[Llvm-bgq-discuss] Implicit memset call

[Jeff Hammond]

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