[Bug tree-optimization/80032] [6/7 Regression] C++ excessive stack usage (no stack reuse)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80032

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
The patch causes some optimization regressions.

A different approach is to put the clobbers in non-conditional context -
something that is valid but with the clobber semantic cannot be done by the
middle-end.  Instead we can do this at gimplification time with, say,

Index: gcc/gimplify.c
===================================================================
--- gcc/gimplify.c      (revision 246216)
+++ gcc/gimplify.c      (working copy)
@@ -6288,10 +6288,13 @@ gimplify_cleanup_point_expr (tree *expr_

 /* Insert a cleanup marker for gimplify_cleanup_point_expr.  CLEANUP
    is the cleanup action required.  EH_ONLY is true if the cleanup should
-   only be executed if an exception is thrown, not on normal exit.  */
+   only be executed if an exception is thrown, not on normal exit.
+   If FORCE_UNCOND is true perform the cleanup unconditionally;  this is
+   only valid for clobbers.  */

 static void
-gimple_push_cleanup (tree var, tree cleanup, bool eh_only, gimple_seq *pre_p)
+gimple_push_cleanup (tree var, tree cleanup, bool eh_only, gimple_seq *pre_p,
+                    bool force_uncond = false)
 {
   gimple *wce;
   gimple_seq cleanup_stmts = NULL;
@@ -6301,7 +6304,8 @@ gimple_push_cleanup (tree var, tree clea
   if (seen_error ())
     return;

-  if (gimple_conditional_context ())
+  if (gimple_conditional_context ()
+      && ! force_uncond)
     {
       /* If we're in a conditional context, this is more complex.  We only
         want to run the cleanup if we actually ran the initialization that
@@ -6426,11 +6430,7 @@ gimplify_target_expr (tree *expr_p, gimp
                                                NULL);
              TREE_THIS_VOLATILE (clobber) = true;
              clobber = build2 (MODIFY_EXPR, TREE_TYPE (temp), temp, clobber);
-             if (cleanup)
-               cleanup = build2 (COMPOUND_EXPR, void_type_node, cleanup,
-                                 clobber);
-             else
-               cleanup = clobber;
+             gimple_push_cleanup (temp, clobber, false, pre_p, true);
            }
          if (asan_poisoned_variables && dbg_cnt (asan_use_after_scope))
            {

which brings down stack usage to GCC 5 levels (so is even better than the
other patch).