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Re: How do I use `native-comp-compiler-options`?

From: Eli Zaretskii
Subject: Re: How do I use `native-comp-compiler-options`?
Date: Sun, 03 Sep 2023 20:06:24 +0300

> From: StrawberryTea <look@strawberrytea.xyz>
> Date: Sun, 03 Sep 2023 11:54:03 -0500
> Hi. So I'm just wondering how to use `native-comp-compiler-options`.
> Does it just take the same values that CFLAGS can take? I'm asking
> because I patched my Emacs to define `native-comp-compiler-options` as
> ```
> `("-march=skylake" "-Ofast" "-g0" "-fno-finite-math-only"
> "-fomit-frame-pointer" "-malign-data=cacheline" "-fira-region=mixed"
> "-fira-algorithm=CB" "-fira-hoist-pressure" "-fira-loop-pressure"
> "-floop-nest-optimize" "-fgraphite-identity")
> ```
> but I don't really notice any difference in performance in usage nor in
> any of the benchmarks in my config file:
> https://github.com/lemonbreezes/cae-emacs/blob/master/benchmarks/seq-cl-dash-benchmarks.org?plain=1#L9
> https://github.com/lemonbreezes/cae-emacs/blob/master/benchmarks/silly-loop.el#L3
> Nor do I see a difference when I compile `silly-loop` with and without
> the above options. I am using `native-comp-speed` 3 and lexical binding
> for all files and my GCC version is
> ```
> gcc (Gentoo 14.0.0 p, commit d0b55776a4e1d2f293db5ba0e4a04aefed055ec4)
> 14.0.0 20230903 (experimental) a7d052b3200c7928d903a0242b8cfd75d131e374
> ```
> I'm not sure if it's because my benchmarks are too trivial or because I
> have `native-comp-speed` 3 so there is not much to optimize beyond that.
> I definitely do notice the numbers change more when I recompile Emacs
> itself with different CFLAGS and I think I did notice a difference
> bumpin `native-comp-speed` up from 2 to 3 but not from setting
> `native-comp-compiler-options`.

I think you will be better off asking these questions the libgccjit
developers (they are on the GCC team) -- they know better which of the
GCC Flags affect native-code emitted by native compilation.

I've added Andrea, in case he has any insights.

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