[gcc r14-4739] rtl-ssa: Support inferring uses of mem in change_insns

[gcc r14-4739] rtl-ssa: Support inferring uses of mem in change_insns

[Alex Coplan]

https://gcc.gnu.org/g:505f1202e3a1a1aecd0df10d0f1620df6fea4ab5

commit r14-4739-g505f1202e3a1a1aecd0df10d0f1620df6fea4ab5
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Mon Oct 16 14:14:40 2023 +0000

    rtl-ssa: Support inferring uses of mem in change_insns
    
    Currently, rtl_ssa::change_insns requires all new uses and defs to be
    specified explicitly.  This turns out to be rather inconvenient for
    forming load pairs in the new aarch64 load pair pass, as the pass has to
    determine which mem def the final load pair consumes, and then obtain or
    create a suitable use (i.e. significant bookkeeping, just to keep the
    RTL-SSA IR consistent).  It turns out to be much more convenient to
    allow change_insns to infer which def is consumed and create a suitable
    use of mem itself.  This patch does that.
    
    gcc/ChangeLog:
    
            * rtl-ssa/changes.cc (function_info::finalize_new_accesses): Add new
            parameter to give final insn position, infer use of mem if it isn't
            specified explicitly.
            (function_info::change_insns): Pass down final insn position to
            finalize_new_accesses.
            * rtl-ssa/functions.h: Add parameter to finalize_new_accesses.

Diff:
---
 gcc/rtl-ssa/changes.cc  | 31 ++++++++++++++++++++++++++++---
 gcc/rtl-ssa/functions.h |  2 +-
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index c48ddd2463cd..523ad60d7d89 100644
--- a/gcc/rtl-ssa/changes.cc
+++ b/gcc/rtl-ssa/changes.cc
@@ -370,8 +370,11 @@ update_insn_in_place (insn_change &change)
 // Finalize the new list of definitions and uses in CHANGE, removing
 // any uses and definitions that are no longer needed, and converting
 // pending clobbers into actual definitions.
+//
+// POS gives the final position of INSN, which hasn't yet been moved into
+// place.
 void
-function_info::finalize_new_accesses (insn_change &change)
+function_info::finalize_new_accesses (insn_change &change, insn_info *pos)
 {
   insn_info *insn = change.insn ();
 
@@ -462,13 +465,34 @@ function_info::finalize_new_accesses (insn_change &change)
   // Add (possibly temporary) uses to m_temp_uses for each resource.
   // If there are multiple references to the same resource, aggregate
   // information in the modes and flags.
+  use_info *mem_use = nullptr;
   for (rtx_obj_reference ref : properties.refs ())
     if (ref.is_read ())
       {
 	unsigned int regno = ref.regno;
 	machine_mode mode = ref.is_reg () ? ref.mode : BLKmode;
 	use_info *use = find_access (unshared_uses, ref.regno);
-	gcc_assert (use);
+	if (!use)
+	  {
+	    // For now, we only support inferring uses of mem.
+	    gcc_assert (regno == MEM_REGNO);
+
+	    if (mem_use)
+	      {
+		mem_use->record_reference (ref, false);
+		continue;
+	      }
+
+	    resource_info resource { mode, regno };
+	    auto def = find_def (resource, pos).prev_def (pos);
+	    auto set = safe_dyn_cast <set_info *> (def);
+	    gcc_assert (set);
+	    mem_use = allocate<use_info> (insn, resource, set);
+	    mem_use->record_reference (ref, true);
+	    m_temp_uses.safe_push (mem_use);
+	    continue;
+	  }
+
 	if (use->m_has_been_superceded)
 	  {
 	    // This is the first reference to the resource.
@@ -656,7 +680,8 @@ function_info::change_insns (array_slice<insn_change *> changes)
 
 	  // Finalize the new list of accesses for the change.  Don't install
 	  // them yet, so that we still have access to the old lists below.
-	  finalize_new_accesses (change);
+	  finalize_new_accesses (change,
+				 placeholder ? placeholder : insn);
 	}
       placeholders[i] = placeholder;
     }
diff --git a/gcc/rtl-ssa/functions.h b/gcc/rtl-ssa/functions.h
index d7da9774213c..73690a0e63b4 100644
--- a/gcc/rtl-ssa/functions.h
+++ b/gcc/rtl-ssa/functions.h
@@ -265,7 +265,7 @@ private:
 
   insn_info *add_placeholder_after (insn_info *);
   void possibly_queue_changes (insn_change &);
-  void finalize_new_accesses (insn_change &);
+  void finalize_new_accesses (insn_change &, insn_info *);
   void apply_changes_to_insn (insn_change &);
 
   void init_function_data ();