[PATCH][AArch64] PR target/71112: Properly create lowpart of pic_offset_table_rtx with -fpie

[Kyrill Tkachov]

Hi all,

This ICE only occurs on big-endian ILP32 -fpie code. The expansion code generates the invalid load:
(insn 6 5 7 (set (reg/f:SI 76)
         (unspec:SI [
                 (mem/u/c:SI (lo_sum:SI (nil)
                         (symbol_ref:SI ("dbs") [flags 0x40] <var_decl 0x7f6e387c0ab0 dbs>)) [0  S4 A8])
             ] UNSPEC_GOTSMALLPIC28K))
      (expr_list:REG_EQUAL (symbol_ref:SI ("dbs") [flags 0x40] <var_decl 0x7f6e387c0ab0 dbs>)
         (nil)))

to load the symbol. Note the (nil) argument to lo_sum.
The buggy hunk meant to take the lowpart of the pic_offset_table_rtx register but it did so by explicitly
constructing a subreg, for which the offset is wrong for big-endian. The right way is to use gen_lowpart which
knows what exactly to do, with this patch we emit:
(insn 6 5 7 (set (reg/f:SI 76)
         (unspec:SI [
                 (mem/u/c:SI (lo_sum:SI (subreg:SI (reg:DI 73) 4)
                         (symbol_ref:SI ("dbs") [flags 0x40] <var_decl 0x7ffb097e6ab0 dbs>)) [0  S4 A8])
             ] UNSPEC_GOTSMALLPIC28K))
      (expr_list:REG_EQUAL (symbol_ref:SI ("dbs") [flags 0x40] <var_decl 0x7ffb097e6ab0 dbs>)
         (nil)))

and everything works fine.

Bootstrapped and tested on aarch64-none-linux-gnu.
Also tested on aarch64_be-none-elf.
Ok for trunk?

Thanks,
Kyrill

2016-11-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR target/71112
     * config/aarch64/aarch64.c (aarch64_load_symref_appropriately,
     case SYMBOL_SMALL_GOT_28K): Use gen_lowpart rather than constructing
     subreg directly.

2016-11-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR target/71112
     * gcc.c-torture/compile/pr71112.c: New test.
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