On Mon, Jul 25, 2016 at 02:28:28PM +0200, Georg-Johann Lay wrote:
> (insn 43 31 44 2 (set (reg:QI 18 r18)
> (const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
> (nil))
> (insn 51 50 52 2 (set (reg:QI 16 r16)
> (const_int 40 [0x28])) bug-combin.c:29 56 {movqi_insn}
> (nil))
> (insn 52 51 61 2 (set (reg:DI 18 r18)
> (ashiftrt:DI (reg:DI 18 r18)
> (reg:QI 16 r16))) bug-combin.c:29 1417 {ashrdi3_insn}
> (expr_list:REG_DEAD (reg:QI 16 r16)
> (nil)))
> /* An arithmetic right shift of a quantity known to be -1 or 0
> is a no-op. */
> if (code == ASHIFTRT
> && (num_sign_bit_copies (varop, shift_mode)
> == GET_MODE_PRECISION (shift_mode)))
> {
> count = 0;
> break;
> }
>
> num_sign_bit_copies() eventually calls
> combine.c::reg_num_sign_bit_copies_for_combine()
>
> which returns const0_rtx because reg 18 is set in insn 43 to const0_rtx.
> Total outcome is that the right shift of reg:DI 18 is transformed to a
> no-op move and cancelled out in the remainder.
Why does num_sign_bit_copies return something bigger than 8?
> IIUC get_last_value should return 0 in this case?
>
> /* If we don't have a value, or if it isn't for this basic block and
> it's either a hard register, set more than once, or it's a live
> at the beginning of the function, return 0.
We do have a value, and it is for this bb.
Segher
