advice statement

Steven M. Haflich smh at mit-eddie.UUCP
Tue Aug 28 22:33:08 AEST 1984

It has been remarked that program flow-control advice given to a
compiler will rarely yield significant speed increase.  This is true for
much code running on normal architectures.  However, pipelined and/or
vector processors can profit greatly by knowing which path to lookahead.
The programmer needn't bother providing advice except for code fragments
of his choosing.  A significant drawback is that programmers are
probably too lazy to provide advice as often as they should.  Some
pipelined RISC machines (which *need* such advice) solve this problem
marvelously by keeping some sort of branch-frequency record right in the
running object code, the microcode updating it on the fly.

Now for a bit of arcane ancient history:  In the middle (early?) 1960s,
IBM 1620 FORTRAN II had a FREQUENCY statement which allowed the
programmer to give branch advice.  (Let me tell you, with the speed of a
1620, the code generator needed all the advice it could get!)  So far as
I know, the FREQUENCY statement was ignored by the compiler and was
never implemented.  But it suggested a principle which still holds:  A
compiler amplifies a programmer, but cannot be truly efficient unless
programmers are willing to tell it enough so it can figure out what
needs to be done.

Steve Haflich

More information about the Comp.lang.c mailing list