"David Mathog" <mat...@caltech.edu> writes:
> Can somebody please explain the behavior of the following program
> to me?
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which is for the development of gcc itself. It would be appropriate on
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> cat >test.c <<EOD
> #include <stdio.h>
> #include <stdlib.h>
> #include <xmmintrin.h>
>
> int main(void){
> register __m128 var;
> fprintf(stdout,"pre %X\n",var);
> var = _mm_setzero_ps();
> fprintf(stdout,"post %X\n",var);
> fprintf(stdout,"zerof %X\n",0.0f);
> exit(EXIT_SUCCESS);
> }
> EOD
> gcc -O0 -g -std=gnu99 -msse -mno-sse2 -m32 -o test test.c
> ./test
> pre FFC5FC98
> post FFC5FC98
> zerof 0
>
>
> gcc --version
> gcc (GCC) 4.4.1
>
> Now I know that var is 16 bytes, not 4, so the %X isn't appropriate to
> show all of it, but apparently it doesn't show any of it, since any 4
> bytes out of the 16 should have been 0.
SSE variables are passed in xmm registers. %X prints an unsigned int
value; unsigned int values are passed in regular registers. The %X is
seeing random garbage, it's not seeing the variable you are passing.
Ian
