[Bug tree-optimization/85757] tree optimizers fail to fully clean up fixed-size memcpy

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85757

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ontop of recent patches:

diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index 32a25b9eb1e..1b672ad204a 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -577,6 +577,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
       else
        defvar = gimple_vdef (temp);
       auto_vec<gimple *, 10> defs;
+      gimple *phi_def = NULL;
       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
        {
          /* Limit stmt walking.  */
@@ -600,7 +601,10 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
                 processing.  */
              if (!bitmap_bit_p (visited,
                                 SSA_NAME_VERSION (PHI_RESULT (use_stmt))))
-               defs.safe_push (use_stmt);
+               {
+                 defs.safe_push (use_stmt);
+                 phi_def = use_stmt;
+               }
            }
          /* If the statement is a use the store is not dead.  */
          else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
@@ -660,12 +671,25 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
       /* Process defs and remove paths starting with a kill from further
          processing.  */
       for (unsigned i = 0; i < defs.length (); ++i)
-       if (stmt_kills_ref_p (defs[i], ref))
-         {
-           if (by_clobber_p && !gimple_clobber_p (defs[i]))
-             *by_clobber_p = false;
+       {
+         gimple *def = defs[i];
+         gimple *use_stmt;
+         use_operand_p use_p;
+         if (stmt_kills_ref_p (def, ref))
+           {
+             if (by_clobber_p && !gimple_clobber_p (def))
+               *by_clobber_p = false;
+             defs.unordered_remove (i);
+           }
+         /* In addition to kills we can remove defs whose only use
+            is another def in defs.  That can only ever be PHIs of which
+            we track a single for simplicity reasons (we fail for multiple
+            PHIs anyways).  */
+         else if (gimple_code (def) != GIMPLE_PHI
+                  && single_imm_use (gimple_vdef (def), &use_p, &use_stmt)
+                  && use_stmt == phi_def)
            defs.unordered_remove (i);
-         }
+       }

       /* If all defs kill the ref we are done.  */
       if (defs.is_empty ())