Support AVRTiny devices in AVR inline assembly by taiki-e · Pull Request #146901 · rust-lang/rust

taiki-e · 2025-09-22T17:59:44Z

Note: This is still draft and this depends on #146900 which adds necessary target feature. (The first commit is from it.)

Follow-up to #131323.

AVR has devices that reduce the number of registers, similar to RISC-V's RV32E, which have different ABI than default. This PR supports such devices in inline assembly.

Refs: AVR-GCC docs https://gcc.gnu.org/wiki/avr-gcc#Reduced_Tiny

On the Reduced Tiny cores (16 GPRs only) several modifications to the ABI above apply:

Call-saved registers are: R18–R19, R28–R29.
Fixed Registers are R16 (tmp_reg) and R17 (zero_reg).

Registers used to pass arguments to functions and return values from functions are R25...R20 (instead of R25...R8).
Values that occupy more than 4 registers are returned in memory (instead of more than 8 registers).
There is only limited library support both from libgcc and AVR-LibC, for example there is no float support and no printf support.

r? @Amanieu

@rustbot label +O-AVR +A-inline-assembly

tgross35 · 2025-09-22T18:44:48Z

Cc target maintainer @Patryk27

compiler/rustc_target/src/asm/avr.rs

Patryk27 · 2025-09-23T17:02:50Z

tests/ui/asm/avr/bad-reg.rs

+        asm!("", out("r15r14") _);
+        //[avrtiny]~^ ERROR on AVRTiny, r[2-15] are unavailable, r16 (scratch register) and r17 (zero register) are reserved by LLVM
+        asm!("", out("r17r16") _);
+        //[avrtiny]~^ ERROR on AVRTiny, r[2-15] are unavailable, r16 (scratch register) and r17 (zero register) are reserved by LLVM


nit: would be nice to have a different message for register pairs, otherwise it takes a moment to parse what the compiler is trying to say

Amanieu · 2025-09-26T17:59:12Z

r=me once the target features land

bors · 2025-11-05T08:32:17Z

☔ The latest upstream changes (presumably #147645) made this pull request unmergeable. Please resolve the merge conflicts.

@Patryk27

…ingjubilee Add avr_target_feature This adds the following unstable target features (tracking issue: rust-lang#146889): - The following two are particularly important for properly supporting inline assembly: - `tinyencoding`: AVR has devices that reduce the number of registers, similar to RISC-V's RV32E. This feature is necessary to support inline assembly in such devices. (see also rust-lang#146901) - `lowbytefirst`: AVR's memory access is per 8-bit, and when writing 16-bit ports, the bytes must be written in a specific order. This order depends on devices, making this feature necessary to write proper inline assembly for such use cases. (see also llvm/llvm-project@2a52876) - The followings help recognizing whether specific instructions are available: - `addsubiw` - `break` - `eijmpcall` - `elpm` - `elpmx` - `ijmpcall` - `jmpcall` - `lpm` - `lpmx` - `movw` - `mul` - `rmw` - `spm` - `spmx` Of these, all except `addsubiw`, `break`, `ijmpcall`, `lpm`, `rmw`, `spm`, and `spmx` have [corresponding conditional codes in avr-libc](https://github.com/search?q=repo%3Aavrdudes%2Favr-libc+%2F__AVR_HAVE_%2F&type=code&p=1). LLVM also has `des` feature, but I excluded it from this PR because [DES](https://en.wikipedia.org/wiki/Data_Encryption_Standard) is insecure. - Report future-incompatible warning (rust-lang#116344) for -C target-feature=-sram and -C target-cpu=<device_without_sram> cases because SRAM is minimum requirement for non-assembly language in both avr-gcc and LLVM. - See rust-lang#146900 (comment) for details. LLVM also has `smallstack`, `wrappingrjmp`, and `memmappedregs` features, but I skipped them because they didn't seem to belong to either of the above categories, but I might have missed something. (The feature names are match with [definitions in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-21.1.0/llvm/lib/Target/AVR/AVRDevices.td).) cc @Patryk27 @Rahix r? workingjubilee @rustbot label +O-AVR +A-target-feature

@Patryk27

…ingjubilee Add avr_target_feature This adds the following unstable target features (tracking issue: rust-lang#146889): - The following two are particularly important for properly supporting inline assembly: - `tinyencoding`: AVR has devices that reduce the number of registers, similar to RISC-V's RV32E. This feature is necessary to support inline assembly in such devices. (see also rust-lang#146901) - `lowbytefirst`: AVR's memory access is per 8-bit, and when writing 16-bit ports, the bytes must be written in a specific order. This order depends on devices, making this feature necessary to write proper inline assembly for such use cases. (see also llvm/llvm-project@2a52876) - The followings help recognizing whether specific instructions are available: - `addsubiw` - `break` - `eijmpcall` - `elpm` - `elpmx` - `ijmpcall` - `jmpcall` - `lpm` - `lpmx` - `movw` - `mul` - `rmw` - `spm` - `spmx` Of these, all except `addsubiw`, `break`, `ijmpcall`, `lpm`, `rmw`, `spm`, and `spmx` have [corresponding conditional codes in avr-libc](https://github.com/search?q=repo%3Aavrdudes%2Favr-libc+%2F__AVR_HAVE_%2F&type=code&p=1). LLVM also has `des` feature, but I excluded it from this PR because [DES](https://en.wikipedia.org/wiki/Data_Encryption_Standard) is insecure. - Report future-incompatible warning (rust-lang#116344) for -C target-feature=-sram and -C target-cpu=<device_without_sram> cases because SRAM is minimum requirement for non-assembly language in both avr-gcc and LLVM. - See rust-lang#146900 (comment) for details. LLVM also has `smallstack`, `wrappingrjmp`, and `memmappedregs` features, but I skipped them because they didn't seem to belong to either of the above categories, but I might have missed something. (The feature names are match with [definitions in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-21.1.0/llvm/lib/Target/AVR/AVRDevices.td).) cc @Patryk27 @Rahix r? workingjubilee @rustbot label +O-AVR +A-target-feature

@Patryk27

…ingjubilee Add avr_target_feature This adds the following unstable target features (tracking issue: rust-lang#146889): - The following two are particularly important for properly supporting inline assembly: - `tinyencoding`: AVR has devices that reduce the number of registers, similar to RISC-V's RV32E. This feature is necessary to support inline assembly in such devices. (see also rust-lang#146901) - `lowbytefirst`: AVR's memory access is per 8-bit, and when writing 16-bit ports, the bytes must be written in a specific order. This order depends on devices, making this feature necessary to write proper inline assembly for such use cases. (see also llvm/llvm-project@2a52876) - The followings help recognizing whether specific instructions are available: - `addsubiw` - `break` - `eijmpcall` - `elpm` - `elpmx` - `ijmpcall` - `jmpcall` - `lpm` - `lpmx` - `movw` - `mul` - `rmw` - `spm` - `spmx` Of these, all except `addsubiw`, `break`, `ijmpcall`, `lpm`, `rmw`, `spm`, and `spmx` have [corresponding conditional codes in avr-libc](https://github.com/search?q=repo%3Aavrdudes%2Favr-libc+%2F__AVR_HAVE_%2F&type=code&p=1). LLVM also has `des` feature, but I excluded it from this PR because [DES](https://en.wikipedia.org/wiki/Data_Encryption_Standard) is insecure. - Report future-incompatible warning (rust-lang#116344) for -C target-feature=-sram and -C target-cpu=<device_without_sram> cases because SRAM is minimum requirement for non-assembly language in both avr-gcc and LLVM. - See rust-lang#146900 (comment) for details. LLVM also has `smallstack`, `wrappingrjmp`, and `memmappedregs` features, but I skipped them because they didn't seem to belong to either of the above categories, but I might have missed something. (The feature names are match with [definitions in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-21.1.0/llvm/lib/Target/AVR/AVRDevices.td).) cc @Patryk27 @Rahix r? workingjubilee @rustbot label +O-AVR +A-target-feature

Add avr_target_feature

18d8866

rustbot assigned Amanieu Sep 22, 2025

taiki-e changed the title ~~Support AVRTiny devices AVR inline assembly~~ Support AVRTiny devices in AVR inline assembly Sep 22, 2025

This was referenced Sep 22, 2025

Tracking Issue for asm_experimental_arch #93335

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Support AVRTiny devices in AVR inline assembly

e2abd1e

taiki-e force-pushed the avrtiny branch from 2a9be64 to e2abd1e Compare September 22, 2025 18:20

Patryk27 reviewed Sep 23, 2025

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Support AVRTiny devices in AVR inline assembly#146901

Support AVRTiny devices in AVR inline assembly#146901
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