[Bug tree-optimization/85017] Missed constant propagation to index of array reference

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85017

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-03-21
          Component|rtl-optimization            |tree-optimization
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's at least smaller.  Probably a cost issue, RTL fwprop is usually handling
this kind of stuff (and of course combine).  fwprop2 sees

(insn 7 3 8 2 (parallel [
            (set (reg:SI 92 [ i ])
                (plus:SI (reg/v:SI 89 [ c ])
                    (const_int 20 [0x14])))
            (clobber (reg:CC 17 flags))
        ]) "t.c":5 221 {*addsi_1}
     (expr_list:REG_DEAD (reg/v:SI 89 [ c ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))
(insn 8 7 9 2 (set (reg:DI 93 [ i ])
        (sign_extend:DI (reg:SI 92 [ i ]))) "t.c":6 149 {*extendsidi2_rex64}
     (expr_list:REG_DEAD (reg:SI 92 [ i ])
        (nil)))
(insn 9 8 14 2 (set (reg:QI 94)
        (mem/j:QI (plus:DI (reg:DI 93 [ i ])
                (symbol_ref:DI ("arr") [flags 0x2]  <var_decl 0x7f6661743e10
arr>)) [0 arr S1 A8])) "t.c":6 88 {*movqi_internal}
     (expr_list:REG_DEAD (reg:DI 93 [ i ])
        (nil)))

where it is probably confused by the sign-extend.  Note on RTL overflow
is well-defined and I'm quite sure movb  arr+20(%rax), %al behaves
differently from addl $20, %eax in that if %eax is 0xffffffff it will
access at 0x100000013 rather than 0x00000013.

This means it is a missed optimization on GIMPLE where we could widen
the add because signed overflow is undefined or perform addressing
mode selection on GIMPLE.