On Wed, Oct 23, 2013 at 9:11 AM, Bernd Edlinger
<bernd.edlin...@hotmail.de> wrote:
> Hi Richard/Joseph,
>
> I noticed, this test case crashes on arm-eabi already witout the patch.
>
> extern void abort (void);
>
> #define test_type unsigned short
> #define MAGIC (unsigned short)0x102u
>
> typedef struct s{
> unsigned char Prefix[1];
> test_type Type;
> }__attribute((__packed__,__aligned__(4))) ss;
>
> volatile ss v;
> ss g;
>
> void __attribute__((noinline))
> foo (test_type u)
> {
> v.Type = u;
> }
>
> test_type __attribute__((noinline))
> bar (void)
> {
> return v.Type;
> }
>
>
> However when compiled with -fno-strict-volatile-bitfields it does not crash,
> but AFAIK the generated code for foo() violates the C++ memory model:
>
> foo:
> @ Function supports interworking.
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> ldr r2, .L2
> ldr r3, [r2]
> bic r3, r3, #16711680
> bic r3, r3, #65280
> orr r3, r3, r0, asl #8
> str r3, [r2]
> bx lr
>
>
> On Intel the generated code uses unaligned access, but is OK for the memory
> model:
>
> foo:
> .LFB0:
> .cfi_startproc
> movw %di, v+1(%rip)
> ret
>
>
> Am I right, or is the code OK for the Memory model?
The C++ memory model says that you may not introduce a data-race
and thus you have to access Type without touching Prefix.
Richard.
> Regards
> Bernd.
