S_clear_placeholders() should call HvHASKFLAGS_off() if no keys remain.

This isn't essential - HvHASKFLAGS() set when there are no keys with flags
merely disables some potential optimisations. (The other way round - not
being set when keys have flags would be a bug).

This is a regression I introduced in Feb 2004 with commit d36773897a6f30fc:
    hv_clear_placeholders now manipulates the linked lists directly, rather
    than using the iterator interface and calling hv_delete
    This will allow hv_delete to be simplified to remove most of the
    special casing related to placeholders.

However several people have looked at the code since then and no-one has
realised that with the logic as-was, this call had to be unreachable.

Also avoid calling  HvPLACEHOLDERS_get() twice - each caller has already
done this, so pass the value in.

MANIFEST

@@ -5878,6 +5878,7 @@ t/op/grep.t			See if grep() and map() work
 t/op/groups.t			See if $( works
 t/op/gv.t			See if typeglobs work
 t/op/hash.t			See if the complexity attackers are repelled
+t/op/hash-clear-placeholders.t	FIXME
 t/op/hash-rt85026.t		See if hash iteration/deletion works
 t/op/hashassign.t		See if hash assignments work
 t/op/hashwarn.t			See if warnings for bad hash assignments work

hv.c

@@ -1882,14 +1882,14 @@ Perl_hv_clear_placeholders(pTHX_ HV *hv)
 }
 
 static void
-S_clear_placeholders(pTHX_ HV *hv, U32 items)
+S_clear_placeholders(pTHX_ HV *hv, const U32 placeholders)
 {
     I32 i;
+    U32 to_find = placeholders;
 
     PERL_ARGS_ASSERT_CLEAR_PLACEHOLDERS;
 
-    if (items == 0)
-        return;
+    assert(to_find);
 
     i = HvMAX(hv);
     do {
@@ -1909,12 +1909,10 @@ S_clear_placeholders(pTHX_ HV *hv, U32 items)
                     hv_free_ent(hv, entry);
                 }
 
-                if (--items == 0) {
+                if (--to_find == 0) {
                     /* Finished.  */
-                    I32 placeholders = HvPLACEHOLDERS_get(hv);
                     HvTOTALKEYS(hv) -= (IV)placeholders;
-                    /* HvUSEDKEYS expanded */
-                    if ((HvTOTALKEYS(hv) - placeholders) == 0)
+                    if (HvTOTALKEYS(hv) == 0)
                         HvHASKFLAGS_off(hv);
                     HvPLACEHOLDERS_set(hv, 0);
                     return;
@@ -1925,7 +1923,7 @@ S_clear_placeholders(pTHX_ HV *hv, U32 items)
         }
     } while (--i >= 0);
     /* You can't get here, hence assertion should always fail.  */
-    assert (items == 0);
+    assert (to_find == 0);
     NOT_REACHED; /* NOTREACHED */
 }
 
@@ -3343,7 +3341,6 @@ Perl_refcounted_he_chain_2hv(pTHX_ const struct refcounted_he *chain, U32 flags)
 
     if (placeholders) {
         clear_placeholders(hv, placeholders);
-        HvTOTALKEYS(hv) -= placeholders;
     }
 
     /* We could check in the loop to see if we encounter any keys with key

t/op/hash-clear-placeholders.t

@@ -0,0 +1,56 @@
+#!/usr/bin/perl -w
+
+BEGIN {
+    chdir 't' if -d 't';
+    require './test.pl';
+    set_up_inc( '../lib' );
+    skip_all_without_dynamic_extension("Devel::Peek");
+    skip_all_without_dynamic_extension("Hash::Util");
+}
+
+use strict;
+use Devel::Peek;
+use Hash::Util qw(lock_keys_plus);
+
+my %hash = (chr 255 => 42, chr 256 => 6 * 9);
+lock_keys_plus(%hash, "baz");
+
+my $tempfile = tempfile();
+
+local *OLDERR;
+open(OLDERR, ">&STDERR") || die "Can't dup STDERR: $!";
+open(STDERR, ">", $tempfile) ||
+    die "Could not open '$tempfile' for write: $^E";
+
+my $sep = "=-=-=-=\n";
+Dump \%hash;
+print STDERR $sep;
+delete $hash{chr 255};
+Internals::hv_clear_placeholders(%hash);
+Dump \%hash;
+print STDERR $sep;
+delete $hash{chr 256};
+Internals::hv_clear_placeholders(%hash);
+Dump \%hash;
+
+open(STDERR, ">&OLDERR") || die "Can't dup OLDERR: $!";
+open(my $fh, "<", $tempfile) ||
+    die "Could not open '$tempfile' for read: $^E";
+local $/;
+my $got = <$fh>;
+
+my ($first, $second, $third) = split $sep, $got;
+
+like($first, qr/\bPERL_MAGIC_rhash\b/, 'first dump has rhash magic');
+like($second, qr/\bPERL_MAGIC_rhash\b/, 'second dump has rhash magic');
+like($third, qr/\bPERL_MAGIC_rhash\b/, 'third dump has rhash magic');
+
+like($first, qr/\bHASKFLAGS\b/, 'first dump has HASHKFLAGS set');
+like($second, qr/\bHASKFLAGS\b/, 'second dump has HASHKFLAGS set');
+unlike($third, qr/\bHASKFLAGS\b/, 'third dump has HASHKFLAGS clear');
+
+like($first, qr/\bMG_LEN = 1\b/, 'first dump has 1 placeholder');
+unlike($second, qr/\bMG_LEN\b/, 'second dump has 0 placeholders');
+unlike($third, qr/\bMG_LEN\b/, 'third dump has 0 placeholders');
+
+done_testing();