[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_

[Chandler Carruth]

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

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.


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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