Re: [PATCH] kbuild: move -pipe to global KBUILD_CFLAGS

[Alex Xu (Hello71)]

Excerpts from Nathan Chancellor's message of February 21, 2020 9:16 pm:
> Hi Alex,
> 
> On Fri, Feb 21, 2020 at 07:38:20PM -0500, Alex Xu (Hello71) wrote:
>> -pipe reduces unnecessary disk wear for systems where /tmp is not a
>> tmpfs, slightly increases compilation speed, and avoids leaving behind
>> files when gcc crashes.
>> 
>> According to the gcc manual, "this fails to work on some systems where
>> the assembler is unable to read from a pipe; but the GNU assembler has
>> no trouble". We already require GNU ld on all platforms, so this is not
>> an additional dependency. LLVM as also supports pipes.
>> 
>> -pipe has always been used for most architectures, this change
>> standardizes it globally. Most notably, arm, arm64, riscv, and x86 are
>> affected.
>> 
>> Signed-off-by: Alex Xu (Hello71) <alex_y_xu@xxxxxxxx>
> 
> Do you have any numbers to show this is actually beneficial from a
> compilation time perspective? I ask because I saw an improvement in
> compilation time when removing -pipe from x86's KBUILD_CFLAGS in
> commit 437e88ab8f9e ("x86/build: Remove -pipe from KBUILD_CFLAGS").
> 
> For what it's worth, clang ignores -pipe so this does not actually
> matter for its integrated assembler.
> 
> That type of change could have been a fluke but I guarantee people
> will care more about any change in compilation time than any of the
> other things that you mention so it might be wise to check on major
> architectures to make sure that it doesn't hurt.
> 
> Cheers,
> Nathan
> 

Sorry, I should've checked the performance first. I have now run:

cd /tmp/linux # previously: make O=/tmp/linux
export MAKEFLAGS=12 # Ryzen 1600, 6 cores, 12 threads
make allnoconfig
for i in {1..10}; do
    make clean >/dev/null
    time make XPIPE=-pipe >/dev/null
    make clean >/dev/null
    time make >/dev/null
done

after patching -pipe to $(XPIPE) in Makefile.

Results (without ld warnings):

make > /dev/null  130.54s user 10.41s system 969% cpu 14.537 total
make XPIPE=-pipe > /dev/null  129.83s user 9.95s system 977% cpu 14.296 total
make > /dev/null  129.73s user 10.28s system 966% cpu 14.493 total
make XPIPE=-pipe > /dev/null  130.04s user 10.63s system 986% cpu 14.252 total
make > /dev/null  129.53s user 10.28s system 972% cpu 14.379 total
make XPIPE=-pipe > /dev/null  130.29s user 10.17s system 983% cpu 14.288 total
make > /dev/null  130.19s user 10.52s system 968% cpu 14.530 total
make XPIPE=-pipe > /dev/null  129.90s user 10.47s system 978% cpu 14.343 total
make > /dev/null  129.50s user 10.81s system 959% cpu 14.620 total
make XPIPE=-pipe > /dev/null  130.37s user 10.60s system 975% cpu 14.446 total
make > /dev/null  129.63s user 10.18s system 972% cpu 14.374 total
make XPIPE=-pipe > /dev/null  131.29s user 9.92s system 1016% cpu 13.899 total
make > /dev/null  129.96s user 10.39s system 961% cpu 14.596 total
make XPIPE=-pipe > /dev/null  131.63s user 10.16s system 1011% cpu 14.015 total
make > /dev/null  129.33s user 10.54s system 970% cpu 14.405 total
make XPIPE=-pipe > /dev/null  129.70s user 10.40s system 976% cpu 14.349 total
make > /dev/null  129.53s user 10.25s system 964% cpu 14.494 total
make XPIPE=-pipe > /dev/null  130.38s user 10.62s system 973% cpu 14.479 total
make > /dev/null  130.73s user 10.08s system 957% cpu 14.704 total
make XPIPE=-pipe > /dev/null  130.43s user 10.62s system 985% cpu 14.309 total
make > /dev/null  130.54s user 10.41s system 969% cpu 14.537 total

There is a fair bit of variance, probably due to cpufreq, schedutil, CPU 
temperature, CPU scheduler, motherboard power delivery, etc. But, I 
think it can be clearly seen that -pipe is, on average, about 0.1 to 0.2 
seconds faster.

I also tried "make defconfig":

make > /dev/null  1238.26s user 102.39s system 1095% cpu 2:02.33 total
make XPIPE=-pipe > /dev/null  1231.33s user 102.52s system 1081% cpu 2:03.29 total
make > /dev/null  1232.92s user 102.07s system 1096% cpu 2:01.71 total
make XPIPE=-pipe > /dev/null  1239.59s user 102.30s system 1096% cpu 2:02.39 total
make > /dev/null  1229.81s user 101.72s system 1093% cpu 2:01.74 total
make XPIPE=-pipe > /dev/null  1234.64s user 101.30s system 1098% cpu 2:01.64 total
make > /dev/null  1228.50s user 104.39s system 1093% cpu 2:01.91 total
make XPIPE=-pipe > /dev/null  1238.78s user 102.57s system 1099% cpu 2:01.99 total
make > /dev/null  1238.26s user 102.39s system 1095% cpu 2:02.33 total

I stopped after this because I needed to use the machine for other 
tasks. The results are less clear, but I think there's not a big 
difference one way or another, at least on my machine.

CPU: Ryzen 1600, overclocked to ~3.8 GHz
RAM: Corsair Vengeance, overclocked to ~3300 MHz, forgot timings
Motherboard: ASRock B450 Pro4

I would speculate that the recent pipe changes have caused a change in 
the relative speed compared to 2018. I am using 5.6.0-rc2 with -O3 
-march=native patches.

Regards,
Alex.