Changes

This project involves extending gcc to support an alwayszero function attribute (see [https://bugzilla.mozilla.org/show_bug.cgi?id=517370 Bug #517370] as well as [http://ehren.wordpress.com/2009/09/30/popping-my-gimples-a-plan/ this blog post] for an overview).

== Project News Possible Bug == Recognizing the call flags  In tree-ssa-ccp.c: /* Compute a default value for variable VAR and store it in the CONST_VAL array. The following rules are used to get default values:  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_tget_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: %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; else 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; } else /* Any other variable defined by an assignment or a PHI node is considered UNDEFINED. */ val.lattice_val = UNDEFINED; } else { /* Otherwise, VAR will never take on a constant value. */ val.lattice_val = VARYING; }  return val;}  In tree-optimize.c: unsigned intexecute_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;}