On Tue, Feb 27, 2024 at 1:50 PM Eric Botcazou <botca...@adacore.com> wrote:
>
> Hi,
>
> this is a regression present on the mainline, 13 and 12 branches. For the
> attached Ada case, it's a tree checking failure on the mainline at -O:
>
> +===========================GNAT BUG DETECTED==============================+
> | 14.0.1 20240226 (experimental) [master r14-9171-g4972f97a265] GCC error:|
> | tree check: expected tree that contains 'decl common' structure, |
> | have 'component_ref' in tree_could_trap_p, at tree-eh.cc:2733 |
> | Error detected around /home/eric/cvs/gcc/gcc/testsuite/gnat.dg/opt104.adb:
>
> Time is a 10-byte record and Packed_Rec.T is placed at bit-offset 65 because
> of the packing. so tree-ssa-dse.cc:setup_live_bytes_from_ref has computed a
> const_size of 88 from ref->offset of 65 and ref->max_size of 80.
>
> Then in tree-ssa-dse.cc:compute_trims:
>
> 411 int last_live = bitmap_last_set_bit (live);
> (gdb) next
> 412 if (ref->size.is_constant (&const_size))
> (gdb)
> 414 int last_orig = (const_size / BITS_PER_UNIT) - 1;
> (gdb)
> 418 *trim_tail = last_orig - last_live;
>
> (gdb) call debug_bitmap (live)
> n_bits = 256, set = {0 1 2 3 4 5 6 7 8 9 10 }
> (gdb) p last_live
> $33 = 10
> (gdb) p const_size
> $34 = 80
> (gdb) p last_orig
> $35 = 9
> (gdb) p *trim_tail
> $36 = -1
>
> In other words, compute_trims is overlooking the alignment adjustments applied
> earlier by setup_live_bytes_from_ref. Moveover it reads:
>
> /* We use sbitmaps biased such that ref->offset is bit zero and the bitmap
> extends through ref->size. So we know that in the original bitmap
> bits 0..ref->size were true. We don't actually need the bitmap, just
> the REF to compute the trims. */
>
> But setup_live_bytes_from_ref used ref->max_size instead of ref->size.
>
> It appears that all the callers of compute_trims assume that ref->offset is
> byte aligned and that the trimmed bytes are relative to ref->size, so the
> patch simply adds an early return if either condition is not fulfilled
>
> Tested on x86-64/Linux, OK for all the affected branches?
OK.
Thanks,
Richard.
>
> 2024-02-27 Eric Botcazou <ebotca...@adacore.com>
>
> * tree-ssa-dse.cc (compute_trims): Fix description. Return early
> if ref->offset is not byte aligned or ref->size is not known to be
> equal to ref->max_size.
> (maybe_trim_complex_store): Fix description.
> (maybe_trim_constructor_store): Likewise.
> (maybe_trim_partially_dead_store): Likewise.
>
>
> 2024-02-27 Eric Botcazou <ebotca...@adacore.com>
>
> * gnat.dg/opt104.ads, gnat.dg/opt104.adb! New test.
>
> --
> Eric Botcazou
