[LLVMdev] Lowering to MMX

[Bill Wendling]

On Oct 20, 2011, at 8:42 AM, Nicolas Capens wrote:

> Hi all,
> 
> I'm working on a graphics project which uses LLVM for dynamic code 
> generation, and I noticed a major performance regression when upgrading 
> from LLVM 2.8 to 3.0-rc1 (LLVM 2.9 didn't support Win64 so I skipped it 
> entirely).
> 
> I found out that the performance regression is due to removing support 
> for lowering 64-bit vector operations to MMX, and using SSE2 instead. My 
> code uses a mix of MMX intrinsics and v4i16 operations, so it ping-pongs 
> back and forth between MMX and SSE2 instructions in the generated code.
> 
> To get more optimal code, I see three options, and I was wondering if 
> someone could share some advice on which approach you think will work best:
> 1) I could use v8i16 or v4i32 instead of v4i16, but then the SSE 
> register pressure would be significantly increased. I already use v4f32 
> operations intensively so having the MMX registers available for 64-bit 
> integer vector operations helps performance quite considerably on the 
> register deprived x86 architecture. There's little to no opportunity for 
> using v8i16 to perform two v4i16 operations simultaneously so that won't 
> make up for the added register pressure. So I'm not keen to implement 
> this option, unless anyone sees some advantages that I missed?

It's my understanding that SSE is by far superior to MMX for a number of reasons, not the least of which is the need to use the expensive EMMS instruction. Instead of guessing about the performance impact, I would encourage you to test this out.

> 2) Since I use MMX intrinsics, I take care of inserting the appropriate 
> EMMS instructions myself as well. So it's absolutely fine to have LLVM 
> lower 64-bit operations into MMX instructions (the way it used to be in 
> LLVM 2.8). Would it be straightforward to re-enable this? I noticed that 
> revision 115243 removes the MMX lowering rules, but I don't know if the 
> rest of LLVM 3.0 would still support them if I simply reverted them. 
> Please note that I'm not an LLVM expert and I'd prefer not having to 
> maintain local changes. Would there be any objection to having an 
> 'EnableMMX' flag (false by default)?

Having the EnableMMX flag is not an option. And the changes are significant, so you wouldn't be able to re-enable the MMX stuff without a major overhaul of the system.

> 3) I believe all MMX instructions are available as intrinsics now? That 
> would allow me to replace all straight LLVM operations with intrinsics. 
> I'm just wondering what the downsides of that would be? I assume I won't 
> get any benefits from instruction combining, but things like dead code 
> elimination still work?

Intrinsics are the only way to go if you want MMX code. We do as much as we can, but to be honest optimizing for MMX is not a high priority for us.

-bw