[LLVMdev] SIMD instructions and memory alignment on X86

Peter Newman
Wed, 17 Jul 2013 01:39:57 -0000

Hello all,

I'm currently in the process of debugging a crash occurring in our
program. In LLVM 3.2 and 3.3 it appears that JIT generated code is
attempting to perform access unaligned memory with a SSE2 instruction.
However this only happens under certain conditions that seem (but may
not be) related to the stacks state on calling the function.

Our program acts as a front-end, using the LLVM C++ API to generate a
JIT generated function. This function is primarily mathematical, so we
use the Vector types to take advantage of SIMD instructions (as well as
a few SSE2 intrinsics).

This worked in LLVM 2.8 but started failing in 3.2 and has continued to
fail in 3.3. It fails with no optimizations applied to the LLVM
Function/Module. It crashes with what is reported as a memory access
error (accessing 0xffffffff), however it's suggested that this is how
the SSE fault raising mechanism appears.

The generated instruction varies, but it seems to often be similar to (I
don't have it in front of me, sorry):
movapd xmm0, xmm[ecx+0x???????]
Where the xmm register changes, and the second parameter is a memory access.
ECX is always set to 0x7ffffff - however I don't know if this is part of
the SSE error reporting process or is part of the situation causing the
error.

I haven't worked out exactly what code path etc is causing this crash.
I'm hoping that someone can tell me if there were any changed
requirements for working with SIMD in LLVM 3.2 (or earlier, we haven't
tried 3.0 or 3.1). I currently suspect the use of GlobalVariable (we
first discovered the crash when using a feature that uses them), however
I have attempted using setAlignment on the GlobalVariables without any
change.

--
Peter N