[cfe-dev] Code generation problem with inline assembly on arm

[Eli Friedman]

On Sat, Aug 4, 2012 at 9:37 AM, Florian Pflug <fgp at phlo.org> wrote:
> Hi
>
> I'm trying to use an inline assembly block to process 16 bytes of data.
> I pass the pointer to the block into the inline assembly via an input
> "m" operand, and return the result via an output "r" operand. For some
> reasons, however, clang schedules my asm *before* the block is initialize,
> and my asm hence reads bogus values.
>
> Basically, I do
>
> const short V1[32] = {…};
> union ptr_t {
>         short e;
>         struct { char bytes[64]; } data;
> };
> int r;
> asm ( "@use 64 bytes at %1, result in %0"
>       : "=r" (r)
>       : "m" (((const ptr_t*)V1)->data));
>
> The complete test case is attached.
>
> The union+struct stuff is supposed to inform clang about the size of
> the referenced memory region, and should be valid under strict aliasing
> rules (since ptr_t contains a member of type short). Note, however, that
> -fno-strict-aliasing does *not* fix the issue. And neither do variations
> of the theme above, like using just a struct, using attribute may_alias,
> …
>
> Whatever I do, clang moves the inline asm into the middle of the
> initialization of V1, as can be seen in the following assembly output,
> generated with "-arch armv7 -mthumb -O3 -fstrict-aliasing".
>         push    {r4, r7, lr}
>         add     r7, sp, #4
>         sub     sp, #68
>         mov     r4, sp
>         bic     r4, r4, #15
>         mov     sp, r4
>         mov.w   r0, <1st element of V1>
>         strh.w  r0, [sp]
>         ...
>         mov.w   r0, <27th element of V1>
>         strh.w  r0, [sp, #52]
>         @ InlineAsm Start
>         @use 64 bytes at [r1], result in r1
>         @ InlineAsm End
>         mov.w   r0, <28th element of V1>
>         strh.w  r0, [sp, #54]
>         ...
>         mov.w   r0, <32th element of V1>
>         strh.w  r0, [sp, #62]
>
> The only way I found to avoid this is to make mark my asm as
> clobbers-memory *and* volatile. Since that causes all variables
> to be reloaded after my asm block, I'd like to find a way to avoid
> that.
>
> Am I doing something wrong, or is this a bug in clang?

It's a bug, specifically http://llvm.org/bugs/show_bug.cgi?id=13504 .

-Eli