introduce --param max-vartrack-expr-depth

[Alexandre Oliva <aoliva at redhat dot com>]

One of my patches for PR 48866 regressed guality/asm-1.c on
x86_64-linux-gnu because what used to be a single complex debug value
expression became a chain of debug temps holding simpler expressions,
and this chain exceeded the default recursion depth in resolving
location expressions.

This patch introduces a param to control this depth, so that one can
trade compile time for better debug info, and bumps up the default depth
a bit, avoiding the regression.

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

for  gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* params.def (PARAM_MAX_VARTRACK_EXPR_DEPTH): New.
	* doc/invoke.texi: Document max-vartrack-expr-depth.
	* var-tracking.c (EXPR_DEPTH): New.
	(reverse_op, vt_expand_loc, vt_expand_loc_dummy): Use it.

Index: gcc/params.def
===================================================================
--- gcc/params.def.orig	2011-05-30 03:16:26.496342009 -0300
+++ gcc/params.def	2011-05-30 03:35:17.852414343 -0300
@@ -839,6 +839,14 @@ DEFPARAM (PARAM_MAX_VARTRACK_SIZE,
 	  "Max. size of var tracking hash tables",
 	  50000000, 0, 0)
 
+/* Set maximum recursion depth for var tracking expression expansion
+   and resolution.  */
+
+DEFPARAM (PARAM_MAX_VARTRACK_EXPR_DEPTH,
+	  "max-vartrack-expr-depth",
+	  "Max. recursion depth for expanding var tracking expressions",
+	  10, 0, 0)
+
 /* Set minimum insn uid for non-debug insns.  */
 
 DEFPARAM (PARAM_MIN_NONDEBUG_INSN_UID,
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi.orig	2011-05-30 03:40:14.716854423 -0300
+++ gcc/doc/invoke.texi	2011-05-30 03:46:29.349288790 -0300
@@ -8866,6 +8866,16 @@ the function.  If the limit is exceeded 
 tracking analysis is completely disabled for the function.  Setting
 the parameter to zero makes it unlimited.
 
+@item max-vartrack-expr-depth
+Sets a maximum number of recursion levels when attempting to map
+variable names or debug temporaries to value expressions.  This trades
+compile time for more complete debug information.  If this is set too
+low, value expressions that are available and could be represented in
+debug information may end up not being used; setting this higher may
+enable the compiler to find more complex debug expressions, but compile
+time may grow exponentially, and even then, it may fail to find more
+usable expressions.  The default is 10.
+
 @item min-nondebug-insn-uid
 Use uids starting at this parameter for nondebug insns.  The range below
 the parameter is reserved exclusively for debug insns created by
Index: gcc/var-tracking.c
===================================================================
--- gcc/var-tracking.c.orig	2011-05-30 03:40:46.135898766 -0300
+++ gcc/var-tracking.c	2011-05-30 03:39:35.745798000 -0300
@@ -5215,6 +5215,8 @@ add_uses_1 (rtx *x, void *cui)
   for_each_rtx (x, add_uses, cui);
 }
 
+#define EXPR_DEPTH (PARAM_VALUE (PARAM_MAX_VARTRACK_EXPR_DEPTH))
+
 /* Attempt to reverse the EXPR operation in the debug info.  Say for
    reg1 = reg2 + 6 even when reg2 is no longer live we
    can express its value as VAL - 6.  */
@@ -5286,7 +5288,7 @@ reverse_op (rtx val, const_rtx expr)
       arg = XEXP (src, 1);
       if (!CONST_INT_P (arg) && GET_CODE (arg) != SYMBOL_REF)
 	{
-	  arg = cselib_expand_value_rtx (arg, scratch_regs, 5);
+	  arg = cselib_expand_value_rtx (arg, scratch_regs, EXPR_DEPTH);
 	  if (arg == NULL_RTX)
 	    return NULL_RTX;
 	  if (!CONST_INT_P (arg) && GET_CODE (arg) != SYMBOL_REF)
@@ -7416,7 +7418,7 @@ vt_expand_loc (rtx loc, htab_t vars, boo
   data.dummy = false;
   data.cur_loc_changed = false;
   data.ignore_cur_loc = ignore_cur_loc;
-  loc = cselib_expand_value_rtx_cb (loc, scratch_regs, 8,
+  loc = cselib_expand_value_rtx_cb (loc, scratch_regs, EXPR_DEPTH,
 				    vt_expand_loc_callback, &data);
 
   if (loc && MEM_P (loc))
@@ -7438,7 +7440,7 @@ vt_expand_loc_dummy (rtx loc, htab_t var
   data.dummy = true;
   data.cur_loc_changed = false;
   data.ignore_cur_loc = false;
-  ret = cselib_dummy_expand_value_rtx_cb (loc, scratch_regs, 8,
+  ret = cselib_dummy_expand_value_rtx_cb (loc, scratch_regs, EXPR_DEPTH,
 					  vt_expand_loc_callback, &data);
   *pcur_loc_changed = data.cur_loc_changed;
   return ret;


-- 
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 Brazil Compiler Engineer