[Info-vax] x86-64 data aligment / faulting

[Joukj]

Arne Vajhøj wrote:
> On 2/27/2022 8:56 PM, Arne Vajhøj wrote:
>> I simply can't get a big difference between aligned and unaligned access.
>>
>> Data at the bottom.
>>
>> I am not saying that there are no cases where unaligned data access
>> make a significant difference.
>>
>> But I have not been able to come up with a case.
> 
> Code below in case anybody wonder WTF I am doing.
> 
> Arne
> 
>       PROGRAM TEST_ALIGN
>       INTEGER*4 N,REP,OFFSET
>       PARAMETER (N=2500000,REP=100,OFFSET=17)
>       BYTE DUMMY(OFFSET)
>       REAL*8 X(N+(OFFSET-1)/8+1)
>       EQUIVALENCE (DUMMY,X)
>       INTEGER*4 I,J,SCALE(4)
>       DATA SCALE/1000,100,10,1/
>       DO 200 J=1,REP
>         DO 100 I=1,N+1
>           X(I)=I
> 100     CONTINUE
> 200   CONTINUE
>       DO 400 J=1,4
>         WRITE(6,700) N/SCALE(J),8*N/SCALE(J),REP*SCALE(J),
>      +               'SEQUENTIAL ACCESS'
>         DO 300 I=1,OFFSET
>             CALL TEST(I,DUMMY(I),N/SCALE(J),REP*SCALE(J),.FALSE.)
> 300     CONTINUE
> 400   CONTINUE
>       DO 600 J=1,4
>         WRITE(6,700) 2**(9+3*J),8*2**(9+3*J),REP*SCALE(J)/6,
>      +               'RANDOM ACCESS'
>         DO 500 I=1,OFFSET
>             CALL TEST(I,DUMMY(I),2**(9+3*J),REP*SCALE(J)/6,.TRUE.)
> 500     CONTINUE
> 600   CONTINUE
> 700   FORMAT(1X,'DATA SIZE = ',I7,' (',I8,' BYTES), REP = ',I6,', ',A)
>       END
> 
> and:
> 
>       SUBROUTINE TEST(IX,X,N,REP,RANACC)
>       INTEGER*4 IX,N,REP
>       REAL*8 X(*)
>       LOGICAL*4 RANACC
>       INTEGER*4 I,T1,T2,DUMMY,RANIX
>       CALL SYSTEM_CLOCK(T1,DUMMY,DUMMY)
>       DO 200 J=1,REP
>         IF(RANACC) RANIX=0
>         DO 100 I=1,N
>           IF(RANACC) THEN
>             RANIX=MOD(401*RANIX+1,N)
>             X(RANIX+1)=I
>           ELSE
>             X(I)=I
>           ENDIF
> 100     CONTINUE
> 200   CONTINUE
>       CALL SYSTEM_CLOCK(T2,DUMMY,DUMMY)
>       IF(RANACC) THEN
>         WRITE(6,300) IX-1,MOD(LOC(X),8),T2-T1
>       ELSE
>         WRITE(6,300) IX-1,MOD(LOC(X),8),T2-T1
>       ENDIF
>       RETURN
> 300   FORMAT(1X,'OFFSET ',I2,' (ADDRESS MOD 8 = ',I1,'): ',I6,' ms')
>       END
> 

As others commented: The compiler may have optimized the allignment away 
by calling subroutine test. Maybe you should force the misalligment by 
using common-blocks instead of passing as variables.

                Jouk