[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]

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.


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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