[gcc r12-4766] Improve backward threading with switches.

[gcc r12-4766] Improve backward threading with switches.

[Aldy Hernandez]

https://gcc.gnu.org/g:113dab2b9d511f3aadc30a6a921fc30bd5f93706

commit r12-4766-g113dab2b9d511f3aadc30a6a921fc30bd5f93706
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Thu Oct 28 11:44:13 2021 +0200

    Improve backward threading with switches.
    
    We've been essentially using find_taken_edge_switch_expr() in the
    backward threader, but this is suboptimal because said function only
    works with singletons.  VRP has a much smarter find_case_label_range
    that works with ranges.
    
    Tested on x86-64 Linux with:
    
            a) Bootstrap & regtests.
    
            b) Verifying we get more threads than before.
    
            c) Asserting that the new code catches everything the old one
            code caught (over a set of bootstrap .ii files).
    
    gcc/ChangeLog:
    
            * tree-ssa-threadbackward.c
            (back_threader::find_taken_edge_switch): Use find_case_label_range
            instead of find_taken_edge.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.dg/tree-ssa/vrp106.c: Adjust for threading.
            * gcc.dg/tree-ssa/vrp113.c: Same.

Diff:
---
 gcc/testsuite/gcc.dg/tree-ssa/vrp106.c | 4 ++--
 gcc/testsuite/gcc.dg/tree-ssa/vrp113.c | 2 --
 gcc/tree-ssa-threadbackward.c          | 8 ++++----
 3 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vrp106.c b/gcc/testsuite/gcc.dg/tree-ssa/vrp106.c
index f25ea9c3826..dc5021a57b5 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/vrp106.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/vrp106.c
@@ -1,6 +1,6 @@
 /* PR tree-optimization/18046  */
-/* { dg-options "-O2 -fdump-tree-vrp-thread1-details" }  */
-/* { dg-final { scan-tree-dump-times "Threaded jump" 1 "vrp-thread1" } }  */
+/* { dg-options "-O2 -fdump-tree-ethread-details" }  */
+/* { dg-final { scan-tree-dump-times "Registering jump thread" 1 "ethread" } }  */
 /* During VRP we expect to thread the true arm of the conditional through the switch
    and to the BB that corresponds to the 7 ... 9 case label.  */
 extern void foo (void);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vrp113.c b/gcc/testsuite/gcc.dg/tree-ssa/vrp113.c
index ab8d91e0f10..dfe4989d313 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/vrp113.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/vrp113.c
@@ -13,5 +13,3 @@ int f(int a) {
       case 7: return 19;
     }
 }
-
-/* { dg-final { scan-tree-dump "return 3;" "vrp1" { xfail *-*-* } } } */
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 6c1b15904ce..456effca5e1 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -195,11 +195,11 @@ back_threader::find_taken_edge_switch (const vec<basic_block> &path,
   if (r.varying_p ())
     return NULL;
 
-  tree val;
-  if (r.singleton_p (&val))
-    return ::find_taken_edge (gimple_bb (sw), val);
+  tree label = find_case_label_range (sw, &r);
+  if (!label)
+    return NULL;
 
-  return NULL;
+  return find_edge (gimple_bb (sw), label_to_block (cfun, CASE_LABEL (label)));
 }
 
 // Same as find_taken_edge, but for paths ending in a GIMPLE_COND.