[cfe-dev] [ARM] Is code generation implemented for __unaligned?

[Roger Ferrer Ibáñez via cfe-dev]

Hi all,

@James thanks a lot for the prompt diagnostic.

@Jan: I raised https://bugs.llvm.org/show_bug.cgi?id=47499 and I'll look
into it ASAP.

I think we want to keep both languages in sync, so bring the C behaviour to
C++.

C++ users that might be impacted by this (clang in Windows I presume) might
want to weigh in just in case this is a bad idea.

My understanding is that __unaligned is a specifier that came from the
Windows Itanium era. A quick check (using James' testcase) shows that x86
assembly output is not impacted. I assume that the X86 backend can be
lenient when it comes to alignment due to what x86 has historically done
there, correct me if I'm wrong.

Kind regards,

Missatge de James Y Knight <jyknight at google.com> del dia dv., 11 de set.
2020 a les 19:50:

> There seems to be *something* implemented, but it looks buggy. Compiling
> the below example (https://godbolt.org/z/7rcErj) with "-O2
> -fms-extensions -emit-llvm" emits:
>   store i64 5, i64* %2, align 1, !tbaa !2
> in C, but
>   store i64 5, i64* %2, align 8, !tbaa !2
> in C++.
>
> struct Foo {
>   unsigned long x;
> };
>
> void foo(__unaligned struct Foo* out){
>   out->x = 5;
> }
>
>
> On Fri, Sep 11, 2020 at 12:07 PM Jan Ole Hüser via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Dear all, Dear Roger Ferrer Ibáñez,
>>
>> at my company we have much Code, which uses the keyword "__unaligned" [1],
>> which is a Microsoft extension.  I have read about an RFC called
>> "Implement
>> code generation for __unaligned" on the mailing list (from Feburary 2017)
>> [2].
>> But I could not find out, if it was actually implemented. Is it
>> implemented? If
>> it is, it does not work for ARM. (Tested with ARMv7-M.)
>>
>> Consider the following Code:
>>
>> ```
>> struct GUID {
>>   unsigned long  Data1;
>>   unsigned short Data2;
>>   unsigned short Data3;
>>   unsigned char  Data4[8];
>> };
>>
>> void load_and_store_uGUID(__unaligned GUID* in, __unaligned GUID* out){
>>         *out = *in;
>> }
>> ```
>>
>> It produces the following assembly:
>>
>> ```
>>         ldm.w   r0, {r2, r3, r12}
>>         ldr     r0, [r0, #12]
>>         stm.w   r1, {r2, r3, r12}
>>         str     r0, [r1, #12]
>>         bx      lr
>> ```
>>
>> I have used the this commandline:
>> C:\Program Files\LLVM\bin\clang++.exe -O2 -S
>> --target=armv7m-none-windows-eabihf -munaligned-access -x c++
>> -mcpu=cortex-M7 -fms-compatibility-version=19.16.27027.1 -fms-compatibility
>> -fms-extensions
>>
>> The problem is, that LDM and STM (load and store multiple) are used, which
>> cannot access one-byte-aligned addresses (aka. unaligned). LDR and STR
>> however
>> can be used for unaligned access.
>>
>> The following C++ code translates to code, which can access unalgined
>> memory.
>> ```
>> extern "C" void* memcpy( void* dest, const void* src, decltype(sizeof(0))
>> count );
>>
>> void load_and_store_memcpy(__unaligned GUID* in, __unaligned GUID* out){
>>         memcpy(reinterpret_cast<char*>(out), reinterpret_cast<char*>(in),
>>                 sizeof(__unaligned GUID));
>> }
>> ```
>>
>> The cast to a byte pointer is necessary.
>>
>> Here is the assembler Code:
>> ```
>>         ldr.w   r12, [r0]
>>         ldr     r3, [r0, #4]
>>         ldr     r2, [r0, #8]
>>         ldr     r0, [r0, #12]
>>         str     r0, [r1, #12]
>>         str     r2, [r1, #8]
>>         str     r3, [r1, #4]
>>         str.w   r12, [r1]
>>         bx      lr
>> ```
>>
>>
>> I could rewrite all that old code to use memcpy or use structs with
>> __attribute__((packed)). But maybe --I hope-- there is some old patch for
>> clang, that didn't get into trunk, but could help me out. Or maybe it is
>> easy
>> to implement. @Roger, is there a patch somewhere?
>>
>> Site Note:
>> The problem with __attribute__((packed)) is, that it cannot be added to an
>> existing type (for example: "typedef __attribute__((packed)) GUID
>> pGUID;"), but
>> the definition has to be changed. Another problem is, that it changes the
>> offset
>> of the members in the struct. Is that called "Record Layout"?
>>
>> Kind regards,
>> Ole
>>
>> [1] https://docs.microsoft.com/en-us/cpp/cpp/unaligned
>> [2] http://lists.llvm.org/pipermail/cfe-dev/2017-February/052739.html
>> Beckhoff Automation GmbH & Co. KG | Managing Director: Dipl. Phys. Hans
>> Beckhoff
>> Registered office: Verl, Germany | Register court: Guetersloh HRA 7075
>>
>>
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>

-- 
Roger Ferrer Ibáñez
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