[Llvm-bgq-discuss] Implicit memset call

[Thomas Gooding]

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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