[Bug rtl-optimization/59138] [4.8/4.9 Regression] possible packed struct miscompile

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59138

--- Comment #4 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
Latent issue in the alias.c machinery: output_dependence returns 0 on

(insn 21 20 22 2 (set (mem/c:DI (const:DI (plus:DI (symbol_ref:DI ("b") [flags
0x2]  <var_decl 0x7ffff6e4d2f8 b>)
                    (const_int 8 [0x8]))) [0 +8 S8 A64])
        (reg:DI 1 dx [99])) pr59138.c:20 85 {*movdi_internal}
     (expr_list:REG_DEAD (reg:DI 1 dx [99])
        (nil)))

(insn 22 21 23 2 (set (mem/c:HI (symbol_ref:DI ("a") [flags 0x2]  <var_decl
0x7ffff6e4d260 a>) [2 a+0 S2 A16])
        (const_int 0 [0])) pr59138.c:21 87 {*movhi_internal}
     (nil))

because base_alias_check returns 0 for different symbols...