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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: auto predicating branches
Date: Wed, 23 Apr 2025 18:09:28 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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Stefan Monnier <monnier@iro.umontreal.ca> writes:
>Of course, nothing prevents a CPU from doing "predicate prediction":
>instead of waiting for an answer to `needle < *base[middle]`, it could
>try and guess whether it will be true or false and thus choose to send
>one of the two addresses (or both) to the memory (and later check the
>prediction and rollback, just like we do with normal branches).

That would subvert the use of predication for getting constant-time
code for avoiding timing side channels.  I.e., not a good idea.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>