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acnpxy
2020 Jul 28
2
_mm_lfence in both pathes of an if/else are hoisted by SimplfyCFG potentially breaking use as a speculation barrier
_mm_lfence was originally documented as a load fence. But in light of
speculative execution vulnerabilities it has started being advertised as a
way to prevent speculative execution. Current Intel Software Development
Manual documents it as "Specifically, LFENCE does not execute until all
prior instructions have completed locally, and no later instruction begins
execution until LFENCE
2020 Aug 09
2
_mm_lfence in both pathes of an if/else are hoisted by SimplfyCFG potentially breaking use as a speculation barrier
Hi Craig,
The review for the similar GPU problem is now up here:
https://reviews.llvm.org/D85603 (+ some other patches on the
Phabricator stack).
>From a pragmatic perspective, the constraints added to program
transforms there are sufficient for what you need. You'd produce IR
such as:
%token = call token @llvm.experimental.convergence.anchor()
br i1 %c, label %then, label %else