[Bug target/64516] New: arm: wrong unaligned load generated

[markus at oberhumer dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64516

            Bug ID: 64516
           Summary: arm: wrong unaligned load generated
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus at oberhumer dot com

Created attachment 34391
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34391&action=edit
unaligned_load_bug.c to reproduce the problem.

arm: bad code generated => run time crash

- gcc 4.9.2 seems to forget the attribute(packed) in some cases.

- please see the disassembly for get16_unaligned().

- put16_unaligned() is correct !

- FWIW, gcc 4.3.5 and clang 3.5 work

~Markus


$ cat unaligned_load_bug.c
typedef struct { char a[2]; } __attribute__((__packed__)) TU2;

// OK: correct (but poor) code generated for the store
void put16_unaligned(void *p, unsigned short v) {
    if (sizeof(TU2) == sizeof(v) && __alignof__(TU2) == 1) {
        *(TU2 *)p = *(const TU2 *)(const void *)(&v);
    }
}

// BUG: incorrect transformation into an aligned load => run time crash !!
unsigned short get16_unaligned(const void *p) {
    unsigned short v;
    if (sizeof(TU2) == sizeof(v) && __alignof__(TU2) == 1) {
        *(TU2 *)(void *)(&v) = *(const TU2 *)p;
    }
    return v;
}

// aligned versions - just for comparison
void put16_aligned(void *p, unsigned short v) {
    *(unsigned short *)p = *(&v);
}
unsigned short get16_aligned(const void *p) {
    unsigned short v;
    *(&v) = *(const unsigned short *)p;
    return v;
}

// EOF

$ arm-linux-gnueabi-gcc-4.9 -v
gcc version 4.9.2 (Ubuntu/Linaro 4.9.2-7ubuntu3)

$ arm-linux-gnueabi-gcc-4.9 -march=armv4 -marm -O2 -Wall -W -Wcast-align -c
unaligned_load_bug.c

$ arm-linux-gnueabi-objdump -d unaligned_load_bug.o

unaligned_load_bug.o:     file format elf32-littlearm

Disassembly of section .text:

00000000 <put16_unaligned>:
   0:   e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
   4:   e24dd00c        sub     sp, sp, #12
   8:   e28d3008        add     r3, sp, #8
   c:   e16310b2        strh    r1, [r3, #-2]!
  10:   e3a02002        mov     r2, #2
  14:   e1a01003        mov     r1, r3
  18:   ebfffffe        bl      0 <memcpy>
  1c:   e28dd00c        add     sp, sp, #12
  20:   e49de004        pop     {lr}            ; (ldr lr, [sp], #4)
  24:   e12fff1e        bx      lr

00000028 <get16_unaligned>:
  28:   e1d000b0        ldrh    r0, [r0]
  2c:   e12fff1e        bx      lr

00000030 <put16_aligned>:
  30:   e1c010b0        strh    r1, [r0]
  34:   e12fff1e        bx      lr

00000038 <get16_aligned>:
  38:   e1d000b0        ldrh    r0, [r0]
  3c:   e12fff1e        bx      lr