Value Range Propagation Patch (Version 3)

[Toon Moene]

John Wehle wrote:

> VRP may be of some help in that it'll remove any addition
> checks which appear later in the loop (assuming that
> the frontend actually creates redundant bounds checks).

Yes, it does - it's far to difficult to keep track in the frontend
whether a certain index already has been checked.

> Keeping in mind that I don't know fortran, or how the
> current frontend works consider:
> 
>       PARAMETER (N=10)
>       DIMENSION A(N)
>       DO I = 1, N
>          A(I) = I
>          A(I) = A(I) + 3
>       ENDDO
>       PRINT*,A
>       END
> 
> VRP is unable to remove the first check since I is a BIV
> and not based on the loop counter.  However, the first
> check does give VRP information about I which it'll use
> to eliminate later bounds checks.

Unfortunately, that doesn't work (yet ?); I've got the feeling VRP gets
confused over the zero-extend using indices on the Alpha (from the -dV
output):

BB 2
    reg 70
        min <integer_cst 0x1204c1d58 0>        max <integer_cst
0x1204c1e18 9>
    reg 78
        min <integer_cst 0x1204c1ef8 10>        max <integer_cst
0x1204c1fb8 4294967295>
    reg 79
        min <integer_cst 0x1204c2098 0>        max <integer_cst
0x1204c2158 1>
    reg 136
        min <integer_cst 0x1204c2238 9>        max <integer_cst
0x1204c22f8 9>

(note 230 27 33 [bb 1] NOTE_INSN_BASIC_BLOCK -1347440721)

(insn 33 230 35 (set (reg:DI 77)
        (plus:DI (reg/v:DI 69)
            (const_int -1 [0xffffffffffffffff]))) 4 {*lda} (nil)
    (nil))

(insn 35 33 36 (set (subreg:DI (reg:SI 76) 0)
        (reg:DI 77)) 204 {reload_inqi_help-1} (nil)
    (nil))

(insn 36 35 38 (set (reg:DI 78)
        (zero_extend:DI (reg:SI 76))) 36 {zero_extendsidi2} (nil)
    (nil))

(insn 38 36 39 (set (reg:DI 79)
        (leu:DI (reg:DI 78)
            (const_int 9 [0x9]))) 126 {*setcc_internal} (nil)
    (nil))

(jump_insn 39 38 232 (set (pc)
        (if_then_else (ne (reg:DI 79)
                (const_int 0 [0x0]))
            (label_ref 64)
            (pc))) 151 {umaxhi3+3} (nil)
    (expr_list:REG_NONNEG (reg:DI 78)
        (nil)))

It would certainly be very useful.

> BTW, why does the current frontend generate both I and a
> separate loop counter?

Because it's easier that way to stick to the wording of the Standard,
which leads to more clarity in the frontend.

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
GNU Fortran 95: http://g95.sourceforge.net/ (under construction)