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[AVR] Replace broken 'avr-unknown-unknown' target with 'avr-unknown-gnu-atmega328' target #74941

Merged
merged 5 commits into from
Aug 27, 2020
Merged

[AVR] Replace broken 'avr-unknown-unknown' target with 'avr-unknown-gnu-atmega328' target #74941

merged 5 commits into from
Aug 27, 2020

Commits on Aug 24, 2020

  1. [AVR] Replace broken 'avr-unknown-unknown' target with 'avr-unknown-g… 

    …nu-atmega328' target

The `avr-unknown-unknown` target has never worked correctly, always trying to invoke
the host linker and failing. It aimed to be a mirror of AVR-GCC's
default handling of the `avr-unknown-unknown' triple (assume bare
minimum chip features, silently skip linking runtime libraries, etc).
This behaviour is broken-by-default as it will cause a miscompiled executable
when flashed.

This patch improves the AVR builtin target specifications to instead
expose only a 'avr-unknown-gnu-atmega328' target. This target system is
`gnu`, as it uses the AVR-GCC frontend along with avr-binutils. The
target triple ABI is 'atmega328'.

In the future, it should be possible to replace the dependency on
AVR-GCC and binutils by using the in-progress AVR LLD and compiler-rt support.
Perhaps at that point it would make sense to add an
'avr-unknown-unknown-atmega328' target as a better default when
implemented.

There is no current intention to add in-tree AVR target specifications for other
AVR microcontrollers - this one can serve as a reference implementation
for other devices via `rustc --print target-spec-json
avr-unknown-gnu-atmega328p`.

There should be no users of the existing 'avr-unknown-unknown' Rust
target as a custom target specification JSON has always been
recommended, and the avr-unknown-unknown target could never pass the
linking step anyway.
    @dylanmckay
    dylanmckay committed Aug 24, 2020
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  2. [AVR] Remove unnecessary arguments passed to the linker for GNU target 

    In general, linking with libc is not required, only libgcc is needed.
As suggested in the code review, a better option for libc support is by
building it into rust-lang/libc directly.

This also removes the '-Os' argument to the linker, which is a NOP.
    @dylanmckay
    dylanmckay committed Aug 24, 2020
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  3. [AVR] Merge the 'freestanding' base target spec into AVR base target … 

    …spec

The 'freestanding' module was only ever used for AVR. It was an
unnecessary layer of abstraction. This commit merges the
'freestanding_base' module into 'avr_gnu_base'.
    @dylanmckay
    dylanmckay committed Aug 24, 2020
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  4. [AVR] Rename the last few remaining references from 'avr-unknown-unkn… 

    …own' to 'avr-unknown-gnu-atmega328'
    @dylanmckay
    dylanmckay committed Aug 24, 2020
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  5. [AVR] Replace 'avr-unknown-unknown' with 'avr-unknown-gnu-atmega328' … 

    …in platform-support.md
    @dylanmckay
    dylanmckay committed Aug 24, 2020
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