Displaying 2 results from an estimated 2 matches for "qmc446".
2020 Jul 28
2
_mm_lfence in both pathes of an if/else are hoisted by SimplfyCFG potentially breaking use as a speculation barrier
...ng test, my intention was to ensure that the body of either
the if or the else would not proceed until any speculation of the branch
had resolved. But SimplifyCFG saw that both control paths started with an
lfence so hoisted it into a single lfence intrinsic before the branch.
https://godbolt.org/z/qMc446 The intrinsic in IR has no properties so it
should be assumed to read/write any memory. But that's not enough to
specify this control flow dependency. gcc also exhibits a similar behavior.
#include <x86intrin.h>
void bar();
void baz();
void foo(int c) {
if (c) {
_mm_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
...ng test, my intention was to ensure that the body of either the if or the else would not proceed until any speculation of the branch had resolved. But SimplifyCFG saw that both control paths started with an lfence so hoisted it into a single lfence intrinsic before the branch. https://godbolt.org/z/qMc446 The intrinsic in IR has no properties so it should be assumed to read/write any memory. But that's not enough to specify this control flow dependency. gcc also exhibits a similar behavior.
> >
> > #include <x86intrin.h>
> >
> > void bar();
> > void baz();...