ -56,6 +56,7 @@ extern bool is_ctrl_stmt (gimple *);
> extern bool is_ctrl_altering_stmt (gimple *);
> extern bool simple_goto_p (gimple *);
> extern bool stmt_ends_bb_p (gimple *);
> +extern bool gimple_unreachable_bb_p (basic_block);
> extern bool assert_unreachable_fallthru_edge_p (edge);
> extern void delete_tree_cfg_annotations (function *);
> extern gphi *get_virtual_phi (basic_block);
> Index: gcc/stmt.c
> ===================================================================
> --- gcc/stmt.c (revision 247291)
> +++ gcc/stmt.c (working copy)
> @@ -30,6 +30,7 @@ along with GCC; see the file COPYING3.
> #include "rtl.h"
> #include "tree.h"
> #include "gimple.h"
> +#include "cfghooks.h"
> #include "predict.h"
> #include "alloc-pool.h"
> #include "memmodel.h"
> @@ -49,6 +50,7 @@ along with GCC; see the file COPYING3.
> #include "expr.h"
> #include "langhooks.h"
> #include "cfganal.h"
> +#include "tree-cfg.h"
> #include "params.h"
> #include "dumpfile.h"
> #include "builtins.h"
> @@ -1007,20 +1009,21 @@ emit_case_dispatch_table (tree index_exp
> = gen_rtx_LABEL_REF (Pmode, label_rtx (n->code_label));
> }
>
> - /* Fill in the gaps with the default. We may have gaps at
> - the beginning if we tried to avoid the minval subtraction,
> - so substitute some label even if the default label was
> - deemed unreachable. */
> - if (!default_label)
> - default_label = fallback_label;
> + /* The dispatch table may contain gaps, including at the beginning of
> + the table if we tried to avoid the minval subtraction. We fill the
> + dispatch table slots associated with the gaps with the default case
> label.
> + However, in the event the default case is unreachable, we then use
> + any label from one of the case statements. */
> + rtx gap_label = (default_label) ? default_label : fallback_label;
> +
> for (i = 0; i < ncases; i++)
> if (labelvec[i] == 0)
> {
> - has_gaps = true;
> - labelvec[i] = gen_rtx_LABEL_REF (Pmode, default_label);
> + has_gaps = true;
> + labelvec[i] = gen_rtx_LABEL_REF (Pmode, gap_label);
> }
>
> - if (has_gaps)
> + if (has_gaps && default_label)
> {
> /* There is at least one entry in the jump table that jumps
> to default label. The default label can either be reached
> @@ -1115,7 +1118,7 @@ void
> expand_case (gswitch *stmt)
> {
> tree minval = NULL_TREE, maxval = NULL_TREE, range = NULL_TREE;
> - rtx_code_label *default_label = NULL;
> + rtx_code_label *default_label;
> unsigned int count, uniq;
> int i;
> int ncases = gimple_switch_num_labels (stmt);
> @@ -1232,9 +1235,20 @@ expand_case (gswitch *stmt)
> case_list, default_label,
> default_prob);
> else
> - emit_case_dispatch_table (index_expr, index_type,
> - case_list, default_label,
> - minval, maxval, range, bb);
> + {
> + /* If the default case is unreachable, then set default_label to NULL
> + so that we omit the range check when generating the dispatch table.
> + We also remove the edge to the unreachable default case. The block
> + itself will be automatically removed later. */
> + if (gimple_unreachable_bb_p (default_edge->dest))
> + {
> + default_label = NULL;
> + remove_edge (default_edge);
> + }
> + emit_case_dispatch_table (index_expr, index_type,
> + case_list, default_label,
> + minval, maxval, range, bb);
> + }
>
> reorder_insns (NEXT_INSN (before_case), get_last_insn (), before_case);
>
> Index: gcc/tree-ssa-ccp.c
> ===================================================================
> --- gcc/tree-ssa-ccp.c (revision 247291)
> +++ gcc/tree-ssa-ccp.c (working copy)
> @@ -2715,7 +2715,8 @@ optimize_unreachable (gimple_stmt_iterat
> }
> else
> {
> - /* Todo: handle other cases, f.i. switch statement. */
> + /* Todo: handle other cases. Note that unreachable switch case
> + statements have already been removed. */
> continue;
> }
>
> Index: gcc/testsuite/gcc.target/powerpc/pr51513.c
> ===================================================================
> --- gcc/testsuite/gcc.target/powerpc/pr51513.c (nonexistent)
> +++ gcc/testsuite/gcc.target/powerpc/pr51513.c (working copy)
> @@ -0,0 +1,25 @@
> +/* { dg-do compile { target { powerpc*-*-linux* } } } */
> +/* { dg-options "-O2 -fjump-tables --param case-values-threshold=1" } */
> +/* Verify we created a jump table. */
> +/* { dg-final { scan-assembler-times "mtctr " 1 } } */
> +/* { dg-final { scan-assembler-times "bctr" 1 } } */
> +/* Verify we eliminated the range check. */
> +/* { dg-final { scan-assembler-not "cmpldi" } } */
> +/* { dg-final { scan-assembler-not "cmplwi" } } */
> +
> +long
> +bug (long cond, long v0, long v1, long v2)
> +{
> + switch (cond)
> + {
> + case 0:
> + return v0;
> + case 1:
> + return v1;
> + case 2:
> + return v2;
> + default:
> + __builtin_unreachable ();
> + }
> + __builtin_abort ();
> +}
> Index: gcc/testsuite/gcc.dg/predict-13.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/predict-13.c (revision 247291)
> +++ gcc/testsuite/gcc.dg/predict-13.c (working copy)
> @@ -10,9 +10,9 @@ int main(int argc, char **argv)
> case 2:
> return 2;
> case 3:
> - __builtin_unreachable();
> + __builtin_abort();
> case 4:
> - __builtin_unreachable();
> + __builtin_abort();
> default:
> return 5;
> }
> Index: gcc/testsuite/gcc.dg/predict-14.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/predict-14.c (revision 247291)
> +++ gcc/testsuite/gcc.dg/predict-14.c (working copy)
> @@ -6,11 +6,11 @@ int main(int argc, char **argv)
> switch (argc)
> {
> case 1:
> - __builtin_unreachable();
> + __builtin_abort();
> case 4:
> - __builtin_unreachable();
> + __builtin_abort();
> default:
> - __builtin_unreachable();
> + __builtin_abort();
> }
>
> return 10;
>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
