Re: x86 code generation question

[Robert Dewar <dewar at gnat dot com>]

Bernd Jendrissek wrote:

> 	cmp ax,bx
> 	sbb dx,dx
> 	; dx is now 0 if ax >= bx, -1 if ax < bx
> 	neg dx

> Have you used superopt?  It finds some pretty surprising sequences to do
> some nifty stuff.  AFAIK some of GCC's sequences are inspired by
> superopt, which, in fact, (and again, IIRC) was written precisely to
> find such beyond-humanly clever sequences for GCC.

I certainly agree that superopt is nice, but this fundamental
use of sbb is not exactly in the category of beyond-humanly
clever :-)

In fact the Realia COBOL generates this all over the place to
avoid jumps (e.g. you can easily do a max or min with this
trick without jumps), and indeed I regard this idiom with
sbb to be in the standard aresenal of asm coding on this
machine.