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install: re-write description of recommended flags, and add size comparison

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Daniel Eklöf 2020-11-22 16:11:01 +01:00
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INSTALL.md
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@ -130,13 +130,20 @@ mkdir -p bld/release && cd bld/release
export CFLAGS="$CFLAGS -O3 -march=native"
meson --buildtype=release --prefix=/usr -Db_lto=true ../..
```
Both `-O3` and `-Db_lto=true` are **highly** recommended.
For performance reasons, I strongly recommend doing a
To optimize for performance, both `-O3`, `-Db_lto=true` and doing a
[PGO](#profile-guided-optimization) (Profile Guided Optimization)
build.
build is recommended.
If you do not want this, just build:
If you instead want to optimize for size, use `-Os` instead of `-O3`.
In general, `-Os` results in the smallest (and slowest) binary. A full
PGO build will be slightly larger than a `-Os` build, but smaller than
a `-O3` build. A partial PGO build will be somewhere in between a full
PGO build and a `-O3` build. See the [size
comparison](#size-comparison).
If you are doing a non-PGO build, just build:
```sh
ninja
@ -151,13 +158,8 @@ for example, uses `CFLAGS` to specify the default set of flags.
Thus, we do `export CFLAGS+="..."` to at least not throw away those
flags.
When packaging, you may want to use the default `CFLAGS` only, but
note this: foot is a performance critical application that relies on
compiler optimizations to perform well.
In particular, with GCC 10.1, it is **very** important `-O3` is used
(and not e.g. `-O2`) when doing a [PGO](#profile-guided-optimization)
build.
**when doing a [PGO](#profile-guided-optimization) build**.
#### Profile Guided Optimization
@ -175,6 +177,12 @@ meson configure -Db_pgo=generate
ninja
```
Remainder: make sure `-O3` is being used.
To get **maximum** performance, you can also add
`-fprofile-partial-training`, but do note that this will result in a
noticeably larger binary (see [size comparison](#size-comparison))
Next, we need to actually generate the profiling data.
There are two ways to do this: a [partial PGO build using a PGO
@ -266,6 +274,15 @@ ninja
Continue reading in [Running the new build](#running-the-new-build)
##### Size comparison
| PGO | default | `-fprofile-partial-training`|
|-----------------|--------:|----------------------------:|
| none (`-O3`) | 340K | - |
| none (`-Os`) | 260K | - |
| partial | 328K | 376K |
| full | 292K | 352K |
### Debug build
```sh