| Issue |
176830
|
| Summary |
[AVR] Register numbers mismatch with GCC, causes errors with AVR-libc
|
| Labels |
libc
|
| Assignees |
|
| Reporter |
GregTheMadMonk
|
Hello!
I've been trying to use LLVM (Clang++) to develop for ATMega32u4 and have stumbled upon being unable to use `libc++`'s `<cstring>`. The error was:
```
/usr/lib/gcc/avr/15.1.0/../../../../avr/include/string.h:270:54: error: unknown register name '30' in asm
270 | : "=r" (__r24) : "0" (__hay), "r" (__r22) : "30", "31");
| ^
/usr/lib/gcc/avr/15.1.0/../../../../avr/include/string.h:669:16: error: unknown register name '30' in asm
669 | : "30", "31", "memory");
| ^
```
which is in the AVR-`libc`'s `string.h` implementation where inline assembly is used to define `strchr`. `libc++` chain-includes this file automatically with
https://github.com/llvm/llvm-project/blob/db48679835d316a420fece71f17a14633497cce3/libcxx/include/string.h#L63-L65
There is no way to disable this, even though (apparently?) all of these functions should be available as Clang's bulitins if another `string.h` isn't present in further include directories:
https://github.com/llvm/llvm-project/blob/db48679835d316a420fece71f17a14633497cce3/libcxx/include/string.h#L94-L96
GCC appears to treat all 8-bit registers separately, so `"30"` and `"31"` correspond to `r30` and `r31`, and `"28"` corresponds to `r28`:
https://github.com/gcc-mirror/gcc/blob/412467a47b9f6f52cc0bfbd8786c5ee5a2c16e7f/gcc/config/avr/avr.h#L424-L429
While LLVM seems to have chosen to use register indices 26+ for register pairs (`"26"` = `r26:r27` = `X`, `"27"` = `r28:r29` = `Y`, `"28"` = `r30:r31` = `Z`, ...)
https://github.com/llvm/llvm-project/blob/9b64509862cbda3db7d4bc03e1f27ed3387e9ee8/clang/lib/Basic/Targets/AVR.h#L78-L98
This creates a discrepancy that makes AVR inline assembly incompatible between GCC and Clang.
Is this intentional? Could this maybe be changed to provide compatibility with AVR libc since it's being included automatically by Clang anyway?
And if it will stay like this, could there maybe be an option to force `libc++`'s `string.h` to use provided builtins instead of going further? Or are they incompatible with AVR?
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