Segfault when using clang 7

[karjonas]

Hi,

It seems like compiling ispc with clang version 7 is giving a segmentation fault. This is the stack trace I get:

Starting program: /home/jonas/Coding/ispc/build/ispc 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Warning: No output file or header file name specified. Program will be compiled and warnings/errors will be issued, but no output will be generated. 
Warning: No --target specified on command-line. Using default system target "avx1-i32x8". 

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff5368dea in ?? () from /usr/lib/libLLVM-7.so
(gdb) bt
#0  0x00007ffff5368dea in ?? () from /usr/lib/libLLVM-7.so
#1  0x00005555556a7dab in llvm::Target::createTargetMachine (this=0x7ffff69ebcc0, TT=..., CPU=..., Features=..., Options=..., RM=..., CM=..., 
    OL=llvm::CodeGenOpt::Default, JIT=false) at /usr/include/llvm/Support/TargetRegistry.h:397
#2  0x00005555556a32ff in Target::Target (this=0x555555f0f840, arch=0x55555576c555 "x86-64", cpu=0x555555f14420 "corei7-avx", isa=0x55555576c16b "avx1-i32x8", 
    pic=false, printTarget=false, genericAsSmth=...) at /home/jonas/Coding/ispc/src/ispc.cpp:1118
#3  0x00005555556ca0d2 in Module::CompileAndOutput (srcFile=0x0, arch=0x0, cpu=0x0, target=0x0, outputFlags=Module::NoFlags, outputType=Module::Object, 
    outFileName=0x0, headerFileName=0x0, includeFileName=0x0, depsFileName=0x0, depsTargetName=0x0, hostStubFileName=0x0, devStubFileName=0x0)
    at /home/jonas/Coding/ispc/src/module.cpp:3187
#4  0x00005555556bcccb in main (Argc=1, Argv=0x7fffffffe8c8) at /home/jonas/Coding/ispc/src/main.cpp:799

If I compile it using gcc it works fine though. I do not know what clang is doing differently. Let me know if you need more information from me.

Regards, Jonas

[dbabokin]

LLVM 7.0 works well for me, except several known fails. Could you provide more details? How to do build LLVM? (this seems to be the key in this case).

[karjonas]

I tried building using CMake, and I tried building using the pkgbuild from arch https://www.archlinux.org/packages/community/x86_64/ispc/ and they both fail if I use clang as the compiler but work fine with gcc. I also made a ticket in the arch linux bug tracker https://bugs.archlinux.org/task/60340?project=5&cat%5B0%5D=33&string=crash. Maybe this issue is specific to Arch, I don't know.

[dbabokin]

I was able to reproduce it. This is reproduced if (1) LLVM 7.0 is used, (2) LLVM is built by gcc, (3) ISPC is built by clang.

This is something new, but we already had a case, when using different compilers for LLVM and ISPC builds caused link problems. So we follow the rule to build LLVM and ISPC with the same compiler. Preferably to clang for that. I.e. if you are building using "alloy.py", you need to use "--selfbuild" switch. When configuring with cmake, define CC and CXX, i.e. run "CC=clang CXX=clang++ cmake ..".

I have no bandwidth to investigate this, so I'm closing the issue. If someone is brave enough to investigate it and report to LLVM, I would applaud.

[ArchangeGabriel]

Upstream is aware (e.g. https://groups.google.com/forum/#!msg/llvm-dev/8Tcz49kTNCw/NI0l_MiZAwAJ), and the issue is more general: “In short, building your project with clang and linking against LLVM libraries built with GCC causes a SEGFAULT because of an ABI incompatibility.”.

[dbabokin]

Thanks for the link! Though it's not clear when they are going to address that. And looks like there's no hope for fixing it in 7.0

[ArchangeGabriel]

Seems this was fixed in 8.0. I’m building ispc against clang again.

[dbabokin]

Thanks for the update!