On 7/29/2022 2:54 AM, Richard Biener via Gcc-patches wrote:
The following makes the backward threader reject threads whose entry
edge is probably never executed according to the profile. That in
particular, for the testcase, avoids threading the irq == 1 check
on the path where irq > 31, thereby avoiding spurious -Warray-bounds
diagnostics
if (irq_1(D) > 31)
goto <bb 3>; [0.00%]
else
goto <bb 4>; [100.00%]
;; basic block 3, loop depth 0, count 0 (precise), probably never executed
_2 = (unsigned long) irq_1(D);
__builtin___ubsan_handle_shift_out_of_bounds (&*.Lubsan_data0, 1, _2);
_3 = 1 << irq_1(D);
mask_4 = (u32) _3;
entry = instance_5(D)->array[irq_1(D)];
capture (mask_4);
if (level_6(D) != 0)
goto <bb 7>; [34.00%]
else
goto <bb 5>; [66.00%]
;; basic block 5, loop depth 0, count 708669600 (estimated locally), maybe
hot if (irq_1(D) == 1)
goto <bb 7>; [20.97%]
else
goto <bb 6>; [79.03%]
Bootstrap and regtest running on x86_64-unknown-linux-gnu.
The testcase in the PR requries both ubsan and sancov so I'm not sure
where to put it but IIRC there were quite some duplicate PRs wrt
threading unlikely paths exposing diagnostics, eventually some
testcase will come out of those (when we identify them). Note
the patch is quite conservative in only disabling likely never
executed paths rather than requiring maybe_hot_edge_p (OTOH those
are somewhat similar in the end).
I'm going to push it when testing finishes but maybe there are some
testcases to adjust.
PR tree-optimization/105679
* tree-ssa-threadbackwards.cc
(back_threader_profitability::profitable_path_p): Avoid threading
when the entry edge is probably never executed.
OK. And more generally I'm absolutely OK with improving the costing
heuristics.
The one thing we need to keep in mind when adding this kind of check is
that we may start seeing new diagnostics from the middle end -- while
the path may not be profitable to thread from a performance standpoint,
it may be useful to thread from a diagnostic standpoint.
But I'd argue that for such cases we really want to move to a model
where the predicate analysis bits are used consistently to prune away
"can't happen" cases for middle end diagnostics rather than relying on
threading to optimize away can't happen cold paths.
Jeff
