On Mon, Jun 19, 2023 at 9:52 AM Jan Hubicka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
> this was suggested earlier somewhere, but I can not find the thread.
> C++ has assume attribute that expands int
> if (conditional)
> __builtin_unreachable ()
> We do not want to account the conditional in inline heuristics since
> we know that it is going to be optimized out.
>
> Bootstrapped/regtested x86_64-linux, will commit it later today if
> thre are no complains.
I think we also had the request to not account the condition feeding
stmts (if they only feed it and have no side-effects). libstdc++ has
complex range comparisons here. Also ...
> gcc/ChangeLog:
>
> * ipa-fnsummary.cc (builtin_unreachable_bb_p): New function.
> (analyze_function_body): Do not account conditionals guarding
> builtin_unreachable calls.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/ipa/fnsummary-1.c: New test.
>
> diff --git a/gcc/ipa-fnsummary.cc b/gcc/ipa-fnsummary.cc
> index a5f5a50c8a5..987da29ec34 100644
> --- a/gcc/ipa-fnsummary.cc
> +++ b/gcc/ipa-fnsummary.cc
> @@ -2649,6 +2649,54 @@ points_to_possible_sra_candidate_p (tree t)
> return false;
> }
>
> +/* Return true if BB is builtin_unreachable.
> + We skip empty basic blocks, debug statements, clobbers and predicts.
> + CACHE is used to memoize already analyzed blocks. */
> +
> +static bool
> +builtin_unreachable_bb_p (basic_block bb, vec<unsigned char> &cache)
> +{
> + if (cache[bb->index])
> + return cache[bb->index] - 1;
> + gimple_stmt_iterator si;
> + auto_vec <basic_block, 4> visited_bbs;
> + bool ret = false;
> + while (true)
> + {
> + bool empty_bb = true;
> + visited_bbs.safe_push (bb);
> + cache[bb->index] = 3;
> + for (si = gsi_start_nondebug_bb (bb);
> + !gsi_end_p (si) && empty_bb;
> + gsi_next_nondebug (&si))
> + {
> + if (gimple_code (gsi_stmt (si)) != GIMPLE_PREDICT
> + && !gimple_clobber_p (gsi_stmt (si))
> + && !gimple_nop_p (gsi_stmt (si)))
> + {
> + empty_bb = false;
> + break;
> + }
> + }
> + if (!empty_bb)
> + break;
> + else
> + bb = single_succ_edge (bb)->dest;
> + if (cache[bb->index])
> + {
> + ret = cache[bb->index] == 3 ? false : cache[bb->index] - 1;
> + goto done;
> + }
> + }
> + if (gimple_call_builtin_p (gsi_stmt (si), BUILT_IN_UNREACHABLE)
> + || gimple_call_builtin_p (gsi_stmt (si), BUILT_IN_UNREACHABLE_TRAP))
... we do code generate BUILT_IN_UNREACHABLE_TRAP, no?
> + ret = true;
> +done:
> + for (basic_block vbb:visited_bbs)
> + cache[vbb->index] = (unsigned char)ret + 1;
> + return ret;
> +}
> +
> /* Analyze function body for NODE.
> EARLY indicates run from early optimization pipeline. */
>
> @@ -2743,6 +2791,8 @@ analyze_function_body (struct cgraph_node *node, bool
> early)
> const profile_count entry_count = ENTRY_BLOCK_PTR_FOR_FN (cfun)->count;
> order = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
> nblocks = pre_and_rev_post_order_compute (NULL, order, false);
> + auto_vec<unsigned char, 10> cache;
> + cache.safe_grow_cleared (last_basic_block_for_fn (cfun));
A sbitmap with two bits per entry would be more space efficient here. bitmap
has bitmap_set_aligned_chunk and bitmap_get_aligned_chunk for convenience,
adding the corresponding to sbitmap.h would likely ease use there as well.
> for (n = 0; n < nblocks; n++)
> {
> bb = BASIC_BLOCK_FOR_FN (cfun, order[n]);
> @@ -2901,6 +2951,24 @@ analyze_function_body (struct cgraph_node *node, bool
> early)
> }
> }
>
> + /* Conditionals guarding __builtin_unreachable will be
> + optimized out. */
> + if (gimple_code (stmt) == GIMPLE_COND)
> + {
> + edge_iterator ei;
> + edge e;
> + FOR_EACH_EDGE (e, ei, bb->succs)
> + if (builtin_unreachable_bb_p (e->dest, cache))
> + {
> + if (dump_file)
> + fprintf (dump_file,
> + "\t\tConditional guarding
> __builtin_unreachable; ignored\n");
> + this_time = 0;
> + this_size = 0;
> + break;
> + }
> + }
> +
> /* TODO: When conditional jump or switch is known to be constant,
> but
> we did not translate it into the predicates, we really can
> account
> just maximum of the possible paths. */
> diff --git a/gcc/testsuite/gcc.dg/ipa/fnsummary-1.c
> b/gcc/testsuite/gcc.dg/ipa/fnsummary-1.c
> new file mode 100644
> index 00000000000..a0ece0c300b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/ipa/fnsummary-1.c
> @@ -0,0 +1,8 @@
> +/* { dg-options "-O2 -fdump-ipa-fnsummary" } */
> +int
> +test(int a)
> +{
> + if (a > 10)
> + __builtin_unreachable ();
> +}
> +/* { dg-final { scan-ipa-dump "Conditional guarding __builtin_unreachable"
> "fnsummary" } } */
