[Bug target/79671] [7 Regression] mapnik miscompilation on armv7hl since r235622

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671

--- Comment #90 from rguenther at suse dot de <rguenther at suse dot de> ---
On April 2, 2017 2:09:07 AM GMT+02:00, "bernd.edlinger at hotmail dot de"
<gcc-bugzilla@gcc.gnu.org> wrote:
>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
>
>Bernd Edlinger <bernd.edlinger at hotmail dot de> changed:
>
>           What    |Removed                     |Added
>----------------------------------------------------------------------------
>  Attachment #41101|0                           |1
>        is obsolete|                            |
>
>--- Comment #89 from Bernd Edlinger <bernd.edlinger at hotmail dot de>
>---
>Created attachment 41103
>  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41103&action=edit
>updated patch
>
>should work in principle with LTO,
>but there is still another bug
>somewhere which makes __attribute__((may_alias))
>and the std::byte handling fail in the end.
>
>consider this test example:
>
>cat t.cc
>#include <cstddef>
>//namespace std { enum class byte: unsigned char {}; };
>struct t  {
>  int x;
>  //std::byte kk[1];
>  unsigned char k;
>} __attribute__((may_alias));
>
>t t1;
>t t2;
>t *t3;
>t *t4;
>
>void __attribute__((noinline, noclone))
>test()
>{
>  t1 = t2;

Can't spot the issue in the RTL you quote but may_alias does not apply to this
assignment.

>  *t4 = *t3;
>}
>
>int main()
>{
>  test();
>}
>
>g++ -std=c++17 -O2 -fdump-rtl-final t.cc
>
>t.cc.309r.final:
>(insn:TI 6 2 7 2 (set (reg:DI 0 ax [orig:90 MEM[(const struct t &
>{ref-all})&t2] ] [90])
>  (mem/c:DI (symbol_ref:DI ("t2") [flags 0x2]  <var_decl 0x7f973b7302d0
>t2>) [0 MEM[(const struct t & {ref-all})&t2]+0 S8 A64])) "t.cc":17 81
>{*movdi_internal}
>     (expr_list:REG_EQUIV (mem/c:DI (symbol_ref:DI ("t2") [flags 0x2] 
><var_decl 0x7f973b7302d0 t2>) [0 MEM[(const struct t & {ref-all})&t2]+0
>S8
>A64])
>        (nil)))
>(insn:TI 7 6 14 2 (set (mem/c:DI (symbol_ref:DI ("t1") [flags 0x2] 
><var_decl
>0x7f973b730240 t1>) [2 t1+0 S8 A64])
>    (reg:DI 0 ax [orig:90 MEM[(const struct t & {ref-all})&t2] ] [90]))
>"t.cc":17 81 {*movdi_internal}
>     (expr_list:REG_DEAD (reg:DI 0 ax [orig:90 MEM[(const struct t &
>{ref-all})&t2] ] [90])
>        (nil)))
>(insn 14 7 10 2 (set (reg/f:DI 0 ax [orig:87 t3.1_1 ] [87])
>(mem/f/c:DI (symbol_ref:DI ("t3") [flags 0x2]  <var_decl 0x7f973b730360
>t3>) [1 t3+0 S8 A64])) "t.cc":18 81 {*movdi_internal}
>    (expr_list:REG_EQUIV (mem/f/c:DI (symbol_ref:DI ("t3") [flags 0x2] 
><var_decl 0x7f973b730360 t3>) [1 t3+0 S8 A64])
>        (nil)))
>(insn:TI 10 14 15 2 (set (reg:DI 1 dx [orig:91 MEM[(const struct t &
>{ref-all})t3.1_1] ] [91])
>(mem:DI (reg/f:DI 0 ax [orig:87 t3.1_1 ] [87]) [0 MEM[(const struct t &
>{ref-all})t3.1_1]+0 S8 A32])) "t.cc":18 81 {*movdi_internal}
>     (expr_list:REG_DEAD (reg/f:DI 0 ax [orig:87 t3.1_1 ] [87])
>        (nil)))
>(insn 15 10 11 2 (set (reg/f:DI 0 ax [orig:88 t4.2_2 ] [88])
>(mem/f/c:DI (symbol_ref:DI ("t4") [flags 0x2]  <var_decl 0x7f973b7303f0
>t4>) [1 t4+0 S8 A64])) "t.cc":18 81 {*movdi_internal}
>    (expr_list:REG_EQUIV (mem/f/c:DI (symbol_ref:DI ("t4") [flags 0x2] 
><var_decl 0x7f973b7303f0 t4>) [1 t4+0 S8 A64])
>        (nil)))
>(insn:TI 11 15 23 2 (set (mem:DI (reg/f:DI 0 ax [orig:88 t4.2_2 ] [88])
>[0
>*t4.2_2+0 S8 A32])
> (reg:DI 1 dx [orig:91 MEM[(const struct t & {ref-all})t3.1_1] ] [91]))
>"t.cc":18 81 {*movdi_internal}
>     (expr_list:REG_DEAD (reg:DI 1 dx [orig:91 MEM[(const struct t &
>{ref-all})t3.1_1] ] [91])
>        (expr_list:REG_DEAD (reg/f:DI 0 ax [orig:88 t4.2_2 ] [88])
>            (nil))))
>
>
>
>I think the access to t1 uses alias set 2, and that is wrong.
>The other accesses are ok.