The instructions are not documented in the Intel SDM but are documented
in the AMD APM as an alias to the group 2, ModRM.reg == 4 variant.
Both AMD and Intel CPUs execute the C[0-1] and D[0-3] instructions as
expected, i.e., like the /4 aliases:
#include <stdio.h>
int main(void)
{
int a = 2;
printf ("a before: %d\n", a);
asm volatile(".byte 0xd0,0xf0" /* SHL %al */
: "+a" (a));
printf("a after : %d\n", a);
return 0;
}
$ ./a.out
a before: 2
a after : 4
