[Bug rtl-optimization/45813] [4.5 Regression] alias analysis problem with -mthumb

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #15 from Mikael Pettersson <mikpe at it dot uu.se> 2012-03-15 
08:10:55 UTC ---
-fno-strict-aliasing makes no difference to the code generated by 4.5-20120308.
 Replacing the type-punning with a union results in both correct-looking and
much tighter code:

unsigned short ReadLE16U_union( volatile unsigned char * ptr )
{
    union {
        unsigned short value;
        unsigned char bytes[2];
    } u;

    u.bytes[0] = ptr[0];
    u.bytes[1] = ptr[1];

    return u.value;
}

ReadLE16U_union:
        push    {r7, lr}
        ldrb    r3, [r0]
        ldrb    r0, [r0, #1]
        add     r7, sp, #0
        mov     sp, r7
        lsl     r0, r0, #8
        orr     r0, r0, r3
        @ sp needed for prologue
        pop     {r7, pc}
        .size   ReadLE16U_union, .-ReadLE16U_union
        .ident  "GCC: (GNU) 4.5.4 20120308 (prerelease)"