[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.tkac...@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.tkac...@arm.com>

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

commit 99052634535958a776a2e7b7c9c6169cb656f5a8
Author: Kyrylo Tkachov <kyrylo.tkac...@arm.com>
Date:   Mon Nov 28 12:26:57 2016 +0000

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

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 686a8e92..aa833c8 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -1299,7 +1299,7 @@ aarch64_load_symref_appropriately (rtx dest, rtx imm,
 	    emit_move_insn (gp_rtx, gen_rtx_HIGH (Pmode, s));
 
 	    if (mode != GET_MODE (gp_rtx))
-	      gp_rtx = simplify_gen_subreg (mode, gp_rtx, GET_MODE (gp_rtx), 0);
+	      gp_rtx = gen_lowpart (mode, gp_rtx);
 	  }
 
 	if (mode == ptr_mode)
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr71112.c b/gcc/testsuite/gcc.c-torture/compile/pr71112.c
new file mode 100644
index 0000000..69e2df6
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr71112.c
@@ -0,0 +1,10 @@
+/* PR target/71112.  */
+/* { dg-additional-options "-fpie" { target pie } } */
+
+extern int dbs[100];
+void f (int *);
+int nscd_init (void)
+{
+  f (dbs);
+  return 0;
+}