[PR58315] reset inlined debug vars at return-to point

[Alexandre Oliva <aoliva@redhat.com>]

This patch fixes a problem that has been with us for several years.
Variable tracking has no idea about the end of the lifetime of inlined
variables, so it keeps on computing locations for them over and over,
even though the computed locations make no sense whatsoever because the
variable can't even be accessed any more.

With this patch, we unbind all inlined variables at the point the
inlined function returns to, so that the locations for those variables
will not be touched any further.

In theory, we could do something similar to non-inlined auto variables,
when they go out of scope, but their decls apply to the entire function
and I'm told gdb sort-of expects the variables to be accessible
throughout the function, so I'm not tackling that in this patch, for I'm
happy enough with what this patch gets us:

- almost 99% reduction in the output asm for the PR testcase

- more than 90% reduction in the peak memory use compiling that testcase

- 63% reduction in the compile time for that testcase

What's scary is that the testcase is not particularly pathological.  Any
function that calls a longish sequence of inlined functions, that in
turn call other inline functions, and so on, something that's not
particularly unusual in C++, will likely observe significant
improvement, as we won't see growing sequences of var_location notes
after each call or so, as var-tracking computes a new in-stack location
for the implicit this argument of each previously-inlined function.

Regstrapped on x86_64-linux-gnu and i686-linux-gnu.  Ok to install?


Reset inlined debug variables at the end of the inlined function

From: Alexandre Oliva <aoliva@redhat.com>

for  gcc/ChangeLog

	PR debug/58315
	* tree-inline.c (reset_debug_binding): New.
	(reset_debug_bindings): Likewise.
	(expand_call_inline): Call it.
---
 gcc/tree-inline.c |   56 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/gcc/tree-inline.c b/gcc/tree-inline.c
index d8abe03..5b58d8b 100644
--- a/gcc/tree-inline.c
+++ b/gcc/tree-inline.c
@@ -4345,6 +4345,60 @@ add_local_variables (struct function *callee, struct function *caller,
       }
 }
 
+/* Add to BINDINGS a debug stmt resetting SRCVAR if inlining might
+   have brought in or introduced any debug stmts for SRCVAR.  */
+
+static inline void
+reset_debug_binding (copy_body_data *id, tree srcvar, gimple_seq *bindings)
+{
+  tree *remappedvarp = id->decl_map->get (srcvar);
+
+  if (!remappedvarp)
+    return;
+
+  if (TREE_CODE (*remappedvarp) != VAR_DECL)
+    return;
+
+  if (*remappedvarp == id->retvar || *remappedvarp == id->retbnd)
+    return;
+
+  tree tvar = target_for_debug_bind (*remappedvarp);
+  if (!tvar)
+    return;
+
+  gdebug *stmt = gimple_build_debug_bind (tvar, NULL_TREE,
+					  id->call_stmt);
+  gimple_seq_add_stmt (bindings, stmt);
+}
+
+/* For each inlined variable for which we may have debug bind stmts,
+   add before GSI a final debug stmt resetting it, marking the end of
+   its life, so that var-tracking knows it doesn't have to compute
+   further locations for it.  */
+
+static inline void
+reset_debug_bindings (copy_body_data *id, gimple_stmt_iterator gsi)
+{
+  tree var;
+  unsigned ix;
+  gimple_seq bindings = NULL;
+
+  if (!gimple_in_ssa_p (id->src_cfun))
+    return;
+
+  if (!opt_for_fn (id->dst_fn, flag_var_tracking_assignments))
+    return;
+
+  for (var = DECL_ARGUMENTS (id->src_fn);
+       var; var = DECL_CHAIN (var))
+    reset_debug_binding (id, var, &bindings);
+
+  FOR_EACH_LOCAL_DECL (id->src_cfun, ix, var)
+    reset_debug_binding (id, var, &bindings);
+
+  gsi_insert_seq_before_without_update (&gsi, bindings, GSI_SAME_STMT);
+}
+
 /* If STMT is a GIMPLE_CALL, replace it with its inline expansion.  */
 
 static bool
@@ -4650,6 +4704,8 @@ expand_call_inline (basic_block bb, gimple stmt, copy_body_data *id)
   	     GCOV_COMPUTE_SCALE (cg_edge->frequency, CGRAPH_FREQ_BASE),
 	     bb, return_block, NULL);
 
+  reset_debug_bindings (id, stmt_gsi);
+
   /* Reset the escaped solution.  */
   if (cfun->gimple_df)
     pt_solution_reset (&cfun->gimple_df->escaped);


-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer