Re: JIT compiling with LLVM v11

Hi,

On 2018-03-09 00:33:03 +1300, Thomas Munro wrote:
> On Wed, Mar 7, 2018 at 3:49 PM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> > make check at today's HEAD of your jit branch crashes on my FreeBSD
> > box. The first thing to crash is this query from point.sql:
> >
> > LOG: server process (PID 87060) was terminated by signal 4: Illegal instruction
> > DETAIL: Failed process was running: SELECT '' AS thirtysix, p1.f1 AS
> > point1, p2.f1 AS point2, p1.f1 <-> p2.f1 AS dist
> > FROM POINT_TBL p1, POINT_TBL p2
> > ORDER BY dist, p1.f1[0], p2.f1[0];
>
> Hmm. It's trying to execute an AVX instruction.

Ah, that's interesting.

> I am not sure if that is real though, because the stack is immediately
> corrupted.

I don't think the stack is corrupted at all, it's just that lldb can't
unwind with functions it doesn't know. To add that capability I've a
pending LLVM patch.

> So either func is not really a function, or it is but was
> compiled for the wrong target. I see that you call
> LLVMCreateTargetMachine() with the result of LLVMGetHostCPUName() as
> cpu. For me that's "ivybridge", so I tried hard coding "generic"
> instead and it didn't help.

Hm.

> I see that you say "" for features, where
> is where one would normally put "avx" to turn on AVX instructions, so
> I think perhaps that theory is entirely bogus.

Could you try a -avx in features and see whether it fixes things?

This kinda suggests an LLVM bug or at least an oddity, but I'll try to
drill down more into this. Is this a native machine or a VM?

I think we can easily fix this by behaving like clang, which uses
llvm::sys::getHostCPUFeatures(HostFeatures) to built the feature list:

// If -march=native, autodetect the feature list.
if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_march_EQ)) {
if (StringRef(A->getValue()) == "native") {
llvm::StringMap<bool> HostFeatures;
if (llvm::sys::getHostCPUFeatures(HostFeatures))
for (auto &F : HostFeatures)
Features.push_back(
Args.MakeArgString((F.second ? "+" : "-") + F.first()));
}
}

which seems easy enough.

Greetings,

Andres Freund