[llvm-commits] [compiler-rt] r159294 - in /compiler-rt/trunk/lib: sanitizer_common/sanitizer_internal_defs.h tsan/rtl/tsan_defs.h tsan/rtl/tsan_flags.cc tsan/rtl/tsan_interceptors.cc tsan/rtl/tsan_md5.cc tsan/rtl/tsan_rtl_report.cc tsan/rtl/tsan_

[Dmitry Vyukov]

On Thu, Jun 28, 2012 at 2:09 AM, Chandler Carruth <chandlerc at google.com>wrote:

> It specifically prevents replacing your implementation of memcpy with a
> call to memcpy, resulting in an infloop. This will actually happen in some
> cases.


Why can't the same happen with a hosted impl?


I personally think we should also provide a way to avoid the introduction
> of *any* calls to memcpy, and instead synthesize the loops directly. That
> said, while we are close to that today, there are still a few places that
> will do this.
>

It would be nice. For some reason I still have a feeling that I am fighting
against implementation details of the particular compiler.



On Wed, Jun 27, 2012 at 3:04 PM, Dmitry Vyukov <dvyukov at google.com> wrote:
>
>> On Thu, Jun 28, 2012 at 1:56 AM, Chandler Carruth <chandlerc at google.com>wrote:
>>
>>> Unless you specify -fno-builtin, the compiler (even the low-level code
>>> generator) is free to insert a call to 'memcpy', assuming that a libc will
>>> eventually show up and provide that function. It may not be happening yet,
>>> but it will be eventually.
>>>
>>> No amount of always-inline annotations help this, because the calls may
>>> get inserted after inlining has taken place.
>>>
>>
>>
>> Since freestanding implementations provide memcpy, how can
>> -ffreestanding/-fno-builtin help with this?... Why a compiler is disallows
>> to insert memcpy's in this case?
>>
>>
>>
>>
>>> On Wed, Jun 27, 2012 at 2:44 PM, Dmitry Vyukov <dvyukov at google.com>wrote:
>>>
>>>>
>>>> On Thu, Jun 28, 2012 at 1:32 AM, Chandler Carruth <chandlerc at google.com
>>>> > wrote:
>>>>
>>>>> On Wed, Jun 27, 2012 at 2:16 PM, Dmitry Vyukov <dvyukov at google.com>wrote:
>>>>>
>>>>>> On Thu, Jun 28, 2012 at 1:07 AM, Chandler Carruth <
>>>>>> chandlerc at google.com> wrote:
>>>>>>
>>>>>>> I suspect you'll need to work still harder than this... =[
>>>>>>
>>>>>>
>>>>>> I know :)
>>>>>> Now I need to understand how exactly it is broken.
>>>>>>
>>>>>
>>>>>  Hmm, I may also need to better understand what you're attempting to
>>>>> achieve with this patch (based on Timur's comment).
>>>>>
>>>>> You define memset, memcpy, etc in a header file -- what callers use
>>>>> them? Most callers moved to 'internal_foo' versions.
>>>>>
>>>>> To answer Timur's question -- yes its ok, but it's hard to control
>>>>> which (instrumented or not-instrumented) gets used if they are inserted by
>>>>> LLVM, and thus have the normal names. It occurs to me that my post-process
>>>>> name-mangling tool would solve this problem as well...
>>>>>
>>>>>
>>>>>>
>>>>>> You'll need to compile with -fno-builtins in order to prevent LLVM's
>>>>>>> optimizer from recognizing your code as equivalent to 'memset' or 'memcpy',
>>>>>>> and replacing it with a call to those functions. =/
>>>>>>>
>>>>>>
>>>>>> I believe we are compiling with -fno-builtin.
>>>>>> You mentioned that I also need -ffreestanding:
>>>>>>
>>>>>
>>>>> Yea, -ffreestanding may be better. It is a superset of -fno-builtin
>>>>>
>>>>>
>>>>>>  "Yes, there are attributes which can be attached to the
>>>>>> non-instrumented memcpy function, provided by the runtime and selected due
>>>>>> to -ffreestanding, which will force inlining. __attribute__((always_inline)),
>>>>>> __attribute__((flatten))"
>>>>>>
>>>>>> Why do I need it?
>>>>>>
>>>>>
>>>>> Need which?
>>>>>
>>>>> You need '-ffreestanding' so that your source code can implement
>>>>> 'memset' itself, and that implementation will be used rather than assuming
>>>>> that at some point 'libc' will get linked in and we can call *its* memset.
>>>>> This is necessary even if your function isn't named 'memset' but something
>>>>> eles.
>>>>>
>>>>> The __attribute__((always_inline)) is needed if you want to force this
>>>>> function to be inlined into its callers. I don't recall off hand why this
>>>>> was important?
>>>>>
>>>>
>>>>
>>>> I need to prevent compiler from inserting memxxx() calls into random
>>>> places of the runtime. At first glance the patch achieves the goal.
>>>> always_inline is required to eliminate calls.
>>>> I do not fully understand why  -fno-builtins/-ffreestanding are
>>>> required. The idea is that the compiler leaves no calls to memxxx
>>>> (everything is inlined).
>>>>
>>>>
>>>>
>>>
>>
>
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