fatal error: error in backend: underestimated function size

[toyowata]

Summary

I got "fatal error: error in backend: underestimated function size" by compiling attached code.

fatal_error_atfe21_1_1.tar.gz

$ clang --target=arm-none-eabi -mcpu=cortex-m0plus -mfloat-abi=soft -g -O3 -std=gnu11 -c ./test_atfe21_1_1_pp.c
fatal error: error in backend: underestimated function size
PLEASE submit a bug report to https://github.com/arm/arm-toolchain/issues and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: clang --target=arm-none-eabi -mcpu=cortex-m0plus -mfloat-abi=soft -g -O3 -std=gnu11 -c ./test_atfe21_1_1_pp.c
1.	<eof> parser at end of file
2.	Code generation
3.	Running pass 'Function Pass Manager' on module './test_atfe21_1_1_pp.c'.
4.	Running pass 'ARM constant island placement and branch shortening pass' on function '@main'
 #0 0x00000001061b3bb4 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1013c3bb4)
 #1 0x00000001061b1af0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1013c1af0)
 #2 0x00000001061b3288 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1013c3288)
 #3 0x0000000106110064 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101320064)
 #4 0x0000000106110010 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101320010)
 #5 0x00000001061aeda0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1013beda0)
 #6 0x0000000104df6658 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100006658)
 #7 0x000000010611a2c8 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x10132a2c8)
 #8 0x000000010611a194 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x10132a194)
 #9 0x00000001050becd4 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1002cecd4)
#10 0x0000000105667fa4 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100877fa4)
#11 0x0000000105a06bdc (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100c16bdc)
#12 0x0000000105a0ce50 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100c1ce50)
#13 0x0000000105a07550 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100c17550)
#14 0x00000001065a24e0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1017b24e0)
#15 0x00000001067f0f64 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101a00f64)
#16 0x00000001076790f0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x1028890f0)
#17 0x0000000106bf1f74 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101e01f74)
#18 0x0000000106b81638 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101d91638)
#19 0x0000000106c62ef0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101e72ef0)
#20 0x0000000104df5aa4 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100005aa4)
#21 0x0000000104df3888 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100003888)
#22 0x00000001069e3cfc (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101bf3cfc)
#23 0x000000010610ffe8 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x10131ffe8)
#24 0x00000001069e37bc (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101bf37bc)
#25 0x00000001069ae920 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101bbe920)
#26 0x00000001069aeae8 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101bbeae8)
#27 0x00000001069c8bb0 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x101bd8bb0)
#28 0x0000000104df2908 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x100002908)
#29 0x0000000104dff2e4 (/Applications/ATfE-21.1.1-Darwin-universal/bin/clang-21+0x10000f2e4)
#30 0x000000018f1f9d54
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 21.1.1
Target: arm-unknown-none-eabi
Thread model: posix
InstalledDir: /Applications/ATfE-21.1.1-Darwin-universal/bin
Arm Toolchain ID: E0058 (1bbd3573)
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: /var/folders/92/d9hjnx8x18nb2yv3zbs34wxm0000gn/T/test_atfe21_1_1_pp-3387d9.c
clang: note: diagnostic msg: /var/folders/92/d9hjnx8x18nb2yv3zbs34wxm0000gn/T/test_atfe21_1_1_pp-3387d9.sh
clang: note: diagnostic msg: Crash backtrace is located in
clang: note: diagnostic msg: /Users/toyowata/Library/Logs/DiagnosticReports/clang-21_<YYYY-MM-DD-HHMMSS>_<hostname>.crash
clang: note: diagnostic msg: (choose the .crash file that corresponds to your crash)
clang: note: diagnostic msg: 

********************

To Reproduce

1. Extract attached file
2. Build the file test_atfe21_1_1_pp.c:

clang --target=arm-none-eabi -mcpu=cortex-m0plus -mfloat-abi=soft -g -O3 -std=gnu11 -c ./test_atfe21_1_1_pp.c

The minimum.c is also a minimum reproducer code.

Expected behavior
The test_atfe21_1_1_pp.c is compiled without fatal error.

Actual behavior
The fatal error occurs and cannot compile the code.

Environment

Arm Toolchain version

$ clang --version
clang version 21.1.1
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /Applications/ATfE-21.1.1-Darwin-universal/bin
Arm Toolchain ID: E0058 (1bbd3573)

Host OS
macOS Tahoe 26.1

$ sw_vers -productVersion
26.1

kernel version

$ uname -a
Darwin xxxxx_MacBook-Pro.local 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:34:05 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6041 arm64

[Additional context]

• Change the optimization level from -O3 to -O2 or lower, the error does not occur.
• Change the target CPU from -mcpu=cortex-m0plus to -mcpu=cortex-m33, the error does not occur.
• The original source code can be found in the official Raspberry Pi Pico example here: https://github.com/raspberrypi/pico-examples/blob/master/adc/joystick_display/joystick_display.c

[voltur01]

Thank you for reporting this! We can confirm this to be an issue in clang itself - we will have a look, likely not for LLVM 22 as the branching is rather close.

[smithp35]

We had a similar problem with the csibe benchmark. With that particular example it looks like LLVM is determining that main never returns, therefore it doesn't need to save main's return address (the link register LR). The function is also estimated not large enough to require LR to be forcibly spilled. When allocating constant pools LLVM determines that because LR is not saved then it cannot safely use a BL for a long branch around the constant pool.

Looking at the example it looks like main does not return there either so it is plausible that this is the case here too. One possible workaround is to put a conditional return in the loop based on something the compiler can't optimise away. For example checking the return value of printf and returning if that is negative (indicating an error from printf).