[SR-6813] LLVM 3.8+ emits NEON instructions on non-NEON ARMv7 processors

[hpux735]

Previous ID	SR-6813
Radar	None
Original Reporter	@hpux735
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, armv7
Assignee	None
Priority	Medium

md5: 3fa4c23803f5dcaafdadeec6ac73a90a

Issue Description:

Beginning at LLVM 3.8, LLVM will generate NEON instructions regardless of whether the processor targeted supports these instructions.

According to this (otherwise unrelated) bug report, this should be fixed in LLVM 3.9, however, while building Swift on these platforms, LLVM 3.9 still generates illegal instructions.

[163/2690] Building AttributesCompatFunc.inc...
FAILED: cd /root/buildSwiftOnARM/build/buildbot_linux/llvm-linux-armv7/lib/IR && /root/buildSwiftOnARM/build/buildbot_li
nux/llvm-linux-armv7/bin/llvm-tblgen -gen-attrs -I /root/buildSwiftOnARM/llvm/lib/IR -I /root/buildSwiftOnARM/llvm/inclu
de /root/buildSwiftOnARM/llvm/lib/IR/AttributesCompatFunc.td -o /root/buildSwiftOnARM/build/buildbot_linux/llvm-linux-ar
mv7/lib/IR/AttributesCompatFunc.inc.tmp
Illegal instruction

I have a build system that exhibits this bug on "the cloud" and I will provide a username and password to anyone willing and able to help with this bug.

[compnerd]

This looks like a bug in the build of LLVM. LLVM can definitely be built on ARM and I believe is done so as part of the buildbot set for LLVM itself.

[hpux735]

Yes, I see your point. You posit that the llvm-3.8 and llvm-3.9 built and distributed in Debian-armhf are broken?

I will try building from source, and see if the problem persists.

[hpux735]

Does anyone have "getting started" instructions that work handy? I'm getting a build error on LLVM:

make[2]: *** [lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/LexicalScopes.cpp.o] Error 1
CMakeFiles/Makefile2:1237: recipe for target 'lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/all' failed
make[1]: *** [lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/all] Error 2
Makefile:149: recipe for target 'all' failed
make: *** [all] Error 2
root@scw-paris-build-1:~/build#
{{ std::forward_as_tuple(Parent, Scope,}}
{{ ^~~~~~~~~~~~~~~~~~~~~}}
/usr/bin/../lib/gcc/arm-linux-gnueabihf/6.3.0/../../../../include/c++/6.3.0/tuple:1403:5: note: candidate template ignored: substitution failure [with _Elements = <llvm::LexicalScope *&, const llvm::DILocalScope *&, nullptr_t,
{{ bool>]}}
{{ forward_as_tuple(_Elements&&... __args) noexcept}}
{{ ^}}

This is following the instructions from llvm.org

[MaxDesiatov]

@hpux735 have you been able to reproduce this with more recent versions of Swift?

[hpux735]

That's a good question. I don't know. We don't have devices that are non-NEON at work anymore, so I don't have a great way to test it.