Re: Chipsandcheese article on the CDC6600

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Sujet : Re: Chipsandcheese article on the CDC6600
De : anton (at) *nospam* mips.complang.tuwien.ac.at (Anton Ertl)
Groupes : comp.arch
Date : 25. Jul 2024, 11:59:16
Autres entêtes
Organisation : Institut fuer Computersprachen, Technische Universitaet Wien
Message-ID : <2024Jul25.125916@mips.complang.tuwien.ac.at>
References : 1 2 3 4 5 6 7 8 9 10 11 12 13
User-Agent : xrn 10.11
Lawrence D'Oliveiro <ldo@nz.invalid> writes:
On Wed, 24 Jul 2024 13:22:46 GMT, Scott Lurndal wrote:
>
Lawrence D'Oliveiro <ldo@nz.invalid> writes:
>
I was wondering how those code patches would
impact on shared code.
 
Global branch prediction, of course.
>
But the characteristics of a program run in one thread/process might not
match those in another.

They might not, or they might.  When the hardware branch predictor
researchers looked into it, they found that there is more synergy than
interference.  Consequently, they did not take measures to avoid
sharing.  And for the approach of patching the hints in the code, the
results of sharing will be beneficial on average, too, because the
only difference from the 2-bit/branch predictor is that the latter is
in microarchitectural state instead of in the code.

Now somebody will point out that sharing makes it possible for an
attacker to train branch predictors in one process to attack a
different process through Spectre and friends.  While preventing
sharing would close that, it does not close training the predictors in
the same thread.

Closing Spectre through invisible speculation (several papers exist
about that) makes it irrelevant (for Spectre) whether the predictors
are shared or not.  Of course, for invisible speculation the permanent
branch predictors must not be updated speculatively, but that's
probably better anyway.

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

Date Sujet#  Auteur
20 Jul 24 * Chipsandcheese article on the CDC660043Thomas Koenig
20 Jul 24 +* Re: Chipsandcheese article on the CDC660039Lawrence D'Oliveiro
21 Jul 24 i`* Re: Chipsandcheese article on the CDC660038John Savard
22 Jul 24 i `* Re: Chipsandcheese article on the CDC660037Lawrence D'Oliveiro
22 Jul 24 i  `* Re: Chipsandcheese article on the CDC660036MitchAlsup1
22 Jul 24 i   `* Re: Chipsandcheese article on the CDC660035Lawrence D'Oliveiro
22 Jul 24 i    `* Re: Chipsandcheese article on the CDC660034Niklas Holsti
22 Jul 24 i     +* Re: Chipsandcheese article on the CDC660026Michael S
22 Jul 24 i     i+- Re: Chipsandcheese article on the CDC66001Andreas Eder
22 Jul 24 i     i+* Re: Chipsandcheese article on the CDC66004MitchAlsup1
22 Jul 24 i     ii`* Re: Chipsandcheese article on the CDC66003Michael S
23 Jul 24 i     ii `* Re: Chipsandcheese article on the CDC66002Lawrence D'Oliveiro
23 Jul 24 i     ii  `- Re: Chipsandcheese article on the CDC66001Michael S
22 Jul 24 i     i+* Re: Chipsandcheese article on the CDC660016Anton Ertl
22 Jul 24 i     ii+* Re: Chipsandcheese article on the CDC66004MitchAlsup1
22 Jul 24 i     iii+- Re: Chipsandcheese article on the CDC66001Michael S
22 Jul 24 i     iii`* Re: Chipsandcheese article on the CDC66002Anton Ertl
22 Jul 24 i     iii `- Re: Chipsandcheese article on the CDC66001MitchAlsup1
22 Jul 24 i     ii+- Re: Chipsandcheese article on the CDC66001Thomas Koenig
22 Jul 24 i     ii+- Re: Chipsandcheese article on the CDC66001Anton Ertl
23 Jul 24 i     ii+* Re: Chipsandcheese article on the CDC66002Lynn Wheeler
23 Jul 24 i     iii`- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro
23 Jul 24 i     ii+- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro
23 Jul 24 i     ii`* Re: Chipsandcheese article on the CDC66006Lawrence D'Oliveiro
24 Jul 24 i     ii `* Re: Chipsandcheese article on the CDC66005Lawrence D'Oliveiro
25 Jul 24 i     ii  `* Re: Chipsandcheese article on the CDC66004Lawrence D'Oliveiro
25 Jul 24 i     ii   `* Re: Chipsandcheese article on the CDC66003Anton Ertl
25 Jul 24 i     ii    `* Re: Chipsandcheese article on the CDC66002MitchAlsup1
26 Jul 24 i     ii     `- Re: Chipsandcheese article on the CDC66001Anton Ertl
23 Jul 24 i     i+* Re: Chipsandcheese article on the CDC66002Lynn Wheeler
23 Jul 24 i     ii`- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro
23 Jul 24 i     i`* Re: Chipsandcheese article on the CDC66002Lawrence D'Oliveiro
23 Jul 24 i     i `- Re: Chipsandcheese article on the CDC66001Michael S
23 Jul 24 i     `* Re: Chipsandcheese article on the CDC66007John Savard
23 Jul 24 i      +* Re: Chipsandcheese article on the CDC66003MitchAlsup1
23 Jul 24 i      i+- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro
23 Jul 24 i      i`- Re: Chipsandcheese article on the CDC66001Lynn Wheeler
23 Jul 24 i      +- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro
23 Jul 24 i      `* Re: Chipsandcheese article on the CDC66002Michael S
23 Jul 24 i       `- Re: Chipsandcheese article on the CDC66001MitchAlsup1
20 Jul 24 `* Re: Chipsandcheese article on the CDC66003Terje Mathisen
21 Jul 24  +- Re: Chipsandcheese article on the CDC66001MitchAlsup1
21 Jul 24  `- Re: Chipsandcheese article on the CDC66001Lawrence D'Oliveiro

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