Re: bit fields && data tearing

Pavel Machek
2014-09-25T16:12:12Z

On Fri 2014-09-05 12:17:16, Peter Hurley wrote:
> On 09/05/2014 08:37 AM, David Laight wrote:
> > From: Peter Hurley
> >> On 09/05/2014 04:30 AM, David Laight wrote:
> >>> I've seen gcc generate 32bit accesses for 16bit structure members on arm.
> >>> It does this because of the more limited range of the offsets for the 16bit access.
> >>> OTOH I don't know if it ever did this for writes - so it may be moot.
> >>
> >> Can you recall the particulars, like what ARM config or what code?
> >>
> >> I tried an overly-simple test to see if gcc would bump up to the word load for
> >> the 12-bit offset mode, but it stuck with register offset rather than immediate
> >> offset. [I used the compiler options for allmodconfig and a 4.8 cross-compiler.]
> >>
> >> Maybe the test doesn't generate enough register pressure on the compiler?
> >
> > Dunno, I would have been using a much older version of the compiler.
> > It is possible that it doesn't do it any more.
> > It might only have done it for loads.
> >
> > The compiler used to use misaligned 32bit loads for structure
> > members on large 4n+2 byte boundaries as well.
> > I'm pretty sure it doesn't do that either.
> >
> > There have been a lot of compiler versions since I was compiling
> > anything for arm.
>
> Yeah, it seems gcc for ARM no longer uses the larger operand size as a
> substitute for 12-bit immediate offset addressing mode, even for reads.
>
> While this test:
>
> struct x {
> short b[12];
> };
>
> short load_b(struct x *p) {
> return p->b[8];
> }
>
> generates the 8-bit immediate offset form,
>
> short load_b(struct x *p) {
> 0: e1d001f0 ldrsh r0, [r0, #16]
> 4: e12fff1e bx lr
>
>
> pushing the offset out past 256:
>
> struct x {
> long unused[64];
> short b[12];
> };
>
> short load_b(struct x *p) {
> return p->b[8];
> }
>
> generates the register offset addressing mode instead of 12-bit immediate:
>
> short load_b(struct x *p) {
> 0: e3a03e11 mov r3, #272 ; 0x110
> 4: e19000f3 ldrsh r0, [r0, r3]
> 8: e12fff1e bx lr

If we rely on new gcc features for correctness, does minimum compiler version need to be updated?
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