Messages in this thread | | | Date | Fri, 4 Jun 2021 11:51:54 -0400 | From | Alan Stern <> | Subject | Re: [RFC] LKMM: Add volatile_if() |
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On Fri, Jun 04, 2021 at 05:42:28PM +0200, Peter Zijlstra wrote: > On Fri, Jun 04, 2021 at 05:22:04PM +0200, Peter Zijlstra wrote: > > On Fri, Jun 04, 2021 at 04:13:57PM +0100, Will Deacon wrote: > > > > > In fact, maybe it's actually necessary to bundle the load and branch > > > together. I looked at some of the examples of compilers breaking control > > > dependencies from memory-barriers.txt and the "boolean short-circuit" > > > example seems to defeat volatile_if: > > > > > > void foo(int *x, int *y) > > > { > > > volatile_if (READ_ONCE(*x) || 1 > 0) > > > WRITE_ONCE(*y, 42); > > > } > > > > Yeah, I'm not too bothered about this. Broken is broken. > > > > If this were a compiler feature, the above would be a compile error. But > > alas, we're not there yet :/ and the best we get to say at this point > > is: don't do that then. > > Ha! Fixed it for you: > > #define volatile_if(cond) if (({ bool __t = (cond); BUILD_BUG_ON(__builtin_constant_p(__t)); volatile_cond(__t); }))
That won't help with more complicated examples, such as:
volatile_if (READ_ONCE(*x) * 0 + READ_ONCE(*y))
Alan
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