It's been three years, let's examine how things have evolved.
I'm using commit db3d11ee3f0, which is a bit behind current master, just
so I can apply my "[PATCH 0/4] Cleanups around qemu-common.h" cleanly.
Markus Armbruster <arm...@redhat.com> writes:
[...]
> = The status quo and why I hate it =
>
> I've seen several schools of thought on use of #include.
>
> There's the "no #include in headers" school: every .c file includes
> exactly the headers it needs, and the prerequisites they need. Cyclic
> inclusion becomes impossible. You can't sweep cyclic dependencies under
> the rug. Headers are read just once per compilation unit. The amount
> of crap you include is clearly visible. However, maintaining the
> #include directives is a drag, not least because their order matters.
> Especially when headers neglect to spell out their dependencies. Or
> they do, but it's wrong.
>
> There's the "headers must be self-contained" school: every header
> includes everything it needs. Headers can be included in any order.
> Sorted #include directives are tidy and easy to navigate. Headers can
> be read multiple times, which can only hurt compilation time.
Our compilers avoid this for headers with proper header guards.
> You need
> to make an effort to avoid cyclic dependencies and excessive inclusion.
>
> And then there's the school of non-thought: when it doesn't compile,
> sprinkle #include on the mess semi-randomly until it does.
>
> We do a bit of all three, but the result looks awfully close to what the
> school of non-thought produces.
>
> Every .c file includes qemu/osdep.h first. For me, a .c file that
> includes nothing but that comes out well over half a Megabyte in >23k
> lines preprocessed. Where does all this crap come from?
>
> #lines KiBytes #files source
> 5233 102 5 QEMU
> 8035 159 70 system
> 7915 224 73 GLib
> 2458 89 1 # lines
> 23641 576 149 total
>
> "# lines" are lines added by the preprocessor so the rest of the
> compiler can keep track of source locations.
#lines KiBytes #files source
375 8 5 QEMU
9722 230 113 system
8212 254 74 GLib
1517 65 N/A # lines
19826 557 192 total
The weight QEMU lost, system + GLib put on.
> Having the compiler wade through almost half a Megabyte of system+GLib
> crap before it begins to consider the code we care about feels wasteful.
> Perhaps we should rethink our approach to including library headers.
No change.
> Of the 102K that are actually our own, just 7K come from include/. 95K
> come from qapi-types.h.
Fixed.
> Judging from the .d files in my build tree, 95% of the .c files include
> qemu-common.h. That makes things a good deal worse.
Down to 90%. My "[PATCH 0/4] Cleanups around qemu-common.h" shrinks it
to less than 10%. Small enough for me not to repeat the measurements
below.
> Without
> NEED_CPU_H, this adds a modest 44K of our own headers, but almost 100K
> of system headers:
>
> #lines KiBytes #files source
> 6938 146 16 QEMU
> 11426 254 74 system
> 7915 224 73 GLib
> 2658 100 1 # lines
> 28937 726 164 total
>
> NEED_CPU_H adds another 120K of our own headers:
>
> #lines KiBytes #files source
> 11534 263 43 QEMU
> 11548 256 78 system
> 7915 225 72 GLib
> 3370 138 1 # lines
> 34367 883 194 total
>
> The average size of a .c file is just over 15KiB. To get to the actual
> C code there, the compiler has to wade through at least 550-880KiB of
> headers. In other words, roughly 2% of the source comes from .c in the
> best case.
>
> But that's not even the worst part. The worst part by far are our
> "touch this and recompile the world" headers.
>
> I find just short 4000 .d files in my build tree.
Some 6400 now, ignoring the .d that don't contain ".o:".
> Guess how many of our
> headers are listed as prerequisites in more than 90% of them (thus
> touching them will recompile the .c file)? *Twenty-two*.
Down to 12 before my "[PATCH 0/4] Cleanups around qemu-common.h", and to
10 afterwards.
> Almost fifty
> recompile more half of the world.
No significant change.
> Naturally, touching osdep.h or anything it includes recompiles the
> world. These are:
>
> config-host.h
> include/glib-compat.h
> include/qapi/error.h
> include/qemu/compiler.h
> include/qemu/osdep.h
> include/qemu/typedefs.h
> include/sysemu/os-posix.h
> qapi-types.h
>
> NEED_CPU_H adds
>
> config-target.h
>
> Fine, except for qapi/error.h and qapi-types.h. The latter is an itch I
> need to scratch urgently. My first patch series will take a swing at
> it.
Both are gone.
New: include/exec/poison.h
[...]
>
> A fun exercise is to count occurences of each header in .d files and
> multiply their number by their size. That's the number of bytes read
> from them when compiling from scratch. Top scorers:
>
> size * count size count
> 525760413 698221 753 trace/generated-tracers.h
> 298039140 93723 3180 qapi-types.h
> 197442619 55759 3541 include/qom/object.h
> 185845916 53884 3449 include/exec/memory.h
> 143750444 36878 3898 /usr/include/glib-2.0/glib/gunicode.h
> 117362690 30643 3830 include/fpu/softfloat.h
> 109783272 28164 3898 /usr/include/glib-2.0/glib/gregex.h
> 105830700 27150 3898 /usr/include/glib-2.0/glib/gvariant.h
> 92972157 123469 753 trace/generated-events.h
> 88706786 22757 3898 /usr/include/glib-2.0/glib/gtestutils.h
After my "[PATCH 0/4] Cleanups around qemu-common.h":
size * count size count
428019130 82789 5170 include/exec/memory.h
336965209 60857 5537 include/qom/object.h
248105382 39121 6342 /usr/include/glib-2.0/glib/gunicode.h
187247550 29525 6342 /usr/include/glib-2.0/glib/gvariant.h
182359220 172037 1060 trace-root.h
178178490 28095 6342 /usr/include/glib-2.0/glib/gregex.h
167477688 71848 2331 qapi/qapi-types-block-core.h
166926546 36947 4518 include/qom/cpu.h
161105826 25403 6342 /usr/include/glib-2.0/glib/gmessages.h
153508110 24205 6342 /usr/include/glib-2.0/glib/gtestutils.h
[...]
>
> = What to do about it =
>
> The immediately obvious thing to do is reduce "recompile the world"
> headers that change frequently. I've started to do that.
>
> Another one is attacking widely included bulky files (see "Top
> scorers"). Some can simply be included less. Others need to be split,
> in particular the generated tracers.
Some progress, notably around trace and QAPI.
> Yet another one is reviewing the way we include system and GLib headers.
Not attempted.
> But our root problem is our undisciplined use of #include. Can we agree
> on a sane set of rules? Here's my proposal:
>
> 1. Have a carefully curated header that's included everywhere first. We
> got that already thanks to Peter: osdep.h.
>
> 2. Headers should normally include everything they need beyond osdep.h.
> If exceptions are needed for some reason, they must be documented in
> the header. If all that's needed from a header is typedefs, put
> those into qemu/typedefs.h instead of including the header.
>
> 3. Cyclic inclusion is forbidden.
>
> Nice to have: "make check" checks 2. and 3.
See my "[RFC v4 0/7] Baby steps towards saner headers".
[...]
Headers that are prerequisites of more than 2000 .o:
6312 config-host.h
2827 include/block/aio.h
4537 include/disas/dis-asm.h
3634 include/exec/cpu-all.h
5177 include/exec/cpu-common.h
3634 include/exec/cpu-defs.h
5401 include/exec/hwaddr.h
5395 include/exec/memattrs.h
5170 include/exec/memory.h
3514 include/exec/memory_ldst.inc.h
3514 include/exec/memory_ldst_cached.inc.h
3514 include/exec/memory_ldst_phys.inc.h
3517 include/exec/ramlist.h
5699 include/fpu/softfloat-types.h
6312 include/glib-compat.h
5429 include/hw/hotplug.h
2423 include/hw/hw.h
5437 include/hw/irq.h
5428 include/hw/qdev-core.h
2520 include/hw/qdev-properties.h
2297 include/hw/qdev.h
2425 include/migration/qemu-file-types.h
2564 include/migration/vmstate.h
2502 include/qapi/error.h
6063 include/qapi/util.h
5939 include/qemu/atomic.h
5494 include/qemu/bitmap.h
5673 include/qemu/bitops.h
5697 include/qemu/bswap.h
6312 include/qemu/compiler.h
3701 include/qemu/coroutine.h
2838 include/qemu/event_notifier.h
5683 include/qemu/host-utils.h
4680 include/qemu/int128.h
3701 include/qemu/lockable.h
3187 include/qemu/log-for-trace.h
2230 include/qemu/log.h
2483 include/qemu/main-loop.h
5568 include/qemu/module.h
5137 include/qemu/notify.h
6312 include/qemu/osdep.h
5561 include/qemu/processor.h
5560 include/qemu/qsp.h
5863 include/qemu/queue.h
5178 include/qemu/rcu.h
5398 include/qemu/rcu_queue.h
5178 include/qemu/sys_membarrier.h
5560 include/qemu/thread-posix.h
5560 include/qemu/thread.h
4356 include/qemu/timer.h
6312 include/qemu/typedefs.h
2008 include/qemu/uuid.h
4518 include/qom/cpu.h
5537 include/qom/object.h
6312 include/sysemu/os-posix.h
2456 include/sysemu/reset.h
2000 include/sysemu/sysemu.h
6062 qapi/qapi-builtin-types.h
2821 qapi/qapi-types-block-core.h
2669 qapi/qapi-types-block.h
3806 qapi/qapi-types-common.h
2860 qapi/qapi-types-crypto.h
2825 qapi/qapi-types-job.h
3038 qapi/qapi-types-misc.h
5249 qapi/qapi-types-run-state.h
3148 qapi/qapi-types-sockets.h
3634 tcg/i386/tcg-target.h
3634 tcg/tcg-mo.h
2369 trace/control-internal.h
2369 trace/control.h
2369 trace/event-internal.h
2361 trace/ftrace.h
5224 x86_64-softmmu/config-target.h
Bulky headers:
size * count size count
17744958 16461 1078 accel/tcg/tcg-runtime.h
32519424 10304 6312 config-host.h
21298950 525900 81 hw/display/trace.h
25070220 1432584 35 hw/net/trace.h
28700355 604218 95 hw/vfio/trace.h
57913922 20486 2827 include/block/aio.h
54939528 30744 1787 include/block/block.h
15698796 53763 292 include/block/block_int.h
12076546 6758 1787 include/block/dirty-bitmap.h
90694630 19990 4537 include/disas/dis-asm.h
22187844 68481 324 include/elf.h
41198658 11337 3634 include/exec/cpu-all.h
19118661 3693 5177 include/exec/cpu-common.h
26052146 7169 3634 include/exec/cpu-defs.h
10698472 11098 964 include/exec/cpu_ldst.h
32181064 19144 1681 include/exec/exec-all.h
14027000 2600 5395 include/exec/memattrs.h
428019130 82789 5170 include/exec/memory.h
12590662 3583 3514 include/exec/memory_ldst.inc.h
13254808 3772 3514 include/exec/memory_ldst_cached.inc.h
18258744 5196 3514 include/exec/memory_ldst_phys.inc.h
37693186 6614 5699 include/fpu/softfloat-types.h
40152640 41480 968 include/fpu/softfloat.h
24408504 3867 6312 include/glib-compat.h
16216423 2987 5429 include/hw/hotplug.h
10178064 1872 5437 include/hw/irq.h
27001315 27137 995 include/hw/pci/pci.h
10509377 10541 997 include/hw/pci/pci_ids.h
84058008 15486 5428 include/hw/qdev-core.h
38099880 15119 2520 include/hw/qdev-properties.h
10165600 4192 2425 include/migration/qemu-file-types.h
140199520 54680 2564 include/migration/vmstate.h
28062432 11216 2502 include/qapi/error.h
14093499 21649 651 include/qapi/visitor.h
122361217 20603 5939 include/qemu/atomic.h
53242354 9691 5494 include/qemu/bitmap.h
91409049 16113 5673 include/qemu/bitops.h
82589409 14497 5697 include/qemu/bswap.h
48009072 7606 6312 include/qemu/compiler.h
32872282 8882 3701 include/qemu/coroutine.h
20290224 11348 1788 include/qemu/hbitmap.h
64189485 11295 5683 include/qemu/host-utils.h
24059880 5141 4680 include/qemu/int128.h
16473456 8474 1944 include/qemu/iov.h
33191317 18553 1789 include/qemu/job.h
11054887 2987 3701 include/qemu/lockable.h
29294434 11798 2483 include/qemu/main-loop.h
14081472 2529 5568 include/qemu/module.h
120111048 19029 6312 include/qemu/osdep.h
14389467 7383 1949 include/qemu/qht.h
142840269 24363 5863 include/qemu/queue.h
24735306 4777 5178 include/qemu/rcu.h
68419650 12675 5398 include/qemu/rcu_queue.h
59775560 10751 5560 include/qemu/thread.h
122708520 28170 4356 include/qemu/timer.h
26573520 4210 6312 include/qemu/typedefs.h
166926546 36947 4518 include/qom/cpu.h
336965209 60857 5537 include/qom/object.h
55562856 55786 996 include/standard-headers/linux/pci_regs.h
12834339 18361 699 include/sysemu/kvm.h
19453584 3082 6312 include/sysemu/os-posix.h
12720000 6360 2000 include/sysemu/sysemu.h
16650270 26429 630 linux-user/qemu.h
53241630 84110 633 linux-user/syscall_defs.h
29144784 1214366 48 migration/trace.h
16888732 5572 6062 qapi/qapi-builtin-types.h
202683208 215544 2821 qapi/qapi-types-block-core.h
11236490 12630 2669 qapi/qapi-types-block.h
15791094 12447 3806 qapi/qapi-types-common.h
21564400 22620 2860 qapi/qapi-types-crypto.h
83229048 82188 3038 qapi/qapi-types-misc.h
11551995 28665 1209 qapi/qapi-types-net.h
25888068 14796 5249 qapi/qapi-types-run-state.h
14465060 13785 3148 qapi/qapi-types-sockets.h
18174224 62526 872 qapi/qapi-types-ui.h
60562920 124872 485 target/arm/cpu.h
18587552 63656 292 target/i386/cpu.h
18575446 36139 514 target/mips/cpu.h
44133336 109512 403 target/ppc/cpu.h
28893934 7951 3634 tcg/i386/tcg-target.h
24384936 51663 472 tcg/tcg-op.h
18304270 11285 1622 tcg/tcg-opc.h
79108184 48772 1622 tcg/tcg.h
64600980 95352 1355 trace-dtrace-root.h
233110135 344074 1355 trace-root.h
22750450 33580 1355 trace-ust-root.h
16220543 6847 2369 trace/control.h
