> that branch prediction got better in the ‘10s and a bunch of techniques that didn’t work before do now.
They got better than they had any right to be, but then we found out that Spectre & Meltdown were vulnerabilities rather than optimizations.
For example, a switch based interpreter was fast as a CGOTO one for a brief period between 2012 and 2018, but suddenly got slower again as the CPUs could no longer rely on branch prediction to do prefetching.
While better predictors allow the speculation window to be larger on average, the the real culprit is that large speculation window. Even if the branch predictor weren't very smart, it will still do well on a program with stable, predictable branches, thus allowing a large speculation window to open up. The vulnerability is that some of those branches guard really important things, like not going out-of-bounds of an array. So a Spectre attack, which works by exploiting a mispredicted branch, is a constructive attack where the gadget is tuned for the branch predictor anyway. The other part of an attack, the windowing gadget, just relies on making a really slow input into a branch. Neither of them would be particularly harder with a dumb predictor.
They got better than they had any right to be, but then we found out that Spectre & Meltdown were vulnerabilities rather than optimizations.
For example, a switch based interpreter was fast as a CGOTO one for a brief period between 2012 and 2018, but suddenly got slower again as the CPUs could no longer rely on branch prediction to do prefetching.