[Llvm-bgq-discuss] Implicit memset call

[Jeff Hammond]

Per http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-May/050112.html,
you might want to try -fno-builtin and/or -ffreestanding.

Jeff

On Thu, Mar 20, 2014 at 11:17 AM, Jeff Hammond <jhammond at anl.gov> wrote:
> As best I can tell, this bug is upstream as well.  I see the same
> thing happen on my Mac with Clang 3.4.
>
> Jeff
>
> On Thu, Mar 20, 2014 at 11:06 AM, Thomas Gooding <tgooding at us.ibm.com> wrote:
>> Hello,
>>
>> I have some code that implements the memset() routine.  It appears that
>> bgclang is getting (too) clever by detecting the setting-loop.  Once it
>> detects this, it decides the "real" memset would be more efficient and calls
>> that routine inplace of the setting-loop.  Very clever, but not optimal for
>> me (and functionally incorrect since in the real code, it results in a
>> recursive bomb calling memset repeatedly)
>>
>> Is there a way to disable this feature?
>>
>> bgclang -g -O2 t.c
>>
>> #include <stdio.h>
>> #include <stdint.h>
>>
>> char buffer[24];
>>
>> void* my_memset(void* dest, int value, size_t n )
>> {
>>     printf("memset: dest=%p size=%ld\n", dest, n);
>>     uint8_t* d = (uint8_t*)dest;
>>     while (n--)
>>         *d++ = value;                      // this while loop replaced with
>> memset() call
>>     return dest;
>> }
>>
>> int main(int argc, char** argv)
>> {
>>     my_memset(buffer, 0, sizeof(buffer));
>>     printf("done\n");
>>     return 0;
>> }
>>
>> 00000000010003c0 <.my_memset>:
>>  10003c0:       7c 08 02 a6     mflr    r0
>>  10003c4:       fb e1 ff f8     std     r31,-8(r1)
>>  10003c8:       f8 01 00 10     std     r0,16(r1)
>>  10003cc:       f8 21 ff 61     stdu    r1,-160(r1)
>> ....
>>  1000408:       7b 84 06 20     clrldi  r4,r28,56
>>  100040c:       7f c3 f3 78     mr      r3,r30
>>  1000410:       7f a5 eb 78     mr      r5,r29
>>  1000414:       48 00 d0 7d     bl      100d490 <.memset>
>>
>> Tom Gooding
>> Senior Engineer / Blue Gene SW Lead / CAPI
>> tgooding at us.ibm.com   507-253-0747
>>
>>
>> _______________________________________________
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>>
>
>
>
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> University of Chicago Computation Institute
> jhammond at anl.gov / jhammond at uchicago.edu / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond



-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at anl.gov / jhammond at uchicago.edu / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond