Messages in this thread | | | Date | Thu, 04 Oct 2018 02:17:24 -0700 | Subject | Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions | From | hpa@zytor ... |
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On October 4, 2018 2:12:22 AM PDT, Ingo Molnar <mingo@kernel.org> wrote: > >* Nadav Amit <namit@vmware.com> wrote: > >> I can run some tests. (@hpa: I thought you asked about the -pipe >overhead; >> perhaps I misunderstood). > >Well, tests are unlikely to show the overhead of extra lines of this >magnitude, unless done very carefully, yet the added bloat exists and >is not even >mentioned by the changelog, it just says: > >Subject: [PATCH v9 02/10] Makefile: Prepare for using macros for inline >asm > > Using macros for inline assembly improves both readability and >compilation decisions that are distorted by big assembly blocks that >use > alternative sections. Compile macros.S and use it to assemble all C > files. Currently, only x86 will use it. > >> I guess you regard to the preprocessing of the assembler. Note that >the C >> preprocessing of macros.S obviously happens only once. That’s the >reason >> I assumed it’s not that expensive. > >True - so first we build macros.s, and that gets included in every C >file build, right? > >macros.s is smaller: 275 lines only in the distro test build I tried, >which looks >a lot better than my first 4,200 lines guesstimate. > >> Anyhow, I remember that we discussed at some point doing something >like >> ‘asm(“.include XXX.s”)’ and somebody said it is not good, but I don’t >> remember why and don’t see any reason it is so. Unless I am missing >> something, I think it is possible to take each individual header and >> preprocess the assembly part of into a separate .s file. Then we can >put in >> the C part of the header ‘asm(".include XXX.s”)’. >> >> What do you think? > >Hm, this looks quite complex - macros.s is better I think. Also, 275 >straight assembly lines is >a lot better than 4,200. > >Another, separate question I wanted to ask: how do we ensure that the >kernel stays fixed? >I.e. is there some tooling we can use to actually measure whether >there's bad inlining decisions >done, to detect all these bad patterns that cause bad GCC code >generation? > >Thanks, > > Ingo
The assembly output from GCC is quite volumious; I doubt tacking a few hundred lines on will matter one iota. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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