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.

2 files changed, 29 insertions, 4 deletions

diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index c48ddd2..523ad60 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 d7da977..73690a0 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 ();