[Bug rtl-optimization/45813] alias analysis problem ?

[mikpe at it dot uu.se]

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

--- Comment #9 from Mikael Pettersson <mikpe at it dot uu.se> 2010-09-29 19:41:08 UTC ---
I can reproduce it with crosses to arm-elf and arm-linux-gnueabi.  The
combination of -mthumb (1 not 2) and -fno-omit-frame-pointer is the trigger.
GCC 4.3.5 works, 4.4 and 4.5 generate broken code, 4.6 works.

gcc-4.3.5 -march=armv5te -mthumb -fno-omit-frame-pointer -Os generates:

ReadLE16U:
        push    {r7, lr}
        ldrb    r3, [r0]
        sub     sp, sp, #8
        add     r7, sp, #0
        add     r2, r7, #6
        strb    r3, [r2]
        ldrb    r3, [r0, #1]
        mov     sp, r7
        add     sp, sp, #8
        strb    r3, [r2, #1]
        ldrh    r0, [r2]
        @ sp needed for prologue
        pop     {r7, pc}

The second byte is correctly stored before the short is read and returned.

gcc-4.4-20100928 and gcc-4.5-20100923 generate:

ReadLE16U:
        push    {r7, lr}
        ldrb    r2, [r0]
        sub     sp, sp, #8
        add     r7, sp, #0
        add     r3, r7, #6
        mov     sp, r7
        strb    r2, [r3]
        add     sp, sp, #8
        ldrb    r2, [r0, #1]
        @ sp needed for prologue
        ldrh    r0, [r3]
        pop     {r7, pc}

The second byte is read but not written before the short is read.

gcc-4.6-20100925 generates:

ReadLE16U:
        push    {r0, r1, r7, lr}
        ldrb    r3, [r0]
        add     r7, sp, #0
        mov     sp, r7
        strb    r3, [r7, #6]
        ldrb    r3, [r0, #1]
        @ sp needed for prologue
        strb    r3, [r7, #7]
        ldrh    r0, [r7, #6]
        pop     {r1, r2, r7, pc}

This is correct.