Re: ASM vs portable code [WAS: Re: Java DES breaker?]

[Bill Frantz <frantz@netcom.com>]

At 9:28 PM -0800 12/14/96, Dale Thorn wrote:
>I remember sitting down with some ASM programmers in the mid 1980's
>(using x86 PCs), and at that time, looking at the Codeview tracings,
>it occurred to me that ASM would nearly always run 2x faster than 'C',
>something that is inherent in the processes.

Modern compiler peephole optimizers are quite good, and there is not much
to be gained by trying to beat them.  The real gains come from being able
to make more restrictive assumptions than a compiler based on your superior
knowledge of the program.

For example, most operating system kernels have a global pointer to the
current process.  Assembly language kernels normally dedicate a register to
hold that pointer.  In C, each separately compiled routine must re-load it
from its memory location because they can not coordinate register usage.
Parameter passing is another place this kind of global register assignment
can improve assembly programs.

Another place where this global view of a program helps is in re-loads
after calling externally compiled routines.  The compiler must assume that
the external routine has changed the variable while a smart programmer can
know better and save the re-load.  Even if the data is in the level 1
cache, most architectures can do at most one memory reference instruction
per cycle, and memory accesses seem to be the critical path for OS kernels.

These optimizations work better with register rich architectures such as
the R4000, Sparc, PowerPC etc. than they do on the popular Intel
architecture because there are more registers to use.

BTW - My experience with Assembler over C is more like 4:1 than 2:1.  YMMV!


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