[AVR] clang LTO causes Arduino examples to break

[beakthoven]

When compiling the Blink example from the Arduino IDE for AVR using Clang with LTO enabled, the program fails to function correctly. The LED blinks once and then remains stuck in the ON position indefinitely. Disabling LTO for wiring.c from Arduino AVR core resolves the issue.

LLVM version: 20.1.0-rc2

Blink example code:

#include <Arduino.h>

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH); 
  delay(1000); 
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000); 
}

Compiler flags used (other than -flto -fno-fat-lto-objects):

--target=avr -c -g -Os -Wall -Wextra --sysroot=/home/dakkshesh/.arduino15/packages/ClangBuiltArduino/tools/cba-avr-sysroot/12022025 -std=gnu11 -ffunction-sections -fdata-sections -MMD -mmcu=atmega2560 -DF_CPU=16000000L

Expected Behavior

The LED should continue blinking at a 1-second interval indefinitely.

Attachments

LLVM IR files for wiring.c with LTO turned on and off.
wiring.c-lto.ll.txt
wiring.c-nonlto.ll.txt

wiring.c: https://github.com/ClangBuiltArduino/core_arduino_avr/blob/8b327d7bede1c1245db99daeba4e168c92c11194/cores/arduino/wiring.c

[foxtran]

Does GCC LTO have the same effect? Just to be sure that is not a problem with C code.

[beakthoven]

Does GCC LTO have the same effect? Just to be sure that is not a problem with C code.

No, AVR GCC works perfectly fine with the same code with and without LTO.

[benshi001]

I am a little confused, I have tried your attached two .ll.txt files, with

wujie@WUJIE-16:~/Downloads$ llc wiring.c-lto.ll.txt -mcpu=atmega2560 -o wiring.c-lto.s.txt
wujie@WUJIE-16:~/Downloads$ llc wiring.c-nonlto.ll.txt -mcpu=atmega2560 wiring.c-nonlto.s.txt

The generated assembly source files only have difference in debug information, but not functional code.

wiring.c-lto.s.txt

wiring.c-nonlto.s.txt

[benshi001]

I also tried to compile wiring.c without debug information,

clang --target=avr  -S  -Os -Wall -Wextra  -std=gnu11 -ffunction-sections -fdata-sections -MMD -mmcu=atmega2560 -DF_CPU=16000000L ./core_arduino_avr/cores/arduino/wiring.c  -I core_arduino_avr/variants/mega/ -o wiring.nolto.s

clang --target=avr  -S  -Os -Wall -Wextra  -std=gnu11 -ffunction-sections -fdata-sections -MMD -mmcu=atmega2560 -DF_CPU=16000000L ./core_arduino_avr/cores/arduino/wiring.c -flto -I core_arduino_avr/variants/mega/ -o wiring.lto.ll
clang --target=avr wiring.lto.ll -S -mmcu=atmega2560 -o wiring.lto.s

The generated two .s files are also same in functional code.

so I guess, there is something wrong in the core libraries, other than wire.c.

[benshi001]

BTW: how did build arduino core library by clang? I used the attached script but failed.

arduino.sh.txt

The failure log is

In file included from /usr/share/arduino/hardware/arduino/avr/cores/arduino/CDC.cpp:20:
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/wdt.h:464:12: error: value '64' out of range for constraint 'I'
  464 |                                 : "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
      |                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/sfr_defs.h:183:27: note: expanded from macro '_SFR_IO_ADDR'
  183 | #define _SFR_IO_ADDR(sfr) (_SFR_MEM_ADDR(sfr) - __SFR_OFFSET)
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/share/arduino/hardware/arduino/avr/cores/arduino/CDC.cpp:20:
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/wdt.h:507:23: error: value '64' out of range for constraint 'I'
  507 |                                 : [WDTREG]  "I"  (_SFR_IO_ADDR(_WD_CONTROL_REG)),
      |                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/sfr_defs.h:183:27: note: expanded from macro '_SFR_IO_ADDR'
  183 | #define _SFR_IO_ADDR(sfr) (_SFR_MEM_ADDR(sfr) - __SFR_OFFSET)
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
clang++: /home/wujie/benshi/llvm-project/clang/include/clang/CodeGen/CGFunctionInfo.h:185: static clang::CodeGen::ABIArgInfo clang::CodeGen::ABIArgInfo::getExtend(clang::QualType, llvm::Type*): Assertion `Ty->isIntegralOrEnumerationType() && "Unexpected QualType"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: clang++ --target=avr -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_NANO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/arduino/avr/cores/arduino -I/usr/share/arduino/hardware/arduino/avr/variants/eightanaloginputs /usr/share/arduino/hardware/arduino/avr/cores/arduino/new.cpp -o /tmp/arduino_build_960004/core/new.cpp.o
1.	/usr/share/arduino/hardware/arduino/avr/cores/arduino/new.cpp:68:1: current parser token 'void'
2.	/usr/share/arduino/hardware/arduino/avr/cores/arduino/new.cpp:59:8: LLVM IR generation of declaration 'operator new'
3.	/usr/share/arduino/hardware/arduino/avr/cores/arduino/new.cpp:59:8: Generating code for declaration 'operator new'
 #0 0x00005aa720523900 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x27fd900)
 #1 0x00005aa7205201ff llvm::sys::RunSignalHandlers() (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x27fa1ff)
 #2 0x00005aa7203fbb38 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007d6b34e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007d6b34e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007d6b34e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007d6b34e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007d6b34e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007d6b34e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x00005aa720c5f084 (anonymous namespace)::AVRABIInfo::computeInfo(clang::CodeGen::CGFunctionInfo&) const AVR.cpp:0:0
#10 0x00005aa720ec80fa clang::CodeGen::CodeGenTypes::arrangeLLVMFunctionInfo(clang::CanQual<clang::Type>, clang::CodeGen::FnInfoOpts, llvm::ArrayRef<clang::CanQual<clang::Type>>, clang::FunctionType::ExtInfo, llvm::ArrayRef<clang::FunctionType::ExtParameterInfo>, clang::CodeGen::RequiredArgs) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x31a20fa)
#11 0x00005aa720ecc97b arrangeLLVMFunctionInfo(clang::CodeGen::CodeGenTypes&, bool, llvm::SmallVectorImpl<clang::CanQual<clang::Type>>&, clang::CanQual<clang::FunctionProtoType>) CGCall.cpp:0:0
#12 0x00005aa720ecec25 clang::CodeGen::CodeGenTypes::arrangeFunctionDeclaration(clang::FunctionDecl const*) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x31a8c25)
#13 0x00005aa720b87325 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x2e61325)
#14 0x00005aa720b82884 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x2e5c884)
#15 0x00005aa720b82efb clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x2e5cefb)
#16 0x00005aa720b8bbd7 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#17 0x00005aa720fe0cd6 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#18 0x00005aa720fce981 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x32a8981)
#19 0x00005aa723021e94 clang::ParseAST(clang::Sema&, bool, bool) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x52fbe94)
#20 0x00005aa721376bd9 clang::FrontendAction::Execute() (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x3650bd9)
#21 0x00005aa7212d151e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x35ab51e)
#22 0x00005aa721496dc8 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x3770dc8)
#23 0x00005aa71ea333bb cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0xd0d3bb)
#24 0x00005aa71ea24e9a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#25 0x00005aa72104211d void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#26 0x00005aa7203fbfe0 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x26d5fe0)
#27 0x00005aa72104476e clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#28 0x00005aa720fed8c1 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x32c78c1)
#29 0x00005aa720fee9e1 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x32c89e1)
#30 0x00005aa720ffeeb4 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0x32d8eb4)
#31 0x00005aa71ea2b13e clang_main(int, char**, llvm::ToolContext const&) (/home/wujie/benshi/llvm-project/build/bin/clang-21+0xd0513e)
#32 0x00005aa71e95bb0b main (/home/wujie/benshi/llvm-project/build/bin/clang-21+0xc35b0b)
#33 0x00007d6b34e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#34 0x00007d6b34e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#35 0x00005aa71ea245c5 _start (/home/wujie/benshi/llvm-project/build/bin/clang-21+0xcfe5c5)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 21.0.0git (git@github.com:llvm/llvm-project.git 254951749febef42620242fd22dfba62f624f79f)
Target: avr
Thread model: posix
InstalledDir: /home/wujie/benshi/llvm-project/build/bin
Build config: +assertions, +expensive-checks
clang++: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang++: note: diagnostic msg: /tmp/new-fd3c2f.cpp
clang++: note: diagnostic msg: /tmp/new-fd3c2f.sh
clang++: note: diagnostic msg: 

********************
/usr/lib/gcc/avr/5.4.0/../../../avr/bin/ar: /tmp/arduino_build_960004/core/CDC.cpp.o: 没有那个文件或目录
/usr/lib/gcc/avr/5.4.0/../../../avr/bin/ar: /tmp/arduino_build_960004/core/new.cpp.o: 没有那个文件或目录

[benshi001]

I think I should fix the above failure first, it indeed is clang's ICE, as the first step to let clang support arduino.

[beakthoven]

I am a little confused, I have tried your attached two .ll.txt files, with

wujie@WUJIE-16:/Downloads$ llc wiring.c-lto.ll.txt -mcpu=atmega2560 -o wiring.c-lto.s.txt
wujie@WUJIE-16:/Downloads$ llc wiring.c-nonlto.ll.txt -mcpu=atmega2560 wiring.c-nonlto.s.txt
The generated assembly source files only have difference in debug information, but not functional code.

wiring.c-lto.s.txt

wiring.c-nonlto.s.txt

Then i think LTO optimizations made during link time are then likely the cause of the issue. I'll try to upload LTO intermediate files shortly in order to debug this.

[beakthoven]

In file included from /usr/share/arduino/hardware/arduino/avr/cores/arduino/CDC.cpp:20:
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/wdt.h:464:12: error: value '64' out of range for constraint 'I'
464 | : "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/sfr_defs.h:183:27: note: expanded from macro '_SFR_IO_ADDR'
183 | #define _SFR_IO_ADDR(sfr) (_SFR_MEM_ADDR(sfr) - __SFR_OFFSET)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/share/arduino/hardware/arduino/avr/cores/arduino/CDC.cpp:20:
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/wdt.h:507:23: error: value '64' out of range for constraint 'I'
507 | : [WDTREG] "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/avr/5.4.0/../../../avr/include/avr/sfr_defs.h:183:27: note: expanded from macro '_SFR_IO_ADDR'
183 | #define _SFR_IO_ADDR(sfr) (_SFR_MEM_ADDR(sfr) - __SFR_OFFSET)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

The constraint error you have encountered is because of a code in AVR libc thats written in a way that makes use of gcc specific behavior.
avr-libc issue: avrdudes/avr-libc#998
llvm issue: #51513

For time being i have applied the following patch in my fork of avr-libc to mitigate the constraint issue since there is no official fix from avr-libc.

However about to the clang crash, I haven't encountered it during my builds so i dont know what we're looking here.

BTW: how did build arduino core library by clang?

Well i wrote this simple bash script to use during initial stage. Now i have completely moved to arduino's build system, the required changes for this were made to the build files in the core lib repo as of commit ClangBuiltArduino/core_arduino_avr@96cc1db. This should be compatible with arduino-cli as well.