On Tue, 1 Apr 2025, Jakub Jelinek wrote:
> Hi!
>
> While working on the previous tailc patch, I've noticed the following
> problem.
> The testcase below fails, because we decide to tail recursion optimize
> the call, but tail recursion (as documented in tree-tailcall.cc) needs to
> add some result multiplication and/or addition if any tail recursion uses
> accumulator, which is added right before the return.
> So, if there are musttail non-recurive calls in the function, successful
> tail recursion optimization will mean we'll later error on the musttail
> calls. musttail recursive calls are ok, those would be tail recursion
> optimized.
>
> So, the following patch punts on all tail recursion optimizations if it
> needs accumulators (add and/or mult) if there is at least one non-recursive
> musttail call.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Do we update cfun->has_musttail at some point for optimized away
musttail calls with respect to inlining/DCE? I do wonder whether
tree-tailcall.cc itself could "quickly" determine whether we have
any musttail calls that we can handle and will not diagnose?
Thanks,
Richard.
> 2025-04-01 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/119493
> * tree-tailcall.cc (tree_optimize_tail_calls_1): Ignore tail recursion
> candidates which need accumulators if there is at least one musttail
> non-recursive call.
>
> * gcc.dg/pr119493-2.c: New test.
>
> --- gcc/tree-tailcall.cc.jj 2025-03-31 13:00:58.288725496 +0200
> +++ gcc/tree-tailcall.cc 2025-03-31 14:34:31.285106333 +0200
> @@ -1373,6 +1373,38 @@ tree_optimize_tail_calls_1 (bool opt_tai
> live_vars = NULL;
> }
>
> + if (cfun->has_musttail)
> + {
> + /* We can't mix non-recursive must tail calls with tail recursive
> + calls which require accumulators, because in that case we have to
> + emit code in between the musttail calls and return, which prevent
> + calling them as tail calls. So, in that case give up on the
> + tail recursion. */
> + for (act = tailcalls; act; act = act->next)
> + if (!act->tail_recursion)
> + {
> + gcall *call = as_a <gcall *> (gsi_stmt (act->call_gsi));
> + if (gimple_call_must_tail_p (call))
> + break;
> + }
> + if (act)
> + for (struct tailcall **p = &tailcalls; *p; )
> + {
> + if ((*p)->tail_recursion && ((*p)->add || (*p)->mult))
> + {
> + struct tailcall *a = *p;
> + *p = (*p)->next;
> + gcall *call = as_a <gcall *> (gsi_stmt (a->call_gsi));
> + maybe_error_musttail (call,
> + _("tail recursion with accumulation "
> + "mixed with musttail "
> + "non-recursive call"), diag_musttail);
> + free (a);
> + }
> + else
> + p = &(*p)->next;
> + }
> + }
> /* Construct the phi nodes and accumulators if necessary. */
> a_acc = m_acc = NULL_TREE;
> for (act = tailcalls; act; act = act->next)
> --- gcc/testsuite/gcc.dg/pr119493-2.c.jj 2025-03-31 14:41:18.977464154
> +0200
> +++ gcc/testsuite/gcc.dg/pr119493-2.c 2025-03-31 14:45:24.327068682 +0200
> @@ -0,0 +1,22 @@
> +/* PR tree-optimization/119493 */
> +/* { dg-do compile { target musttail } } */
> +/* { dg-options "-O2 -fdump-tree-tailr1-details" } */
> +/* { dg-final { scan-tree-dump-times "tail recursion with accumulation mixed
> with musttail non-recursive call" 2 "tailr1" } } */
> +
> +[[gnu::noipa]] int
> +bar (int x, int y)
> +{
> + return x + y;
> +}
> +
> +[[gnu::noinline, gnu::noclone]] int
> +foo (int x, int y)
> +{
> + if (x < 10)
> + [[gnu::musttail]] return bar (x, y);
> + if (y & 2)
> + return foo (x - 1, y) * 2;
> + if (y & 1)
> + [[gnu::musttail]] return foo (x - 1, y);
> + return foo (x - 1, y) * 3;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
