The following two instructions:

Add REG, CONST

Sub REG, -CONST

are *NOT* equivalent! They set the carry flag in opposite directions. I think this guy is just getting lucky in that most programs don't check the carry flag. However, source that looks like this:

A -= 3;

if (A > 0) ...

which *could* crop up, can test the carry flag if you are using a good optimizing compiler. The guy has a good idea (there are *OTHER* redundancies in the x86 instruction set) but replacing ADD with SUB is just completely broken.

--

http://bstring.sourceforge.net/

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Link to this comment: http://www.securityfocus.com/comments/articles/2623/18523#18523