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Re: [PATCH] reimplement -fstrict-volatile-bitfields v4, part 1/2


On Wed, Oct 23, 2013 at 9:11 AM, Bernd Edlinger
<bernd.edlinger@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.


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