Hi Floks,
At the bottom of this post is a simple test kernel that does a memset on a shared memory array on the device . However, copying the memory to the host and examining it shows that the memset does not always work.
On debug builds (compiled with –G0) memset always works.
On non-debug builds (compiled without –G0) memset never works on a GT 240
On non-debug builds (compiled without –G0) memset fails on a GTX 460 unless the memory is getting set to zero.
In other words, on GTX 460 non-debug builds memset(rgBuffer,5…) always fails but memset(rgBuffer,0,…) always works.
Examining the ptx files for each compilation shows the most probable culprit.
Without -G0 (version that fails):
.loc 28 107 0
mov.s32 %r5, %r1;
mov.s32 %r6, 500;
$Lt_0_3842:
// Loop body line 107, nesting depth: 1, iterations: 500
st.shared.u32 [%r5+0], %r7; <------ ** r7 is not initialized anywhere in the ptx file
add.u32 %r5, %r5, 4;
sub.s32 %r6, %r6, 1;
mov.u32 %r8, 0;
setp.ne.s32 %p2, %r6, %r8;
@%p2 bra $Lt_0_3842;
With -G0 (version that works):
$L_0_2562:
.loc 28 107 0
mov.s32 %r14, 0; <---------**Notice how this version initializes the register used for the memset
mov.u32 %r15, __cuda___cuda_local_var_67239_29_rgBuffer4;
mov.s32 %r16, %r15;
mov.s32 %r17, 500;
$L_0_2818:
.loc 28 107 0
st.shared.u32 [%r16+0], %r14;
add.u32 %r16, %r16, 4;
sub.s32 %r17, %r17, 1;
mov.u32 %r18, 0;
setp.gt.s32 %p3, %r17, %r18;
@%p3 bra $L_0_2818;
Notice how the version that works initializes the register used (second line) for the memset and the version that does not work fails to do this. Adding the missing register initialization to the ptx file of the version that fails and manually building the executable fixes the problem.
Is this a known issue? (I could not find anything in the forums)
How come GTX 460 seems to always work even without the register initialization?
Kernel code:
#define BUFFSIZE 500
global void MemsetTestCUDA( int* piBuffer )
{
shared int rgBuffer[BUFFSIZE];
int i;
for( i = 0; i < BUFFSIZE; i++ )
{
rgBuffer[i] = 1; // make sure non zero
}
memset( rgBuffer, 0, sizeof(int) * BUFFSIZE ); // zero out memory
for( i = 0; i < BUFFSIZE; i++ )
{
piBuffer[i] = rgBuffer[i]; // copy to client buffer
}
}
#include <windows.h>
#include <stdio.h>
void MemsetTest()
{
void* pvBuffer;
int i;
int rgBuffer[BUFFSIZE];
cudaMalloc( &pvBuffer, sizeof(int) * BUFFSIZE );
MemsetTestCUDA<<< 1, 1>>>( (int*)pvBuffer );
cudaMemcpy( rgBuffer, pvBuffer, sizeof(int) * BUFFSIZE, cudaMemcpyDeviceToHost );
for( i = 0; i < BUFFSIZE; i++ )
{
if( rgBuffer[i] != 0 )
{
goto Error;
}
}
printf("\r\n---------------------- MemsetTest Passed--------------------------" );
return;
Error:
printf(“\r\n---------------------- MemsetTest Failed!--------------------------” );
}
