GCC alwayszero function attribute

From CDOT Wiki
Jump to: navigation, search

Project Name

GCC alwayszero function attribute

Project Description

This project involves extending gcc to support an attribute for functions which always return zero (see Bug #517370). This attribute rectifies the fact that GCC cannot determine the results of a call to a virtual member function at compile time (for example).

Project Leader(s)

Ehren Metcalfe

Project Contributor(s)

Project Details

Relevant blog posts

10: Reworking my analysis

9: Firefox build issues with GCC 4.5

8: Analysis and type madness

7: i686 vs x86-64 issues

6: Creating a GCC optimization plugin

5: A GCC Hack (My 0.1 Release)

4: A Gimple Call Flags Mystery

3: Adding a new function attribute to GCC

2: Popping My GIMPLES (a plan)

1: Building GCC from trunk

Breakdown of Releases


This involved getting acquainted with GCC as well as my initial plan (posts 1-2).


A new attribute and a proof of concept optimization were hacked into GCC (posts 3-5).


The optimization was developed as a separate compiled plugin (post 6). A number of refinements were made to the plugin. Post 7 contains a critical bug fix. Post 8 addresses a minor type safety issue. A Dehydra analysis was introduced in post 8 for identifying alwayszero functions.


A preliminary Treehydra analysis for alwayszero functions based on outparams.js was introduced in post 10. Post 9 details alternative steps required to run the plugin on mozilla-central.

Note: I'd be comfortable including the last two posts in either 0.2 or 0.3. If my 'release style', to be generous, for 0.2 is found lacking, they should perhaps count for 0.2.

Possible Bug

Calling gimple_call_flags on a gimple stmt representing an indirect call to a virtual method (with a function attribute) always returns 0. This is even though the correct nonzero value of the call flags can be recognized from other parts of gcc.

For example:

tree-ssa-ccp.c modified with a debugging statement:

/* Compute a default value for variable VAR and store it in the
   CONST_VAL array.  The following rules are used to get default

   1- Global and static variables that are declared constant are
      considered CONSTANT.

   2- Any other value is considered UNDEFINED.  This is useful when
      considering PHI nodes.  PHI arguments that are undefined do not
      change the constant value of the PHI node, which allows for more
      constants to be propagated.

   3- Variables defined by statements other than assignments and PHI
      nodes are considered VARYING.

   4- Initial values of variables that are not GIMPLE registers are
      considered VARYING.  */

static prop_value_t
get_default_value (tree var)
  tree sym = SSA_NAME_VAR (var);
  prop_value_t val = { UNINITIALIZED, NULL_TREE };
  gimple stmt;

  stmt = SSA_NAME_DEF_STMT (var);

  /* My debugging statment */

  if (is_gimple_call (stmt))
    fprintf(stderr, "Call flags from tree-ssa-ccp.c: %d\n",
        gimple_call_flags (stmt));

  /* End My Debugging Statement */

  if (gimple_nop_p (stmt))
      /* Variables defined by an empty statement are those used
   before being initialized.  If VAR is a local variable, we
   can assume initially that it is UNDEFINED, otherwise we must
   consider it VARYING.  */
      if (is_gimple_reg (sym) && TREE_CODE (sym) != PARM_DECL)
  val.lattice_val = UNDEFINED;
  val.lattice_val = VARYING;
  else if (is_gimple_assign (stmt)
     /* Value-returning GIMPLE_CALL statements assign to
        a variable, and are treated similarly to GIMPLE_ASSIGN.  */
     || (is_gimple_call (stmt)
         && gimple_call_lhs (stmt) != NULL_TREE)
     || gimple_code (stmt) == GIMPLE_PHI)
      tree cst;
      if (gimple_assign_single_p (stmt)
    && DECL_P (gimple_assign_rhs1 (stmt))
    && (cst = get_symbol_constant_value (gimple_assign_rhs1 (stmt))))
    val.lattice_val = CONSTANT;
    val.value = cst;
  /* Any other variable defined by an assignment or a PHI node
     is considered UNDEFINED.  */
  val.lattice_val = UNDEFINED;
      /* Otherwise, VAR will never take on a constant value.  */
      val.lattice_val = VARYING;

  return val;

tree-optimize.c modified with a debugging statement:

unsigned int
execute_fixup_cfg (void)
  basic_block bb;
  gimple_stmt_iterator gsi;
  int todo = gimple_in_ssa_p (cfun) ? TODO_verify_ssa : 0;

  if (cfun->eh)
    FOR_EACH_BB (bb)
  for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
      gimple stmt = gsi_stmt (gsi);
      tree decl = is_gimple_call (stmt)
                  ? gimple_call_fndecl (stmt)
      : NULL;

      /* My debugging statement */
      if (decl)
        fprintf(stderr, "Call flags from tree-optimize.c: %d\n",
            gimple_call_flags (stmt));
      /* End my debugging statement */

      if (decl
    && gimple_call_flags (stmt) & (ECF_CONST
                 | ECF_PURE 
                 | ECF_LOOPING_CONST_OR_PURE))
    if (gimple_in_ssa_p (cfun))
        todo |= TODO_update_ssa | TODO_cleanup_cfg;
        mark_symbols_for_renaming (stmt);
              update_stmt (stmt);

      maybe_clean_eh_stmt (stmt);

  if (gimple_purge_dead_eh_edges (bb))
          todo |= TODO_cleanup_cfg;

  /* Dump a textual representation of the flowgraph.  */
  if (dump_file)
    gimple_dump_cfg (dump_file, dump_flags);

  return todo;

Sample program:

/* blah.h */
class Blah {
  virtual int blah() __attribute__((nothrow));

/* blah.cpp */
#include "blah.h"

int Blah::blah()
  return 0;

/* main.cpp */
#include "blah.h"
int main()
  Blah b;
  Blah* p = &b;
  int x;
  x = p->blah();
  return 0;

Compiling sample program with gcc trunk build containing above debugging statements:

$ ~/gcc/dist/bin/g++ blah.cpp main.cpp -O2
Call flags from tree-optimize.c: 64
Call flags from tree-ssa-ccp.c: 0
Call flags from tree-optimize.c: 64
Call flags from tree-optimize.c: 64