Inlined code
Chris Johns
chrisj at rtems.org
Tue Aug 7 00:43:08 UTC 2018
On 07/08/2018 05:14, Joel Sherrill wrote:
>
> On Mon, Aug 6, 2018 at 1:21 AM, Chris Johns <chrisj at rtems.org
> <mailto:chrisj at rtems.org>> wrote:
>
> On 06/08/2018 16:12, Christian Mauderer wrote:
> > Am 06.08.2018 um 07:31 schrieb Chris Johns:
> >> On 06/08/2018 10:51, Chris Johns wrote:
> >>> On 05/08/2018 19:39, Christian Mauderer wrote:
> >>>> Am 05.08.2018 um 04:00 schrieb Chris Johns:
> >>>>> Hi,
> >>>>>
> >>>>> I have been working on migrating covoar in the rtems-tools repo to
> DWARF. The
> >>>>> goal is remove objdump parsing and to get accurate details about the
> functions
> >>>>> being covered. This is an unfunded task.
> >>>>>
> >>>>> The work has resulted in a close examination of inlined code in RTEMS
> and what I
> >>>>> saw alarmed me so I have added a report to the rtems-exeinfo tool in
> rtems-tools
> >>>>> (the change is to be posted for review once I get the coverage tests
> running).
> >>>>>
> >>>>> A summary report for hello.exe on RTEMS 5 for SPARC is:
> >>>>>
> >>>>> inlined funcs : 1412
> >>>>> total funcs : 1956
> >>>>> % inline funcs : 72%
> >>>>> total size : 174616
> >>>>> inline size : 81668
> >>>>> % inline size : 46%
> >>>>>
> >>>>> This is a small application so it could be argued that skews the
> figures. A
> >>>>> large C/C++ application built with -O2 running on RTEMS 4.11 ARM
> reports the
> >>>>> inline usage as:
> >>>>>
> >>>>> inlined funcs : 10370
> >>>>> total funcs : 17700
> >>>>> % inline funcs : 58%
> >>>>> total size : 3066240
> >>>>> inline size : 1249514
> >>>>> % inline size : 40%
> >>>>>
> >>>>> This does not seem right to me.
> >>>>>
> >>>>> The report is new and there could be issues in the DWARF handling that
> feeds
> >>>>> this report however I am posting this to start a discussion on the
> topic of
> >>>>> inlining.
> >>>>>
> >>>>> I attach the report for hello.exe. The `-i` option generates the
> inline report.
> >>>>>
> >>>>> The first section is a summary showing the total number of functions
> in the
> >>>>> executable that have machine code and are flagged as inline. The
> report lists
> >>>>> the percentage of functions that are inlined and the percentage of
> machine code
> >>>>> that is inlined. The values seem high to me.
> >>>>>
> >>>>> The second table lists inline functions that are repeated sorted from the
> >>>>> largest foot print to the smallest. The first column the total size of
> machine
> >>>>> code in the executable and the second column the number of instances.
> >>>>>
> >>>>> The third table is the list of inline functions sorted from largest
> machine code
> >>>>> footprint to smallest. The second column are flags of which there is
> one. A `E`
> >>>>> indicates the inline function is also external which means the
> compiler has
> >>>>> created an external reference to this function, ie an address-of is
> being taken.
> >>>>> The third column is the address in the executable so you can take a
> look with
> >>>>> objdump at the machine code.
> >>>>>
> >>>>> We need to ask some important question in relation to inlining. It is
> cheap to
> >>>>> add and we all feel the code we add needs to be fast and needs to be
> inlined but
> >>>>> does it really need to be inlined?
> >>>>>
> >>>>> Some pieces of code do need to be inlined and the overhead is just that an
> >>>>> overhead, for example in the large C/C++ application there is a low level
> >>>>> volatile hardware write routing with close to 300 instances and a code
> size of
> >>>>> 10K. This code needs to be inlined for performance reasons but should
> the size
> >>>>> on average be 40 bytes, I doubt it.
> >>>>>
> >>>>> Can we be more judicious with our use of the inline keyword?
> >>>>>
> >>>>> Is the performance gain we really expect or is the actual overhead of
> a call
> >>>>> frame not worth saving?
> >>>>>
> >>>>> What are the real costs of inlining a piece of code? It adds size to the
> >>>>> executable and depending on the code being inlined it complicates coverage
> >>>>> analysis be adding extra branch points.
> >>>>>
> >>>>> The metrics to determine what should be inlined is complicated and I
> do not
> >>>>> think we have a suitable policy in place. I believe it is time we to
> create one.
> >>>>>
> >>>>> The issue is not limited to our code, gcc, newlib and libstdc++ seem
> to have
> >>>>> some code that should be looked at more closely. For example
> __udivmoddi4, and
> >>>>> __sprint_r.
> >>>>>
> >>>>> Chris
> >>>>>
> >>>>>
> >>>>
> >>>> Hello Chris,
> >>>>
> >>>> I just took a look at one of the first function in your list: __sprint_r
> >>>>
> >>>>
> https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libc/stdio/vfprintf.c;h=c4bf2dbe31da64462ecccec97c8e901e4ffadd44;hb=HEAD#l403
> <https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libc/stdio/vfprintf.c;h=c4bf2dbe31da64462ecccec97c8e901e4ffadd44;hb=HEAD#l403>
> >>>>
> >>>> As far as I can see, there is no explicit inline key word for that
> >>>> function. So in that case, the compiler decided that it would be a good
> >>>> idea to inline that function.
> >>>
> >>> Thanks and yes. At this point in time I cannot tell what is happening
> and I am
> >>> not sure the tool is reporting accurate data, I need to investigate.
> >>
> >> I have updated the tool and report to show which inline functions are:
> >>
> >> - inlined by compiler
> >> - declared inline and not inlined
> >> - declared inline and inlined
> >>
> >> I have also fixed a quick hack I had where the size was the span from the
> low PC
> >> to the high PC, this was wrong. Inlined code can be split and moved when
> >> inlining creating a discontinuous address range. The size in the report
> is now
> >> the number of machine code bytes.
> >>
> >> The report will show any functions not inlined when asked to be inlined.
> We do
> >> not have any.
> >>
> >> The 'C' flag in the inlined table shows which functions the compiler has
> inlined.
> >>
> >> Chris
> >>
> >
> > With that list it is now much clearer which functions would be relevant
> > for a potential review.
>
> Yes. FYI the C/C++ RTEMS 4.11 app now gives:
>
> inlined funcs : 10370
> total funcs : 17700
> % inline funcs : 58%
> total size : 2296354
> inline size : 479628
> % inline size : 20%
>
> This level of reduction is more inline with what I expected.
>
> >
> >>>
> >>>> I'm not sure whether I might just haven't seen it but is there a
> >>>> possibility to distinguish between functions that have been inlined by
> >>>> the compiler and ones that have been inlined due to the "inline" keyword
> >>>> without looking at every definition?
> >>>
> >>> I am not sure. The DWARF data is complex and detailed and I view this
> initial
> >>> step into the area of using DWARF to perform static analysis of RTEMS
> >>> executables as green.
> >>>
> >>> DWARF does provide declaration attributes. I need to review the DWARF
> data and
> >>> standard to determine if we can tell what is declared inline and what
> has been
> >>> inlined. I think it would be good to know.
> >>>
> >>>> Did you try compiling with size optimization? I would expect that the
> >>>> compiler would inline far less functions and maybe even ignore some
> >>>> "inline" keywords. As far as I know it's more of a hint to the compiler.
> >>>
> >>> Not yet. A complete tool build with those options is a lot of effort and
> I am
> >>> still not comfortable the report is accurate. I think this is something that
> >>> should be done at some point. I think it would create an interesting
> data point.
> >>>
> >>>> I would only worry about functions that are still inlined if size
> >>>> optimization is selected.
> >>>
> >>> I think we need to review the functions we currently have tagged as
> inline. I
> >>> think the only way we can do this is with real data.
> >>>
> >>>> That's the case when I tell the compiler to
> >>>> make the program as small as possible. In all other cases I want some
> >>>> well balanced optimum between speed and size. Inlining small functions
> >>>> is OK in that case if you ask me.
> >>>
> >>> How do you define this, ie what is the inline policy we use?
> >>> How do you audit this?
> >
> > Both questions are not simple to answer. It is most likely a case by
> > case decision. I think there are roughly two reasons for inlining:
> >
> > - The code is short enough that it is smaller if it is inlined compared
> > to a function call.
> >
> > - There is some performance reason.
> >
> > Anything else?
>
> Not for inlining rather for not inlining? I had a discussion with Joel last week
> about coverage and he said he had previously reviewed the inlines to reduce the
> branch counts because it complicates coverage. I would prefer he talks about
> this, it is his area.
>
>
> Thanks for the lead-in Chris.
>
> First, there are multiple types of coverage. Statement, Decision coverage,
> MC/DC, and object coverage. Based on various limiting factors which I
> am happy to discuss in more detail if someone is interested, we went
> with object coverage. I will briefly remind everyone of the limitations
> and guidelines we went into this with:
>
> + We didn't want instrumented code. Test as you fly, fly as you test.
> Plus there would need to be a way to get information generated by
> the instrumentation off the target.
>
> + Without gcc support for identifying sub-expressions in complex
> logical expressions, there is no way to do DC or MCDC. Further,
> we would have had to have a tool to interpret the non-existent
> information and know which entries in the truth tables for every
> expression had to be exercised for DC and MCDC.
>
> + gcov's reports are primarily statement coverage. And you can't get
> gcov reports without gcov output from runs which we still are working
> on. Also in terms of DO-178, statement coverage is generally Level C
> type coverage.
>
> We wanted higher level coverage for RTEMS and thus object
> coverage with emphasis on every generated branch statement being
> needed and exercisable as taken and not taken. This approach
> ensured we didn't have dead code.
>
> Plus this type of information could easily be obtained from simulator
> traces from Qemu and tsim. covoar was written to combine and
> correlate the information back to assembly and source and generate
> the reports.
>
> That's what led to the current coverage approach. Chris has been
> working to eliminate the use of command line tools (e.g. nm, objdump,
> etc) in favor of using DWARF directly.
>
> That's the background of the approach and how we got reports.
>
> Also remember that RTEMS testing is primarily via the public APIs
> which combined with coverage ensures we actually need all the
> internal code. If you can't reach it from a public API, it is dead.
>
> But the insights into RTEMS coding practices began when we started
> trying to improve the coverage percentages. Key point:
>
> IF CODE IS DUPLICATED AT THE SOURCE OR OBJECT LEVEL, IT HAS TO
> BE FULLY TESTED AT EACH INSTANCE OR INSTANTIATION.
>
> At the source level, sometimes we factored out a common helper method
> to avoid testing the same logic via multiple APIs. This reduced the number
> of test cases needed.
>
> We looked at a lot of generated assembly. Sometimes we would see
> large methods being inlined multiple times. This would increase the overall
> size of an RTEMS application. But size is not the only impact of inlining.
> If an inlined method has one or more if's in it, then the branch paths
> it includes are introduced EVERYWHERE it is inlined. When we
> had _Thread_Dispatch_enable, I recall it was used > 100 times and
> includes a branch in it. There was a build option to not inline this
> routine to avoid needing to add over 100 test cases.
>
> There is a delicate code size and complexity dance when deciding
> whether to inline something or not. There isn't an easy or obvous
> answer.
>
> + How large is the method?
>
> + How many times is it used?
>
> + How many branches/loops does it have (e.g. cyclomatic complexity)?
I would like to add Capstone [*] to rtems-tools to allow us to look into the
actual code and measure these sort of things.
[*] https://www.capstone-engine.org/
Chris
>
> In general, very small methods with no branches can be inlined in less
> code than the subroutine call takes. So these tend to be a win.
>
> Over a certain size, the number of times used multiplied by the inlined
> size can have a negative impact on code size. We saw this a lot when
> doing the code coverage improvements.
>
> If the inlined method has branches, is it possible or cost effective
> to write test cases for the branches introduced at EVERY use?
>
> I came away believing a few things:
>
> + The MISRA rule for using simple, single condition branch logic
> is really good for code coverage analysis. It simplifies it. Statement
> coverage and branch coverage become closer to equivalent. The
> resulting assembly code is also much easier to match to source.
>
> + Inlining a method with conditional logic has to be looked at
> very carefully. You don't want to inline something and find yourself
> having exploded the required number of test cases. Again, look
> at the number of times a method is used but be cautious about
> conditional expressions in inline methods.
>
> + Inlining doesn't always improve performance measurably. It
> can increase code size, code to branch around, etc.
>
> For a current example of something that is very likely a problem
> from a object code test coverage and code size increase view,
> consider _Thread_Dispatch_enable. It is inlined 41 times.
> Chris and I looked at a place where it was inlined. It was about
> 80 bytes. 41*80 ==> 3280 bytes. Based on size along, it would
> save ~3K across the entire RTEMS API object code to not inline it
> at all.
>
> Then I looked at the cyclomatic complexity of _Thread_Dispatch_enable().
> From Wikipedia:
>
> Cyclomatic complexity is a software metric, used to indicate the complexity
> of a program. It is a quantitative measure of the number of linearly independent
> paths through a program's source code.
>
> I used http://www.lizard.ws/ [1] to analyse this method. If you take the conditional
> compilation out of the code, the cyclomatic complexity is 4. This means that
> every place this is inlined, it introduces 3 extra paths to test. 41*3 ==> 123.
>
> My recommendation would be to evaluate the impact of not inlining this
> method at all. The negative impact on testing and code size are pretty high.
>
> Unfortunately, what I wrote up is just the guidelines. Things change over
> time. An inlined method that is a nice encapsulation when written could
> turn out to be too large to be inlined if 10 other use cases get added.
>
> I lean to a method can be inlined if it is small and no more than one
> branch statement in assembly. You can be more lenient if used only
> once or twice. But lean to NO branches on an inlined method if the
> number of uses is high.
>
> Ideally the inlined code is no larger than the code for the call itself
> and there are no branches. But going above that is not a reason to
> not inline.
>
> I know this was long but there are a lot of trade-offs. We want RTEMS to
> be as small as possible and as testable as possible. We don't have an
> army of people to write test cases. And I don't want to need that.
>
> Also I have focused on the things that the RTEMS community has control
> over. Chris' tool gives us a broader view and we can likely help improve
> things more broadly.
>
> [1] You can cut and paste code into this via the web. So pretty easy to
> look at a single method.
>
>
>
>
> Chris
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