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    SubjectRe: PROPOSAL: Extend inline asm syntax with size spec
    Hi Segher,

    On Sun, 7 Oct 2018, Segher Boessenkool wrote:

    > On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote:
    > > this is an attempt to see whether gcc's inline asm heuristic when
    > > estimating inline asm statements' cost for better inlining can be
    > > improved.
    >
    > GCC already estimates the *size* of inline asm, and this is required
    > *for correctness*. So any workaround that works against this will only
    > end in tears.

    You're right and wrong. GCC can't even estimate the size of mildly
    complicated inline asms right now, so your claim of it being necessary for
    correctness can't be true in this absolute form. I know what you try to
    say, but still, consider inline asms like this:

    insn1
    .section bla
    insn2
    .previous

    or
    invoke_asm_macro foo,bar

    in both cases GCCs size estimate will be wrong however you want to count
    it. This is actually the motivating example for the kernel guys, the
    games they play within their inline asms make the estimates be wildly
    wrong to a point it interacts with the inliner.

    > So I guess the real issue is that the inline asm size estimate for x86
    > isn't very good (since it has to be pessimistic, and x86 insns can be
    > huge)?

    No, see above, even if we were to improve the size estimates (e.g. based
    on some average instruction size) the kernel examples would still be off
    because they switch sections back and forth, use asm macros and computed
    .fill directives and maybe further stuff. GCC will never be able to
    accurately calculate these sizes (without an built-in assembler which
    hopefully noone proposes).

    So, there is a case for extending the inline-asm facility to say
    "size is complicated here, assume this for inline decisions".

    > > Now, Richard suggested doing something like:
    > >
    > > 1) inline asm ("...")
    >
    > What would the semantics of this be?

    The size of the inline asm wouldn't be counted towards the inliner size
    limits (or be counted as "1").

    > I don't like 2) either. But 1) looks interesting, depends what its
    > semantics would be? "Don't count this insn's size for inlining decisions",
    > maybe?

    TBH, I like the inline asm (...) suggestion most currently, but what if we
    want to add more attributes to asms? We could add further special
    keywords to the clobber list:
    asm ("...." : : : "cc,memory,inline");
    sure, it might seem strange to "clobber" inline, but if we reinterpret the
    clobber list as arbitrary set of attributes for this asm, it'd be fine.

    > Another option is to just force inlining for those few functions where
    > GCC currently makes an inlining decision you don't like. Or are there
    > more than a few?

    I think the examples I saw from Boris were all indirect inlines:

    static inline void foo() { asm("large-looking-but-small-asm"); }
    static void bar1() { ... foo() ... }
    static void bar2() { ... foo() ... }
    void goo (void) { bar1(); } // bar1 should have been inlined

    So, while the immediate asm user was marked as always inline that in turn
    caused users of it to become non-inlined. I'm assuming the kernel guys
    did proper measurements that they _really_ get some non-trivial speed
    benefit by inlining bar1/bar2, but for some reasons (I didn't inquire)
    didn't want to mark them all as inline as well.


    Ciao,
    Michael.

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