[LLVMdev] How to prevent insertion of memcpy()

[Jeffrey Yasskin]

On Tue, May 29, 2012 at 10:50 AM, Chandler Carruth <chandlerc at google.com> wrote:
> On Tue, May 29, 2012 at 10:46 AM, Dmitry Vyukov <dvyukov at google.com> wrote:
>>
>> On Tue, May 29, 2012 at 9:40 PM, Chandler Carruth <chandlerc at google.com>
>> wrote:
>>>>>>>
>>>>>>> > How do I disable that feature? I've tried -fno-builtin and/or
>>>>>>> > -ffreestanding
>>>>>>> > with no success.
>>>>>>> clang (as well as gcc) requires that freestanding environment
>>>>>>> provides
>>>>>>> memcpy, memmove, memset and memcmp.
>>>>>>>
>>>>>>> PS: Consider emailing cfedev, not llvmdev.
>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Thanks. I've emailed cfe-dev.
>>>>>> We absolutely need clang/llvm to not insert the calls into our code.
>>>>>
>>>>>
>>>>> This really isn't possible.
>>>>>
>>>>> The C++ standard essentially requires the compiler to insert calls to
>>>>> memcpy for certain code patterns.
>>>>>
>>>>> What do you really need here? Clearly you have some way of handling
>>>>> when the user writes memcpy; what is different about Clang or LLVM inserting
>>>>> memcpy?
>>>>
>>>>
>>>> I need it for ThreadSanitizer runtime. In particular
>>>>
>>>> http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/tsan/rtl/tsan_interceptors.cc?view=annotate
>>>> line 1238. But I had similar problems in other places.
>>>> Both memory access processing and signal handling are quite tricky, we
>>>> can't allow recursion.
>>>
>>>
>>> The first thing to think about is that you *do* need to use -fno-builtin
>>> / -ffreestanding when compiling the runtime because it provides its own
>>> implementations of memcpy.
>>
>>
>> We used both at some points in time, but the problem is that they do not
>> help to solve the problem. I think we use -fno-builtin now, I am not sure
>> about -ffreestanding.
>>
>>> The second is that there is no way to write fully generic C++ code w/o
>>> inserting calls to memcpy. =/ If you are writing your memcpy implementation,
>>> you'll have to go to great lengths to use C constructs that are guaranteed
>>> to not cause this behavior, or to manually call an un-instrumented memcpy
>>> implementation. I don't know of any easy ways around this.
>>
>>
>> What are these magic constructs. I had problems with both struct copies
>> and for loops.
>
>
> Don't copy things by value ever. =/ It is really, *really* hard to do this.
> If at all possible, I would build your runtime against an un-instrumented
> memcpy (perhaps defined within the runtime), and then use aliases or other
> techniques to wrap the instrumented functions in the exported names
> necessary for use when intercepting memcpy calls from the instrumented
> program.

There are some other platforms that absolutely can't tolerate function
calls. Do they have an attribute or pass to tell LLVM to inline any
functions it or clang inserts? Could Dmitry do the same thing?