On Mon, Jul 8, 2024 at 7:00 PM Andi Kleen <a...@linux.intel.com> wrote:
>
> When musttail is set, make tree-tailcall give error messages
> when it cannot handle a call. This avoids vague "other reasons"
> error messages later at expand time when it sees a musttail
> function not marked tail call.
>
> In various cases this requires delaying the error until
> the call is discovered.
>
> Also print more information on the failure to the dump file.
This looks OK now.
Thanks,
Richard.
> gcc/ChangeLog:
>
> PR83324
> * tree-tailcall.cc (maybe_error_musttail): New function.
> (suitable_for_tail_opt_p): Report error reason.
> (suitable_for_tail_call_opt_p): Report error reason.
> (find_tail_calls): Accept basic blocks with abnormal edges.
> Delay reporting of errors until the call is discovered.
> Move top level suitability checks to here.
> (tree_optimize_tail_calls_1): Remove top level checks.
> ---
> gcc/tree-tailcall.cc | 187 +++++++++++++++++++++++++++++++++++--------
> 1 file changed, 154 insertions(+), 33 deletions(-)
>
> diff --git a/gcc/tree-tailcall.cc b/gcc/tree-tailcall.cc
> index 43e8c25215cb..a68079d4f507 100644
> --- a/gcc/tree-tailcall.cc
> +++ b/gcc/tree-tailcall.cc
> @@ -40,9 +40,11 @@ along with GCC; see the file COPYING3. If not see
> #include "tree-eh.h"
> #include "dbgcnt.h"
> #include "cfgloop.h"
> +#include "intl.h"
> #include "common/common-target.h"
> #include "ipa-utils.h"
> #include "tree-ssa-live.h"
> +#include "diagnostic-core.h"
>
> /* The file implements the tail recursion elimination. It is also used to
> analyze the tail calls in general, passing the results to the rtl level
> @@ -131,14 +133,20 @@ static tree m_acc, a_acc;
>
> static bitmap tailr_arg_needs_copy;
>
> +static void maybe_error_musttail (gcall *call, const char *err);
> +
> /* Returns false when the function is not suitable for tail call optimization
> - from some reason (e.g. if it takes variable number of arguments). */
> + from some reason (e.g. if it takes variable number of arguments). CALL
> + is call to report for. */
>
> static bool
> -suitable_for_tail_opt_p (void)
> +suitable_for_tail_opt_p (gcall *call)
> {
> if (cfun->stdarg)
> - return false;
> + {
> + maybe_error_musttail (call, _("caller uses stdargs"));
> + return false;
> + }
>
> return true;
> }
> @@ -146,35 +154,47 @@ suitable_for_tail_opt_p (void)
> /* Returns false when the function is not suitable for tail call optimization
> for some reason (e.g. if it takes variable number of arguments).
> This test must pass in addition to suitable_for_tail_opt_p in order to
> make
> - tail call discovery happen. */
> + tail call discovery happen. CALL is call to report error for. */
>
> static bool
> -suitable_for_tail_call_opt_p (void)
> +suitable_for_tail_call_opt_p (gcall *call)
> {
> tree param;
>
> /* alloca (until we have stack slot life analysis) inhibits
> sibling call optimizations, but not tail recursion. */
> if (cfun->calls_alloca)
> - return false;
> + {
> + maybe_error_musttail (call, _("caller uses alloca"));
> + return false;
> + }
>
> /* If we are using sjlj exceptions, we may need to add a call to
> _Unwind_SjLj_Unregister at exit of the function. Which means
> that we cannot do any sibcall transformations. */
> if (targetm_common.except_unwind_info (&global_options) == UI_SJLJ
> && current_function_has_exception_handlers ())
> - return false;
> + {
> + maybe_error_musttail (call, _("caller uses sjlj exceptions"));
> + return false;
> + }
>
> /* Any function that calls setjmp might have longjmp called from
> any called function. ??? We really should represent this
> properly in the CFG so that this needn't be special cased. */
> if (cfun->calls_setjmp)
> - return false;
> + {
> + maybe_error_musttail (call, _("caller uses setjmp"));
> + return false;
> + }
>
> /* Various targets don't handle tail calls correctly in functions
> that call __builtin_eh_return. */
> if (cfun->calls_eh_return)
> - return false;
> + {
> + maybe_error_musttail (call, _("caller uses __builtin_eh_return"));
> + return false;
> + }
>
> /* ??? It is OK if the argument of a function is taken in some cases,
> but not in all cases. See PR15387 and PR19616. Revisit for 4.1. */
> @@ -182,7 +202,10 @@ suitable_for_tail_call_opt_p (void)
> param;
> param = DECL_CHAIN (param))
> if (TREE_ADDRESSABLE (param))
> - return false;
> + {
> + maybe_error_musttail (call, _("address of caller arguments taken"));
> + return false;
> + }
>
> return true;
> }
> @@ -402,16 +425,42 @@ propagate_through_phis (tree var, edge e)
> return var;
> }
>
> +/* Report an error for failing to tail convert must call CALL
> + with error message ERR. Also clear the flag to prevent further
> + errors. */
> +
> +static void
> +maybe_error_musttail (gcall *call, const char *err)
> +{
> + if (gimple_call_must_tail_p (call))
> + {
> + error_at (call->location, "cannot tail-call: %s", err);
> + /* Avoid another error. ??? If there are multiple reasons why tail
> + calls fail it might be useful to report them all to avoid
> + whack-a-mole for the user. But currently there is too much
> + redundancy in the reporting, so keep it simple. */
> + gimple_call_set_must_tail (call, false); /* Avoid another error. */
> + gimple_call_set_tail (call, false);
> + }
> + if (dump_file)
> + {
> + print_gimple_stmt (dump_file, call, 0, TDF_SLIM);
> + fprintf (dump_file, "Cannot convert: %s\n", err);
> + }
> +}
> +
> /* Argument for compute_live_vars/live_vars_at_stmt and what
> compute_live_vars
> returns. Computed lazily, but just once for the function. */
> static live_vars_map *live_vars;
> static vec<bitmap_head> live_vars_vec;
>
> /* Finds tailcalls falling into basic block BB. The list of found tailcalls
> is
> - added to the start of RET. When ONLY_MUSTTAIL is set only handle
> musttail. */
> + added to the start of RET. When ONLY_MUSTTAIL is set only handle musttail.
> + Update OPT_TAILCALLS as output parameter. */
>
> static void
> -find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
> +find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail,
> + bool &opt_tailcalls)
> {
> tree ass_var = NULL_TREE, ret_var, func, param;
> gimple *stmt;
> @@ -426,8 +475,23 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> tree var;
>
> if (!single_succ_p (bb))
> - return;
> + {
> + /* If there is an abnormal edge assume it's the only extra one.
> + Tolerate that case so that we can give better error messages
> + for musttail later. */
> + if (!has_abnormal_or_eh_outgoing_edge_p (bb))
> + {
> + if (dump_file)
> + fprintf (dump_file, "Basic block %d has extra exit edges\n",
> + bb->index);
> + return;
> + }
> + if (!cfun->has_musttail)
> + return;
> + }
>
> + bool bad_stmt = false;
> + gimple *last_stmt = nullptr;
> for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
> {
> stmt = gsi_stmt (gsi);
> @@ -441,6 +505,8 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> || is_gimple_debug (stmt))
> continue;
>
> + if (!last_stmt)
> + last_stmt = stmt;
> /* Check for a call. */
> if (is_gimple_call (stmt))
> {
> @@ -448,6 +514,12 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> /* Handle only musttail calls when not optimizing. */
> if (only_musttail && !gimple_call_must_tail_p (call))
> return;
> + if (bad_stmt)
> + {
> + maybe_error_musttail (call,
> + _("memory reference or volatile after call"));
> + return;
> + }
> ass_var = gimple_call_lhs (call);
> break;
> }
> @@ -462,19 +534,37 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> /* If the statement references memory or volatile operands, fail. */
> if (gimple_references_memory_p (stmt)
> || gimple_has_volatile_ops (stmt))
> - return;
> + {
> + if (dump_file)
> + {
> + fprintf (dump_file, "Cannot handle ");
> + print_gimple_stmt (dump_file, stmt, 0);
> + }
> + bad_stmt = true;
> + if (!cfun->has_musttail)
> + break;
> + }
> }
>
> + if (bad_stmt)
> + return;
> +
> if (gsi_end_p (gsi))
> {
> edge_iterator ei;
> /* Recurse to the predecessors. */
> FOR_EACH_EDGE (e, ei, bb->preds)
> - find_tail_calls (e->src, ret, only_musttail);
> + find_tail_calls (e->src, ret, only_musttail, opt_tailcalls);
>
> return;
> }
>
> + if (!suitable_for_tail_opt_p (call))
> + return;
> +
> + if (!suitable_for_tail_call_opt_p (call))
> + opt_tailcalls = false;
> +
> /* If the LHS of our call is not just a simple register or local
> variable, we can't transform this into a tail or sibling call.
> This situation happens, in (e.g.) "*p = foo()" where foo returns a
> @@ -489,13 +579,30 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> if (ass_var
> && !is_gimple_reg (ass_var)
> && !auto_var_in_fn_p (ass_var, cfun->decl))
> - return;
> + {
> + maybe_error_musttail (call, _("return value in memory"));
> + return;
> + }
> +
> + if (cfun->calls_setjmp)
> + {
> + maybe_error_musttail (call, _("caller uses setjmp"));
> + return;
> + }
>
> /* If the call might throw an exception that wouldn't propagate out of
> cfun, we can't transform to a tail or sibling call (82081). */
> - if (stmt_could_throw_p (cfun, stmt)
> - && !stmt_can_throw_external (cfun, stmt))
> + if ((stmt_could_throw_p (cfun, stmt)
> + && !stmt_can_throw_external (cfun, stmt)) || !single_succ_p (bb))
> + {
> + if (stmt == last_stmt)
> + maybe_error_musttail (call,
> + _("call may throw exception that does not
> propagate"));
> + else
> + maybe_error_musttail (call,
> + _("code between call and return"));
> return;
> + }
>
> /* If the function returns a value, then at present, the tail call
> must return the same type of value. There is conceptually a copy
> @@ -524,7 +631,10 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> if (result_decl
> && may_be_aliased (result_decl)
> && ref_maybe_used_by_stmt_p (call, result_decl, false))
> - return;
> + {
> + maybe_error_musttail (call, _("tail call must be same type"));
> + return;
> + }
>
> /* We found the call, check whether it is suitable. */
> tail_recursion = false;
> @@ -605,6 +715,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> {
> if (local_live_vars)
> BITMAP_FREE (local_live_vars);
> + maybe_error_musttail (call, _("call invocation refers to
> locals"));
> return;
> }
> else
> @@ -613,6 +724,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> if (bitmap_bit_p (local_live_vars, *v))
> {
> BITMAP_FREE (local_live_vars);
> + maybe_error_musttail (call, _("call invocation refers to
> locals"));
> return;
> }
> }
> @@ -658,17 +770,21 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> continue;
>
> if (gimple_code (stmt) != GIMPLE_ASSIGN)
> - return;
> + {
> + maybe_error_musttail (call, _("unhandled code after call"));
> + return;
> + }
>
> /* This is a gimple assign. */
> par ret = process_assignment (as_a <gassign *> (stmt), gsi,
> &tmp_m, &tmp_a, &ass_var, to_move_defs);
> - if (ret == FAIL)
> - return;
> + if (ret == FAIL || (ret == TRY_MOVE && !tail_recursion))
> + {
> + maybe_error_musttail (call, _("return value changed after call"));
> + return;
> + }
> else if (ret == TRY_MOVE)
> {
> - if (! tail_recursion)
> - return;
> /* Do not deal with checking dominance, the real fix is to
> do path isolation for the transform phase anyway, removing
> the need to compute the accumulators with new stmts. */
> @@ -716,16 +832,26 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> if (ret_var
> && (ret_var != ass_var
> && !(is_empty_type (TREE_TYPE (ret_var)) && !ass_var)))
> - return;
> + {
> + maybe_error_musttail (call, _("call uses return slot"));
> + return;
> + }
>
> /* If this is not a tail recursive call, we cannot handle addends or
> multiplicands. */
> if (!tail_recursion && (m || a))
> - return;
> + {
> + maybe_error_musttail (call, _("operations after non tail recursive
> call"));
> + return;
> + }
>
> /* For pointers only allow additions. */
> if (m && POINTER_TYPE_P (TREE_TYPE (DECL_RESULT (current_function_decl))))
> - return;
> + {
> + maybe_error_musttail (call,
> + _("tail recursion with pointers can only use
> additions"));
> + return;
> + }
>
> /* Move queued defs. */
> if (tail_recursion)
> @@ -1112,17 +1238,12 @@ tree_optimize_tail_calls_1 (bool opt_tailcalls, bool
> only_mustcall)
> tree param;
> edge_iterator ei;
>
> - if (!suitable_for_tail_opt_p ())
> - return 0;
> - if (opt_tailcalls)
> - opt_tailcalls = suitable_for_tail_call_opt_p ();
> -
> FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR_FOR_FN (cfun)->preds)
> {
> /* Only traverse the normal exits, i.e. those that end with return
> statement. */
> if (safe_is_a <greturn *> (*gsi_last_bb (e->src)))
> - find_tail_calls (e->src, &tailcalls, only_mustcall);
> + find_tail_calls (e->src, &tailcalls, only_mustcall, opt_tailcalls);
> }
>
> if (live_vars)
> --
> 2.45.2
>
