On Sat, Jun 22, 2024 at 9:01 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.
>
> gcc/ChangeLog:
>
> * tree-tailcall.cc (maybe_error_musttail): New function.
> (bb_get_succ_edge_count): New function.
> (find_tail_calls): Add error reporting. Handle EH edges
> for error reporting.
> ---
> gcc/tree-tailcall.cc | 116 +++++++++++++++++++++++++++++++++++++------
> 1 file changed, 102 insertions(+), 14 deletions(-)
>
> diff --git a/gcc/tree-tailcall.cc b/gcc/tree-tailcall.cc
> index 0c6df10e64f7..4687e20e61d0 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
> @@ -402,6 +404,41 @@ 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);
> + }
> +}
> +
> +/* Count succ edges for BB and return in NUM_OTHER and NUM_EH. */
> +
> +static void
> +bb_get_succ_edge_count (basic_block bb, int &num_other, int &num_eh)
> +{
> + edge e;
> + edge_iterator ei;
> + num_eh = 0;
> + num_other = 0;
> + FOR_EACH_EDGE (e, ei, bb->succs)
> + if (e->flags & EDGE_EH)
> + num_eh++;
> + else
> + num_other++;
> +}
> +
> /* 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;
> @@ -426,8 +463,16 @@ find_tail_calls (basic_block bb, struct tailcall **ret,
> bool only_musttail)
> tree var;
>
> if (!single_succ_p (bb))
> - return;
> + {
> + int num_eh, num_other;
> + bb_get_succ_edge_count (bb, num_eh, num_other);
> + /* Allow EH edges so that we can give a better
> + error message later. */
Please instead use has_abnormal_or_eh_outgoing_edge_p (bb) instead
to avoid adding another function like this. Also only continue searching
for a musttail call if cfun->has_musttail
> + if (num_other != 1)
> + return;
> + }
>
> + bool bad_stmt = false;
> for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
> {
> stmt = gsi_stmt (gsi);
> @@ -448,6 +493,12 @@ 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;
> }
> @@ -462,9 +513,14 @@ 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;
> + {
> + bad_stmt = true;
break here when !cfun->has_musttail?
> + }
> }
> + if (bad_stmt)
> + return;
> +
> if (gsi_end_p (gsi))
> {
> edge_iterator ei;
> @@ -489,13 +545,26 @@ 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))
This reports for the found stmt while above we reject any intermediate
non-fallthru control flow. I would suggest to, in the above BB check,
record a gimple *last = last_stmt (bb) and if last == stmt report this reason
but otherwise "control altering statement between call and return"?
> + {
> + maybe_error_musttail (call,
> + _("call may throw exception that does not
> propagate"));
> 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 +593,10 @@ 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"));
? "call uses the return slot"?
Otherwise looks OK.
> + return;
> + }
>
> /* We found the call, check whether it is suitable. */
> tail_recursion = false;
> @@ -605,6 +677,7 @@ 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 +686,7 @@ 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 +732,21 @@ 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 +794,26 @@ 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 must be the same type"));
> + 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)
> --
> 2.45.2
>
