X-Git-Url: http://nsz.repo.hu/git/?a=blobdiff_plain;f=ir%2Fir%2Firopt.c;h=b9a1246f9e858cfddfdf88bc5f5075ed980e5038;hb=428ca393b64f346987ae1abf54017cdbff4a2605;hp=a31b138e9cda5f00911c40f50d8727e5128348e3;hpb=eb69bf6043d29071581499f9be23a4b59fb61214;p=libfirm diff --git a/ir/ir/iropt.c b/ir/ir/iropt.c index a31b138e9..b9a1246f9 100644 --- a/ir/ir/iropt.c +++ b/ir/ir/iropt.c @@ -53,16 +53,22 @@ value_of (ir_node *n) return tarval_bad; } +/** + * return the value of a Constant + */ static tarval *computed_value_Const(ir_node *n) { return get_Const_tarval(n); } +/** + * return the value of a 'sizeof' SymConst + */ static tarval *computed_value_SymConst(ir_node *n) { - if ((get_SymConst_kind(n) ==symconst_size) && + if ((get_SymConst_kind(n) == symconst_size) && (get_type_state(get_SymConst_type(n))) == layout_fixed) - return new_tarval_from_long(get_type_size_bytes(get_SymConst_type(n)), mode_Is); + return new_tarval_from_long(get_type_size_bytes(get_SymConst_type(n)), get_irn_mode(n)); return tarval_bad; } @@ -207,8 +213,8 @@ static tarval *computed_value_And(ir_node *n) } else { tarval *v; - if ( (tarval_classify ((v = computed_value (a))) == TV_CLASSIFY_NULL) - || (tarval_classify ((v = computed_value (b))) == TV_CLASSIFY_NULL)) { + if ( (classify_tarval ((v = computed_value (a))) == TV_CLASSIFY_NULL) + || (classify_tarval ((v = computed_value (b))) == TV_CLASSIFY_NULL)) { return v; } } @@ -227,8 +233,8 @@ static tarval *computed_value_Or(ir_node *n) return tarval_or (ta, tb); } else { tarval *v; - if ( (tarval_classify ((v = computed_value (a))) == TV_CLASSIFY_ALL_ONE) - || (tarval_classify ((v = computed_value (b))) == TV_CLASSIFY_ALL_ONE)) { + if ( (classify_tarval ((v = computed_value (a))) == TV_CLASSIFY_ALL_ONE) + || (classify_tarval ((v = computed_value (b))) == TV_CLASSIFY_ALL_ONE)) { return v; } } @@ -311,7 +317,7 @@ static tarval *computed_value_Rot(ir_node *n) tarval *tb = value_of(b); if ((ta != tarval_bad) && (tb != tarval_bad)) { - /* return tarval_rot (ta, tb); */ + return tarval_rot (ta, tb); } return tarval_bad; } @@ -346,12 +352,13 @@ static tarval *computed_value_Proj(ir_node *n) 3. The predecessors are Allocs or void* constants. Allocs never return NULL, they raise an exception. Therefore we can predict the Cmp result. */ - if (get_irn_op(a) == op_Cmp) { + switch (get_irn_opcode(a)) { + case iro_Cmp: aa = get_Cmp_left(a); ab = get_Cmp_right(a); if (aa == ab) { /* 1.: */ - /* This is a tric with the bits used for encoding the Cmp + /* This is a trick with the bits used for encoding the Cmp Proj numbers, the following statement is not the same: return new_tarval_from_long ((get_Proj_proj(n) == Eq), mode_b) */ return new_tarval_from_long ((get_Proj_proj(n) & Eq), mode_b); @@ -366,8 +373,8 @@ static tarval *computed_value_Proj(ir_node *n) return new_tarval_from_long (get_Proj_proj(n) & flags, mode_b); } } else { /* check for 3.: */ - ir_node *aaa = skip_nop(skip_Proj(aa)); - ir_node *aba = skip_nop(skip_Proj(ab)); + ir_node *aaa = skip_Id(skip_Proj(aa)); + ir_node *aba = skip_Id(skip_Proj(ab)); if ( ( (/* aa is ProjP and aaa is Alloc */ (get_irn_op(aa) == op_Proj) @@ -393,18 +400,54 @@ static tarval *computed_value_Proj(ir_node *n) return new_tarval_from_long (get_Proj_proj(n) & Ne, mode_b); } } - } else if (get_irn_op(a) == op_DivMod) { + break; + + case iro_DivMod: + { tarval *tb = value_of(b = get_DivMod_right(a)); tarval *ta = value_of(a = get_DivMod_left(a)); if ((ta != tarval_bad) && (tb != tarval_bad) && (get_irn_mode(a) == get_irn_mode(b))) { if (tb == get_mode_null(get_tarval_mode(tb))) /* div by zero: return tarval_bad */ return tarval_bad; - if (get_Proj_proj(n)== 0) /* Div */ + if (get_Proj_proj(n)== pn_DivMod_res_div) return tarval_div(ta, tb); - else /* Mod */ + else if (get_Proj_proj(n)== pn_DivMod_res_mod) return tarval_mod(ta, tb); } + break; + } + + case iro_Div: + { + tarval *tb = value_of(b = get_Div_right(a)); + tarval *ta = value_of(a = get_Div_left(a)); + + if ((ta != tarval_bad) && (tb != tarval_bad) && (get_irn_mode(a) == get_irn_mode(b))) { + if (tb == get_mode_null(get_tarval_mode(tb))) /* div by zero: return tarval_bad */ + return tarval_bad; + if (get_Proj_proj(n)== pn_Div_res) + return tarval_div(ta, tb); + } + break; + } + + case iro_Mod: + { + tarval *tb = value_of(b = get_Mod_right(a)); + tarval *ta = value_of(a = get_Mod_left(a)); + + if ((ta != tarval_bad) && (tb != tarval_bad) && (get_irn_mode(a) == get_irn_mode(b))) { + if (tb == get_mode_null(get_tarval_mode(tb))) /* div by zero: return tarval_bad */ + return tarval_bad; + if (get_Proj_proj(n)== pn_Mod_res) + return tarval_mod(ta, tb); + } + break; + } + + default: + return tarval_bad; } return tarval_bad; } @@ -412,6 +455,8 @@ static tarval *computed_value_Proj(ir_node *n) /** * If the parameter n can be computed, return its value, else tarval_bad. * Performs constant folding. + * + * @param n The node this should be evaluated */ tarval *computed_value(ir_node *n) { @@ -482,7 +527,7 @@ static ir_node *equivalent_node_Block(ir_node *n) { ir_node *oldn = n; - /* The Block constructor does not call optimize, but mature_block + /* The Block constructor does not call optimize, but mature_immBlock calls the optimization. */ assert(get_Block_matured(n)); @@ -494,7 +539,7 @@ static ir_node *equivalent_node_Block(ir_node *n) Remaining Phi nodes are just Ids. */ if ((get_Block_n_cfgpreds(n) == 1) && (get_irn_op(get_Block_cfgpred(n, 0)) == op_Jmp)) { - ir_node *predblock = get_nodes_Block(get_Block_cfgpred(n, 0)); + ir_node *predblock = get_nodes_block(get_Block_cfgpred(n, 0)); if (predblock == oldn) { /* Jmp jumps into the block it is in -- deal self cycle. */ n = new_Bad(); DBG_OPT_DEAD; @@ -504,7 +549,7 @@ static ir_node *equivalent_node_Block(ir_node *n) } else if ((get_Block_n_cfgpreds(n) == 1) && (get_irn_op(skip_Proj(get_Block_cfgpred(n, 0))) == op_Cond)) { - ir_node *predblock = get_nodes_Block(get_Block_cfgpred(n, 0)); + ir_node *predblock = get_nodes_block(get_Block_cfgpred(n, 0)); if (predblock == oldn) { /* Jmp jumps into the block it is in -- deal self cycle. */ n = new_Bad(); DBG_OPT_DEAD; @@ -524,7 +569,7 @@ static ir_node *equivalent_node_Block(ir_node *n) (get_irn_mode(get_Cond_selector(get_Proj_pred(a))) == mode_b)) { /* Also a single entry Block following a single exit Block. Phis have twice the same operand and will be optimized away. */ - n = get_nodes_Block(a); DBG_OPT_IFSIM; + n = get_nodes_block(a); DBG_OPT_IFSIM; } } else if (get_opt_unreachable_code() && (n != current_ir_graph->start_block) && @@ -551,7 +596,7 @@ static ir_node *equivalent_node_Jmp(ir_node *n) { /* GL: Why not same for op_Raise?? */ /* unreachable code elimination */ - if (is_Bad(get_nodes_Block(n))) + if (is_Bad(get_nodes_block(n))) n = new_Bad(); return n; @@ -607,7 +652,7 @@ static ir_node *equivalent_node_neutral_zero(ir_node *n) /* If this predecessors constant value is zero, the operation is unnecessary. Remove it: */ - if (tarval_classify (tv) == TV_CLASSIFY_NULL) { + if (classify_tarval (tv) == TV_CLASSIFY_NULL) { n = on; DBG_OPT_ALGSIM1; } @@ -628,7 +673,7 @@ static ir_node *equivalent_node_left_zero(ir_node *n) ir_node *a = get_binop_left(n); ir_node *b = get_binop_right(n); - if (tarval_classify(computed_value(b)) == TV_CLASSIFY_NULL) { + if (classify_tarval(computed_value(b)) == TV_CLASSIFY_NULL) { n = a; DBG_OPT_ALGSIM1; } @@ -673,9 +718,9 @@ static ir_node *equivalent_node_Mul(ir_node *n) ir_node *b = get_Mul_right(n); /* Mul is commutative and has again an other neutral element. */ - if (tarval_classify (computed_value (a)) == TV_CLASSIFY_ONE) { + if (classify_tarval (computed_value (a)) == TV_CLASSIFY_ONE) { n = b; DBG_OPT_ALGSIM1; - } else if (tarval_classify (computed_value (b)) == TV_CLASSIFY_ONE) { + } else if (classify_tarval (computed_value (b)) == TV_CLASSIFY_ONE) { n = a; DBG_OPT_ALGSIM1; } return n; @@ -690,7 +735,7 @@ static ir_node *equivalent_node_Div(ir_node *n) ir_node *b = get_Div_right(n); /* Div is not commutative. */ - if (tarval_classify(computed_value(b)) == TV_CLASSIFY_ONE) { /* div(x, 1) == x */ + if (classify_tarval(computed_value(b)) == TV_CLASSIFY_ONE) { /* div(x, 1) == x */ /* Turn Div into a tuple (mem, bad, a) */ ir_node *mem = get_Div_mem(n); turn_into_tuple(n, 3); @@ -713,9 +758,9 @@ static ir_node *equivalent_node_And(ir_node *n) if (a == b) { n = a; /* And has it's own neutral element */ - } else if (tarval_classify(computed_value(a)) == TV_CLASSIFY_ALL_ONE) { + } else if (classify_tarval(computed_value(a)) == TV_CLASSIFY_ALL_ONE) { n = b; - } else if (tarval_classify(computed_value(b)) == TV_CLASSIFY_ALL_ONE) { + } else if (classify_tarval(computed_value(b)) == TV_CLASSIFY_ALL_ONE) { n = a; } if (n != oldn) DBG_OPT_ALGSIM1; @@ -921,11 +966,14 @@ static ir_node *equivalent_node_Proj(ir_node *n) if ( get_Proj_proj(n) <= get_Tuple_n_preds(a) ) { n = get_Tuple_pred(a, get_Proj_proj(n)); DBG_OPT_TUPLE; } else { - assert(0); /* This should not happen! */ - n = new_Bad(); +// assert(0); /* This should not happen! */ +// n = new_Bad(); + dump_ir_block_graph(current_ir_graph, "-CRASH"); + printf(">>>%d\n", get_irn_node_nr(n)); + exit(1); } } else if (get_irn_mode(n) == mode_X && - is_Bad(get_nodes_Block(n))) { + is_Bad(get_nodes_block(n))) { /* Remove dead control flow -- early gigo. */ n = new_Bad(); } @@ -1114,7 +1162,7 @@ static ir_node *transform_node_DivMod(ir_node *n) set_Tuple_pred(n, pn_DivMod_X_except, new_Bad()); /* no exception */ set_Tuple_pred(n, pn_DivMod_res_div, a); set_Tuple_pred(n, pn_DivMod_res_mod, b); - assert(get_nodes_Block(n)); + assert(get_nodes_block(n)); } return n; @@ -1133,7 +1181,7 @@ static ir_node *transform_node_Cond(ir_node *n) (get_opt_unreachable_code())) { /* It's a boolean Cond, branching on a boolean constant. Replace it by a tuple (Bad, Jmp) or (Jmp, Bad) */ - jmp = new_r_Jmp(current_ir_graph, get_nodes_Block(n)); + jmp = new_r_Jmp(current_ir_graph, get_nodes_block(n)); turn_into_tuple(n, 2); if (ta == tarval_b_true) { set_Tuple_pred(n, pn_Cond_false, new_Bad()); @@ -1143,7 +1191,7 @@ static ir_node *transform_node_Cond(ir_node *n) set_Tuple_pred(n, pn_Cond_true, new_Bad()); } /* We might generate an endless loop, so keep it alive. */ - add_End_keepalive(get_irg_end(current_ir_graph), get_nodes_Block(n)); + add_End_keepalive(get_irg_end(current_ir_graph), get_nodes_block(n)); } else if ((ta != tarval_bad) && (get_irn_mode(a) == mode_Iu) && (get_Cond_kind(n) == dense) && @@ -1152,21 +1200,21 @@ static ir_node *transform_node_Cond(ir_node *n) Also this tuple might get really big... I generate the Jmp here, and remember it in link. Link is used when optimizing Proj. */ - set_irn_link(n, new_r_Jmp(current_ir_graph, get_nodes_Block(n))); + set_irn_link(n, new_r_Jmp(current_ir_graph, get_nodes_block(n))); /* We might generate an endless loop, so keep it alive. */ - add_End_keepalive(get_irg_end(current_ir_graph), get_nodes_Block(n)); + add_End_keepalive(get_irg_end(current_ir_graph), get_nodes_block(n)); } else if ((get_irn_op(a) == op_Eor) && (get_irn_mode(a) == mode_b) - && (tarval_classify(computed_value(get_Eor_right(a))) == TV_CLASSIFY_ONE)) { + && (classify_tarval(computed_value(get_Eor_right(a))) == TV_CLASSIFY_ONE)) { /* The Eor is a negate. Generate a new Cond without the negate, simulate the negate by exchanging the results. */ - set_irn_link(n, new_r_Cond(current_ir_graph, get_nodes_Block(n), + set_irn_link(n, new_r_Cond(current_ir_graph, get_nodes_block(n), get_Eor_left(a))); } else if ((get_irn_op(a) == op_Not) && (get_irn_mode(a) == mode_b)) { /* A Not before the Cond. Generate a new Cond without the Not, simulate the Not by exchanging the results. */ - set_irn_link(n, new_r_Cond(current_ir_graph, get_nodes_Block(n), + set_irn_link(n, new_r_Cond(current_ir_graph, get_nodes_block(n), get_Not_op(a))); } return n; @@ -1180,16 +1228,16 @@ static ir_node *transform_node_Eor(ir_node *n) if ((get_irn_mode(n) == mode_b) && (get_irn_op(a) == op_Proj) && (get_irn_mode(a) == mode_b) - && (tarval_classify (computed_value (b)) == TV_CLASSIFY_ONE) + && (classify_tarval (computed_value (b)) == TV_CLASSIFY_ONE) && (get_irn_op(get_Proj_pred(a)) == op_Cmp)) /* The Eor negates a Cmp. The Cmp has the negated result anyways! */ - n = new_r_Proj(current_ir_graph, get_nodes_Block(n), get_Proj_pred(a), + n = new_r_Proj(current_ir_graph, get_nodes_block(n), get_Proj_pred(a), mode_b, get_negated_pnc(get_Proj_proj(a))); else if ((get_irn_mode(n) == mode_b) - && (tarval_classify (computed_value (b)) == TV_CLASSIFY_ONE)) + && (classify_tarval (computed_value (b)) == TV_CLASSIFY_ONE)) /* The Eor is a Not. Replace it by a Not. */ /* ????!!!Extend to bitfield 1111111. */ - n = new_r_Not(current_ir_graph, get_nodes_Block(n), a, mode_b); + n = new_r_Not(current_ir_graph, get_nodes_block(n), a, mode_b); return n; } @@ -1206,7 +1254,7 @@ static ir_node *transform_node_Not(ir_node *n) && (get_irn_mode(a) == mode_b) && (get_irn_op(get_Proj_pred(a)) == op_Cmp)) /* We negate a Cmp. The Cmp has the negated result anyways! */ - n = new_r_Proj(current_ir_graph, get_nodes_Block(n), get_Proj_pred(a), + n = new_r_Proj(current_ir_graph, get_nodes_block(n), get_Proj_pred(a), mode_b, get_negated_pnc(get_Proj_proj(a))); return n; @@ -1227,7 +1275,7 @@ static ir_node *transform_node_Proj(ir_node *proj) b = get_Div_right(n); tb = computed_value(b); - if (tb != tarval_bad && tarval_classify(tb) != TV_CLASSIFY_NULL) { /* div(x, c) && c != 0 */ + if (tb != tarval_bad && classify_tarval(tb) != TV_CLASSIFY_NULL) { /* div(x, c) && c != 0 */ ir_node *div, *proj; ir_node *a = get_Div_left(n); ir_node *mem = get_Div_mem(n); @@ -1236,9 +1284,9 @@ static ir_node *transform_node_Proj(ir_node *proj) set_optimize(0); { div = new_rd_Div(get_irn_dbg_info(n), current_ir_graph, - get_nodes_Block(n), get_irg_initial_mem(current_ir_graph), a, b); + get_nodes_block(n), get_irg_initial_mem(current_ir_graph), a, b); - proj = new_r_Proj(current_ir_graph, get_nodes_Block(n), div, get_irn_mode(a), pn_Div_res); + proj = new_r_Proj(current_ir_graph, get_nodes_block(n), div, get_irn_mode(a), pn_Div_res); } set_optimize(rem); @@ -1252,7 +1300,7 @@ static ir_node *transform_node_Proj(ir_node *proj) b = get_Mod_right(n); tb = computed_value(b); - if (tb != tarval_bad && tarval_classify(tb) != TV_CLASSIFY_NULL) { /* mod(x, c) && c != 0 */ + if (tb != tarval_bad && classify_tarval(tb) != TV_CLASSIFY_NULL) { /* mod(x, c) && c != 0 */ ir_node *mod, *proj; ir_node *a = get_Mod_left(n); ir_node *mem = get_Mod_mem(n); @@ -1261,9 +1309,9 @@ static ir_node *transform_node_Proj(ir_node *proj) set_optimize(0); { mod = new_rd_Mod(get_irn_dbg_info(n), current_ir_graph, - get_nodes_Block(n), get_irg_initial_mem(current_ir_graph), a, b); + get_nodes_block(n), get_irg_initial_mem(current_ir_graph), a, b); - proj = new_r_Proj(current_ir_graph, get_nodes_Block(n), mod, get_irn_mode(a), pn_Mod_res); + proj = new_r_Proj(current_ir_graph, get_nodes_block(n), mod, get_irn_mode(a), pn_Mod_res); } set_optimize(rem); @@ -1277,7 +1325,7 @@ static ir_node *transform_node_Proj(ir_node *proj) b = get_DivMod_right(n); tb = computed_value(b); - if (tb != tarval_bad && tarval_classify(tb) != TV_CLASSIFY_NULL) { /* DivMod(x, c) && c != 0 */ + if (tb != tarval_bad && classify_tarval(tb) != TV_CLASSIFY_NULL) { /* DivMod(x, c) && c != 0 */ ir_node *div_mod, *proj_div, *proj_mod; ir_node *a = get_Mod_left(n); ir_node *mem = get_Mod_mem(n); @@ -1286,10 +1334,10 @@ static ir_node *transform_node_Proj(ir_node *proj) set_optimize(0); { div_mod = new_rd_DivMod(get_irn_dbg_info(n), current_ir_graph, - get_nodes_Block(n), get_irg_initial_mem(current_ir_graph), a, b); + get_nodes_block(n), get_irg_initial_mem(current_ir_graph), a, b); - proj_div = new_r_Proj(current_ir_graph, get_nodes_Block(n), div_mod, get_irn_mode(a), pn_DivMod_res_div); - proj_mod = new_r_Proj(current_ir_graph, get_nodes_Block(n), div_mod, get_irn_mode(a), pn_DivMod_res_mod); + proj_div = new_r_Proj(current_ir_graph, get_nodes_block(n), div_mod, get_irn_mode(a), pn_DivMod_res_div); + proj_mod = new_r_Proj(current_ir_graph, get_nodes_block(n), div_mod, get_irn_mode(a), pn_DivMod_res_mod); } set_optimize(rem); @@ -1300,6 +1348,29 @@ static ir_node *transform_node_Proj(ir_node *proj) set_Tuple_pred(n, pn_DivMod_res_mod, proj_mod); } break; + + case iro_Cond: + if (get_opt_unreachable_code()) { + b = get_Cond_selector(n); + tb = computed_value(b); + + if (tb != tarval_bad && mode_is_int(get_tarval_mode(tb))) { + /* we have a constant switch */ + long num = get_Proj_proj(proj); + + if (num != get_Cond_defaultProj(n)) { /* we cannot optimize default Proj's yet */ + if (get_tarval_long(tb) == num) { + /* Do NOT create a jump here, or we will have 2 control flow ops + * in a block. This case is optimized away in optimize_cf(). */ + return proj; + } + else + return new_Bad(); + } + } + } + return proj; + default: /* do nothing */ return proj; @@ -1403,7 +1474,7 @@ static ir_node *transform_node_Or(ir_node *or) tv4 = get_Const_tarval(c4); tv = tarval_or(tv4, tv2); - if (tarval_classify(tv) != TV_CLASSIFY_ALL_ONE) { + if (classify_tarval(tv) != TV_CLASSIFY_ALL_ONE) { /* have at least one 0 at the same bit position */ return or; } @@ -1466,7 +1537,7 @@ static ir_op *firm_set_default_transform_node(ir_op *op) CASE(Eor); CASE(Not); CASE(Proj); - CASE(Store); + CASE(Store); /* dangerous, see todo */ CASE(Or); default: op->transform_node = NULL; @@ -1598,8 +1669,8 @@ vt_cmp (const void *elt, const void *key) if (irn_arity_a != get_irn_arity(b)) return 1; - /* for block-local cse and pinned nodes: */ - if (!get_opt_global_cse() || (get_op_pinned(get_irn_op(a)) == pinned)) { + /* for block-local cse and op_pin_state_pinned nodes: */ + if (!get_opt_global_cse() || (get_op_pinned(get_irn_op(a)) == op_pin_state_pinned)) { if (get_irn_n(a, -1) != get_irn_n(b, -1)) return 1; } @@ -1701,7 +1772,7 @@ identify (pset *value_table, ir_node *n) } /** - * During construction we set the pinned flag in the graph right when the + * During construction we set the op_pin_state_pinned flag in the graph right when the * optimization is performed. The flag turning on procedure global cse could * be changed between two allocations. This way we are safe. */ @@ -1710,7 +1781,7 @@ identify_cons (pset *value_table, ir_node *n) { ir_node *old = n; n = identify(value_table, n); if (get_irn_n(old, -1) != get_irn_n(n, -1)) - set_irg_pinned(current_ir_graph, floats); + set_irg_pinned(current_ir_graph, op_pin_state_floats); return n; } @@ -1844,7 +1915,7 @@ optimize_node (ir_node *n) /** common subexpression elimination **/ /* Checks whether n is already available. */ /* The block input is used to distinguish different subexpressions. Right - now all nodes are pinned to blocks, i.e., the cse only finds common + now all nodes are op_pin_state_pinned to blocks, i.e., the cse only finds common subexpressions within a block. */ if (get_opt_cse()) n = identify_cons (current_ir_graph->value_table, n); @@ -1928,7 +1999,7 @@ optimize_in_place_2 (ir_node *n) /** common subexpression elimination **/ /* Checks whether n is already available. */ /* The block input is used to distinguish different subexpressions. Right - now all nodes are pinned to blocks, i.e., the cse only finds common + now all nodes are op_pin_state_pinned to blocks, i.e., the cse only finds common subexpressions within a block. */ if (get_opt_cse()) { n = identify (current_ir_graph->value_table, n); @@ -1967,7 +2038,7 @@ optimize_in_place (ir_node *n) assert(get_irg_phase_state(current_ir_graph) != phase_building); if (get_opt_global_cse()) - set_irg_pinned(current_ir_graph, floats); + set_irg_pinned(current_ir_graph, op_pin_state_floats); if (get_irg_outs_state(current_ir_graph) == outs_consistent) set_irg_outs_inconsistent(current_ir_graph); /* Maybe we could also test whether optimizing the node can