Document the default value of target-cpu

What is the default value of target-cpu if no -C target-cpu= is passed to rustc? Is it native, generic, something else?

It should probably be documented around here:

rust/src/doc/rustc/src/codegen-options/index.md

Lines 572 to 580 in 1151ea6

    
           ## target-cpu 
        
           This instructs `rustc` to generate code specifically for a particular processor. 
        
           You can run `rustc --print target-cpus` to see the valid options to pass 
        
           here. Each target has a default base CPU. Special values include: 
        
           * `native` can be passed to use the processor of the host machine. 
        
           * `generic` refers to an LLVM target with minimal features but modern tuning.

I believe it is documented there where it says "Each target has a default base CPU.". Every target has a different default target-cpu. I do not believe there is an option to explicitly use the default.

rustc --print target-cpus could perhaps indicate which one is the default. I don't think the rustc book is the appropriate place to document the specs for every target.

The default rustc target is host and therefor the default target-cpu is native right? rustc --print target-cpus currently displays the host processor, e.g.

Available CPUs for this target:
    native         - Select the CPU of the current host (currently skylake).

No, native is different from the default. For example, the default CPU for x86_64-unknown-linux-gnu is x86-64, which is compatible with the very first generation of x86-64 CPUs. The "native" CPU on my machine is znver1. The default is hard-coded into rustc for each target, typically picking the oldest CPU that is possible for that target.

@rustbot

O, gotcha. Would it make more sense for --print target-cpus to display the default target cpu for the default target or for --print target list to show the default target cpu for each target? I guess showing the cpu for each target might be too much information, but I'm wondering how one could look up the cpu for any taget...

@rustbot claim

It might be reasonable to add a note next to the default CPU in the target-cpus list in a fashion similar to the native entry. But I am not on the compiler team, so it is not my call. Maybe something like:

    cannonlake
    cascadelake
    cooperlake
    core-avx-i
    core-avx2
    core2         - Default for the x86_64-apple-darwin target
    corei7
    corei7-avx
    emeraldrapids

You can get the default cpu today on nightly by running rustc --print=target-spec-json.

Rollup merge of rust-lang#110876 - mj10021:issue-110647-fix, r=b-naber

Addressed in #110876.

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Document the default value of `target-cpu` #110647

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	## target-cpu

	This instructs `rustc` to generate code specifically for a particular processor.

	You can run `rustc --print target-cpus` to see the valid options to pass
	here. Each target has a default base CPU. Special values include:

	* `native` can be passed to use the processor of the host machine.
	* `generic` refers to an LLVM target with minimal features but modern tuning.

Document the default value of target-cpu #110647

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ehuss commented on Apr 21, 2023

mj10021 commented on Apr 26, 2023

ehuss commented on Apr 26, 2023

mj10021 commented on Apr 26, 2023

ehuss commented on Apr 26, 2023

workingjubilee commented on Jul 22, 2023

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Document the default value of `target-cpu` #110647