[Csnd-dev] plugins: illegal instruction

Date	2026-10-31 18:45
From	James Hearon
Subject	[Csnd-dev] plugins: illegal instruction
	re: plugins(libbeosc, libklib, lipathtools, etc.) and getting "illegal instruction" after building or downloading linux binaries, I did further inspection using gdb and found my problem is the sources CMakeLists.txt is using AVX2, which my older processor doesn't support. if(LINUX AND CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64\|i[3-6]86\|AMD64") #addflag(-mavx2 HAS_AVX2) #causes illegal instruction error XXX for older cpus if(HAS_AVX2) message(STATUS "AVX2 enabled for Linux (x86)") endif() endif() Also needed to add target_compile_options for each CMakeLists.txt for each library: option(BUILD_BEOSC_OPCODES "Build beosc opcodes" ON) if(BUILD_BEOSC_OPCODES) make_plugin(beosc beosc.c) target_compile_options(beosc PRIVATE -mno-avx2) #avoid building with AVX2 endif() And is some cases a conditional general compiler directive: make_plugin(else src/else.c) #avoid building using AVX2 if(CMAKE_C_COMPILER_ID MATCHES "GNU\|Clang") target_compile_options(else PRIVATE -mno-avx2) endif() ...then rebuilt and seems working.

Date

2026-10-31 19:22

From

Victor Lazzarini <000010b17ddd988e-dmarc-request@LISTSERV.HEANET.IE>

Subject

Re: [Csnd-dev] [EXTERNAL] [Csnd-dev] plugins: illegal instruction

Makes sense, we should fix CMakeLists.txt to allow for that.

Prof. Victor Lazzarini

Maynooth University

Ireland

On 31 Oct 2026, at 18:45, James Hearon <j_hearon@hotmail.com> wrote:

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re: plugins(libbeosc, libklib, lipathtools, etc.) and getting "illegal instruction" after building or downloading linux binaries, I did further inspection using gdb and found my problem is the sources CMakeLists.txt is using AVX2, which my older processor doesn't support.

if(LINUX AND CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|i[3-6]86|AMD64")

#addflag(-mavx2 HAS_AVX2) #causes illegal instruction error XXX for older cpus

if(HAS_AVX2)

message(STATUS "AVX2 enabled for Linux (x86)")

endif()

endif()

Also needed to add target_compile_options for each CMakeLists.txt for each library:

option(BUILD_BEOSC_OPCODES "Build beosc opcodes" ON)

if(BUILD_BEOSC_OPCODES)

make_plugin(beosc beosc.c)

target_compile_options(beosc PRIVATE -mno-avx2) #avoid building with AVX2

endif()

And is some cases a conditional general compiler directive:

make_plugin(else src/else.c)

#avoid building using AVX2

if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")

target_compile_options(else PRIVATE -mno-avx2)

endif()

...then rebuilt and seems working.

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Date

2026-10-31 19:51

From

Victor Lazzarini <000010b17ddd988e-dmarc-request@LISTSERV.HEANET.IE>

Subject

Re: [Csnd-dev] [EXTERNAL] [Csnd-dev] plugins: illegal instruction

Ok I got all confused because you said 'plugins' and mentioned the plugins from csound/plugins. However I see your problem is nothing to do with the csound/plugins repository but with csound/csound.

This problem looks like the csound library being built with -mavx2 globally, rather than a problem specific to the external plugin repository.

libbeosc, libklib, and lipathtools are Csound opcode libraries, not part of the separate csound/plugins repository.

In Csound 7.0.0, USE_AVX2 defaults to ON (CMakeLists.txt line 236), and lines 474–502 add -mavx2 to the global C and C++ flags. That allows every translation unit, including opcode libraries, to contain AVX2 instructions. There is no runtime CPU check, so a pre-AVX2 CPU raises SIGILL at the first such instruction.

The target-level -mno-avx2 workaround works because target options are appended after the global flags, but patching each library is fragile. It does not cover dependencies or flags injected through CFLAGS/CXXFLAGS, it does not work on MSVC, and it only disables AVX2 rather than other possible instruction requirements.

On current develop, commit f5bac72c0 makes AVX2 an explicit opt-in: USE_AVX2 defaults to OFF. CI also builds with the default and runs under QEMU with a Nehalem CPU, which has SSE2 but no AVX or AVX2, to catch this regression.

That fix is not in tag 7.0.0, so 7.0.0 source builds and release binaries still have AVX2 enabled by default. Here's what you should do

1) ‎When building 7.0.0 from source, configure with -DUSE_AVX2=OFF. If reusing an existing build directory, set it explicitly and verify it took effect:

cmake -S . -B build -DUSE_AVX2=OFF -DCMAKE_BUILD_TYPE=Release

grep USE_AVX2 build/CMakeCache.txt

2) When using downloaded 7.0.0 binaries, use a build from current develop or the next release containing the fix. Rebuilding only the plugin libraries will not remove AVX2 from the Csound core binaries.

Or, alternatively, just build from the latest develop. There is nothing to be done from our end.

========================================

Prof. Victor Lazzarini

Director

Maynooth University Arts and Humanities Institute

From: Csound-developers <CSOUND-DEV@LISTSERV.HEANET.IE> on behalf of James Hearon <j_hearon@HOTMAIL.COM>
Sent: Saturday 31 October 2026 18:45
To: CSOUND-DEV@LISTSERV.HEANET.IE <CSOUND-DEV@LISTSERV.HEANET.IE>
Subject: [EXTERNAL] [Csnd-dev] plugins: illegal instruction

You don't often get email from j_hearon@hotmail.com. Learn why this is important

Warning

This email originated from outside of Maynooth University's Mail System. Do not reply, click links or open attachments unless you recognise the sender and know the content is safe.

re: plugins(libbeosc, libklib, lipathtools, etc.) and getting "illegal instruction" after building or downloading linux binaries, I did further inspection using gdb and found my problem is the sources CMakeLists.txt is using AVX2, which my older processor doesn't support.

if(LINUX AND CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|i[3-6]86|AMD64")

#addflag(-mavx2 HAS_AVX2) #causes illegal instruction error XXX for older cpus

if(HAS_AVX2)

message(STATUS "AVX2 enabled for Linux (x86)")

endif()

Also needed to add target_compile_options for each CMakeLists.txt for each library:

option(BUILD_BEOSC_OPCODES "Build beosc opcodes" ON)

if(BUILD_BEOSC_OPCODES)

make_plugin(beosc beosc.c)

target_compile_options(beosc PRIVATE -mno-avx2) #avoid building with AVX2

endif()

And is some cases a conditional general compiler directive:

make_plugin(else src/else.c)

#avoid building using AVX2

if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")

target_compile_options(else PRIVATE -mno-avx2)

endif()

...then rebuilt and seems working.

Registered charity number 20037130