[gcc r13-5652] sched-deps, cselib: Fix up some -fcompare-debug issues and regressions [PR108463]

[gcc r13-5652] sched-deps, cselib: Fix up some -fcompare-debug issues and regressions [PR108463]

[Jakub Jelinek]

https://gcc.gnu.org/g:465a9c51e7d5bafa7a81195b5af20f2a54f22210

commit r13-5652-g465a9c51e7d5bafa7a81195b5af20f2a54f22210
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Feb 2 13:52:45 2023 +0100

    sched-deps, cselib: Fix up some -fcompare-debug issues and regressions [PR108463]
    
    On Sat, Jan 14, 2023 at 08:26:00AM -0300, Alexandre Oliva via Gcc-patches wrote:
    > The testcase used to get scheduled differently depending on the
    > presence of debug insns with MEMs.  It's not clear to me why those
    > MEMs affected scheduling, but the cselib pre-canonicalization of the
    > MEM address is not used at all when analyzing debug insns, so the
    > memory allocation and lookup are pure waste.  Somehow, avoiding that
    > waste fixes the problem, or makes it go latent.
    
    Unfortunately, this patch breaks the following testcase.
    The code in sched_analyze_2 did 2 things:
    1) cselib_lookup_from_insn
    2) shallow_copy_rtx + cselib_subst_to_values_from_insn
    Now, 1) is precondition of 2), we can only subst the VALUEs if we
    have actually looked the address up, but as can be seen on that testcase,
    we are relying on at least the 1) to be done because we subst the values
    later on even on DEBUG_INSNs and actually use those when needed.
    cselib_subst_to_values_from_insn mostly just replaces stuff in the
    returned rtx, except for:
          /* This used to happen for autoincrements, but we deal with them
             properly now.  Remove the if stmt for the next release.  */
          if (! e)
            {
              /* Assign a value that doesn't match any other.  */
              e = new_cselib_val (next_uid, GET_MODE (x), x);
            }
    which is like that since 2011, I hope it is never reachable and we should
    in stage1 replace that with gcc_assert or just remove (then it will
    segfault on following
          return e->val_rtx;
    ).
    
    So, I (as done in the patch below) reinstalled the 1) and not 2) for
    DEBUG_INSNs.  This fixed the new testcase, but broke again the PR106746
    testcases.
    
    I've spent a day debugging that and found the problem is that as documented
    in a large comment in cselib.cc above n_useless_values variable definition,
    we spend quite a few effort on making sure that VALUEs created on
    DEBUG_INSNs don't affect the cselib decisions for non-DEBUG_INSNs such as
    pruning of useless values etc., but if a VALUE created that way is then
    looked up/needed from non-DEBUG_INSNs, we promote it to non-debug.
    
    The reason for -fcompare-debug failure is that there is one large DEBUG_INSN
    with 16 MEMs in it mostly with addresses that so far didn't appear in the IL
    otherwise.  Later on, we see an instruction storing into MEM destination
    and invalidate that MEM.  Unfortunately, there is a bug caused by the
    introduction of SP_DERIVED_VALUE_P where alias.cc isn't able to disambiguate
    MEMs with sp + optional offset in address vs. MEMs with address being a
    VALUE having SP_DERIVED_VALUE_P + constant (or the SP_DERIVED_VALUE_P
    itself), which ought to be possible when REG_VALUES (REGNO
    (stack_pointer_rtx)) has SP_DERIVED_VALUE_P + constant location.  Not sure
    if I should try to fix that in stage4 or defer for stage1.
    Anyway, the cselib_invalidate_mem call because of this invalidates basically
    all MEMs with the exception of 5 which have MEM_EXPRs that guarantee
    non-aliasing with the sp based store.
    Unfortunately, n_useless_values which in my understanding should be always
    the same between -g and -g0 compilations diverges, has 3 more useless values
    for -g.
    
    Now, these were initially VALUEs created for DEBUG_INSN lookups.  As I said,
    cselib.cc has code to promote such VALUEs (well, their location elements) to
    non-debug if they are looked up from non-DEBUG_INSNs.  The problem is that
    when looking some completely unrelated MEM from a non-DEBUG_INSN we run into
    a hash collision and so call cselib_hasher::equal to check if the unrelated
    MEM is equal to the one from DEBUG_INSN only element.  The equal static
    member function calls rtx_equal_for_cselib_1 and if that returns true,
    promotes the location to non-DEBUG, otherwise returns false.  So far so
    good.  But rtx_equal_for_cselib_1 internally performs various other cselib
    lookups, all done with the non-DEBUG_INSN cselib_current_insn, so they
    all promote to non-debug.  And that is wrong, because if it was -g0
    compilation, such hashtable entry wouldn't be there at all (or would be
    but wouldn't contain that locs element), so with -g0 we wouldn't call
    that rtx_equal_for_cselib_1 at all.  So, I think we need to pretend
    that such lookup which only happens with -g and not -g0 actually comes
    from some DEBUG_INSN (note, the lookups rtx_equal_for_cselib_1 does
    are always with create = 0).
    The cselib.cc part of the patch does that.
    
    BTW, I'm not really sure how:
              if (num_mems < param_max_cselib_memory_locations
                  && ! canon_anti_dependence (x, false, mem_rtx,
                                              GET_MODE (mem_rtx), mem_addr))
                {
                  has_mem = true;
                  num_mems++;
                  p = &(*p)->next;
                  continue;
                }
    num_mems cap can actually work correctly for -fcompare-debug,
    I'd think we would need to differentiate between num_debug_mems and
    num_mems depending on if setting_insn is non-NULL DEBUG_INSN or not.
    That was one of my suspicions on this testcase, but the number of MEMs
    was small enough for the param in either case (especially because of
    the above mentioned missed non-aliasings).  But as implemented, I think
    if we have tons of non-aliased MEMs from DEBUG_INSN setting_insns,
    we could unchain lots more non-DEBUG MEMs with -g than with -g0.
    
    2023-02-02  Jakub Jelinek  <jakub@redhat.com>
    
            PR debug/106746
            PR rtl-optimization/108463
            PR target/108484
            * cselib.cc (cselib_current_insn): Move declaration earlier.
            (cselib_hasher::equal): For debug only locs, temporarily override
            cselib_current_insn to their l->setting_insn for the
            rtx_equal_for_cselib_1 call, so that unsuccessful comparisons don't
            promote some debug locs.
            * sched-deps.cc (sched_analyze_2) <case MEM>: For MEMs in DEBUG_INSNs
            when using cselib call cselib_lookup_from_insn on the address but
            don't substitute it.
    
            * gcc.dg/pr108463.c: New test.

Diff:
---
 gcc/cselib.cc                   | 28 +++++++++++++++++++++-------
 gcc/sched-deps.cc               |  9 ++++++++-
 gcc/testsuite/gcc.dg/pr108463.c | 13 +++++++++++++
 3 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/gcc/cselib.cc b/gcc/cselib.cc
index 4a9b8d9d48d..065867b4a84 100644
--- a/gcc/cselib.cc
+++ b/gcc/cselib.cc
@@ -80,6 +80,10 @@ struct expand_value_data
 
 static rtx cselib_expand_value_rtx_1 (rtx, struct expand_value_data *, int);
 
+/* This is a global so we don't have to pass this through every function.
+   It is used in new_elt_loc_list to set SETTING_INSN.  */
+static rtx_insn *cselib_current_insn;
+
 /* There are three ways in which cselib can look up an rtx:
    - for a REG, the reg_values table (which is indexed by regno) is used
    - for a MEM, we recursively look up its address and then follow the
@@ -143,11 +147,25 @@ cselib_hasher::equal (const cselib_val *v, const key *x_arg)
   /* We don't guarantee that distinct rtx's have different hash values,
      so we need to do a comparison.  */
   for (l = v->locs; l; l = l->next)
-    if (rtx_equal_for_cselib_1 (l->loc, x, memmode, 0))
+    if (l->setting_insn && DEBUG_INSN_P (l->setting_insn)
+	&& (!cselib_current_insn || !DEBUG_INSN_P (cselib_current_insn)))
       {
-	promote_debug_loc (l);
-	return true;
+	rtx_insn *save_cselib_current_insn = cselib_current_insn;
+	/* If l is so far a debug only loc, without debug stmts it
+	   would never be compared to x at all, so temporarily pretend
+	   current instruction is that DEBUG_INSN so that we don't
+	   promote other debug locs even for unsuccessful comparison.  */
+	cselib_current_insn = l->setting_insn;
+	bool match = rtx_equal_for_cselib_1 (l->loc, x, memmode, 0);
+	cselib_current_insn = save_cselib_current_insn;
+	if (match)
+	  {
+	    promote_debug_loc (l);
+	    return true;
+	  }
       }
+    else if (rtx_equal_for_cselib_1 (l->loc, x, memmode, 0))
+      return true;
 
   return false;
 }
@@ -158,10 +176,6 @@ static hash_table<cselib_hasher> *cselib_hash_table;
 /* A table to hold preserved values.  */
 static hash_table<cselib_hasher> *cselib_preserved_hash_table;
 
-/* This is a global so we don't have to pass this through every function.
-   It is used in new_elt_loc_list to set SETTING_INSN.  */
-static rtx_insn *cselib_current_insn;
-
 /* The unique id that the next create value will take.  */
 static unsigned int next_uid;
 
diff --git a/gcc/sched-deps.cc b/gcc/sched-deps.cc
index a9214f67432..2aa6623ad2e 100644
--- a/gcc/sched-deps.cc
+++ b/gcc/sched-deps.cc
@@ -2605,7 +2605,14 @@ sched_analyze_2 (class deps_desc *deps, rtx x, rtx_insn *insn)
 
     case MEM:
       {
-	if (!DEBUG_INSN_P (insn))
+	if (DEBUG_INSN_P (insn) && sched_deps_info->use_cselib)
+	  {
+	    machine_mode address_mode = get_address_mode (x);
+
+	    cselib_lookup_from_insn (XEXP (x, 0), address_mode, 1,
+				     GET_MODE (x), insn);
+	  }
+	else if (!DEBUG_INSN_P (insn))
 	  {
 	    /* Reading memory.  */
 	    rtx_insn_list *u;
diff --git a/gcc/testsuite/gcc.dg/pr108463.c b/gcc/testsuite/gcc.dg/pr108463.c
new file mode 100644
index 00000000000..6e94e7daed9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr108463.c
@@ -0,0 +1,13 @@
+/* PR rtl-optimization/108463 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fsched2-use-superblocks -fcompare-debug -Wno-psabi" } */
+
+typedef int __attribute__((__vector_size__ (32))) V;
+int a;
+
+void
+foo (V v)
+{
+  a--;
+  v = (V) v;
+}