Re: ccache isn't working with xbps-src

[pullmoll <pullmoll@users.noreply.github.com>]

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New comment by pullmoll on void-packages repository

https://github.com/void-linux/void-packages/issues/26508#issuecomment-730645253

Comment:
1) You probably cannot expect a single hit between chromium-86 and chromium-87 because headers changed and a change in a central header (think `version.h`) prevents a cache hit.
2) You need to make sure to re-bootstrap your environment, i.e. do `./xbps-src zap` and `./xbps-src binary-bootstrap` again to make the `etc/conf` (not `etc/config` AFAICT) change take effect.
3) Any change in the compiler binary invalidates the entire current cache, because a checksum of the compiler binary (`/usr/bin/clang++` in this case) is used to make sure that compilation results will indeed be identical. So the update of llvm10 to llvm11 already invalidated all your existing cache entries for `clang` / `clang++`. You can clean and zap the cache (`ccache -C` and `ccache -Z`) in such a case, except perhaps if you also have lots of `gcc` and `g++` cache entries.

I have a separate 1TB SSD for ccache and currently have 653 GiB of 768 GiB used with a cache hit rate of 8.06%. I'm building literally tens of thousands of packages for all architectures and can confirm that `ccache` indeed works as it is supposed to.

Just to show you the figures this is the output of `ccache -s`. In my user's environment I have `CCACHE_DIR=/work/void/ccache` i.e. the same directory which is used from inside the chroot:
```
cache directory                     /work/void/ccache
primary config                      /work/void/ccache/ccache.conf
secondary config (readonly)         /etc/ccache.conf
stats updated                       Thu Nov 19 22:29:31 2020
cache hit (direct)                819345
cache hit (preprocessed)          178710
cache miss                      11381469
cache hit rate                      8.06 %
called for link                   994196
called for preprocessing          994011
multiple source files               1762
compiler produced stdout            9438
compiler produced no output           77
compiler produced empty output     10314
compile failed                    200101
ccache internal error                613
preprocessor error                145901
can't use precompiled header       89380
bad compiler arguments            162657
unsupported source language        62376
autoconf compile/link             867964
unsupported compiler option        93317
unsupported code directive          1051
output to stdout                       4
could not write to output file         6
no input file                     345577
cleanups performed                  2489
files in cache                   8008769
cache size                         654.3 GB
max cache size                     768.0 GB
```

Edit2: This script is my `$HOME/bin/ccwatch` and I use to watch ccache working...
```bash
#!/bin/bash
CCACHE_DIR=/work/void/ccache
if [ -n "$1" ]; then
        CCACHE_DIR="$1"
fi
export CCACHE_DIR
watch -n 1 ccache -s
```