Messages in this thread | | | Date | Fri, 9 Apr 2021 22:09:26 +0200 | From | Peter Zijlstra <> | Subject | Re: static_branch/jump_label vs branch merging |
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On Fri, Apr 09, 2021 at 03:21:49PM -0400, David Malcolm wrote: > [Caveat: I'm a gcc developer, not a kernel expert] > > But it's not *quite* a global constant, or presumably you would be > simply using a global constant, right? As the optimizer gets smarter, > you don't want to have it one day decide that actually it really is > constant, and optimize away everything at compile-time (e.g. when LTO > is turned on, or whatnot).
Right; as I said, the result is not a constant, but any invocation ever, will return the same result. Small but subtle difference :-)
> I get the impression that you're resorting to assembler because you're > pushing beyond what the C language can express.
Of course :-) I tend to always push waaaaay past what C considers sane. Lets say I'm firmly in the C-as-Optimizing-Assembler camp :-)
> Taking things to a slightly higher level, am I right in thinking that > what you're trying to achieve is a control flow construct that almost > always takes one of the given branches, but which can (very rarely) be > switched to permanently take one of the other branches, and that you > want the lowest possible overhead for the common case where the > control flow hasn't been touched yet?
Correct, that's what it is. We do runtime code patching to flip the branch if/when needed. We've been doing this for many many years now.
The issue of today is all this clever stuff defeating some simple optimizations.
> (and presumably little overhead for when it > has been?)... and that you want to be able to merge repeated such > conditionals.
This.. So the 'static' branches have been upstream and in use ever since GCC added asm-goto, it was in fact the driving force to get asm-goto implemented. This was 2010 according to git history.
So we emit, using asm goto, either a "NOP5" or "JMP.d32" (x86 speaking), and a special section entry into which we encode the key address and the instruction address and the jump target.
GCC, not knowing what the asm does, only sees the 2 edges and all is well.
Then, at runtime, when we decide we want the other edge for a given key, we iterate our section and rewrite the code to either nop5 or jmp.d32 with the correct jump target.
> It's kind of the opposite of "volatile" - something that the user is > happy for the compiler to treat as not changing much, as opposed to > something the user is warning the compiler about changing from under > it. A "const-ish" value?
Just so. Encoded in text, not data.
> Sorry if I'm being incoherent; I'm kind of thinking aloud here.
No problem, we're way outside of what is generally considered normal, and I did somewhat assume people were familiar with our 'dodgy' construct (some on this list are more than others).
I hope it's all a little clearer now.
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