[Issue 9097] New: Value range propagation to disable some array bound tests

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9097

           Summary: Value range propagation to disable some array bound
                    tests
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2012-11-29 11:17:33 PST ---
A little test program:


void main(string[] args) {
    size_t a = 6000 + args.length;
    int[200] array;
    array[a % 100] = 1;
}


-----------------------

Compiled with "dmd -O -inline":

__Dmain:
L0:     sub ESP, 0338h
        mov ECX, 0C8h
        push EDI
        mov EAX, 0340h[ESP]
        add EAX, 01770h
        mov 0Ch[ESP], EAX
        xor EAX, EAX
        lea EDI, 01Ch[ESP]
        rep
        stosd
        mov EAX, 0Ch[ESP]
        mov ECX, 064h
        xor EDX, EDX
        div ECX
        cmp EDX, 0C8h
        mov 8[ESP], EDX
        jb  L47
        mov EAX, 4
        call near ptr _D5test27__arrayZ
L47:    mov EDX, 8[ESP]
        xor EAX, EAX
        mov dword ptr 01Ch[EDX*4][ESP], 1
        pop EDI
        add ESP, 0338h
        ret

_D5test27__arrayZ:
L0:     push EAX
        mov ECX, offset FLAT:_D5test212__ModuleInfoZ
        push EAX
        push ECX
        call near ptr __d_array_bounds

-----------------------

Compiled with "dmd -O -inline -release -noboundscheck":

__Dmain:
        sub ESP, 0330h
        mov ECX, 0C8h
        push EDI
        mov EAX, 0338h[ESP]
        add EAX, 01770h
        mov 4[ESP], EAX
        xor EAX, EAX
        lea EDI, 014h[ESP]
        rep
        stosd
        mov EAX, 4[ESP]
        mov ECX, 064h
        xor EDX, EDX
        div ECX
        xor EAX, EAX
        mov dword ptr 014h[EDX*4][ESP], 1
        pop EDI
        add ESP, 0330h
        ret

-----------------------

Value range propagation is able to tell that the value of "a % 100" can't be
outside the 0..200 bounds of the array, so there is no point in performing the
bound test even in nonrelease mode.

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