[LLVMdev] Heads up! New SROA implementation is going on-by-default today!

David Tweed David.Tweed at arm.com
Mon Sep 24 09:05:26 PDT 2012

Actually, I may have been wrong about this: I'm currently checking it out. I'll post again if I'm convinced any of them do occur with the latest repository head.

(Many source changes result in a build time on a native ARM for LLVM is, umm, really quite significant, and I've been trying various ways to speed the process up and I currently think I made a mistake in the latest "build optimization" so that the compiled object files got "stuck" corresponding to a point in the middle of last weekend even when I thought I'd updated to the head of the repository. Those failures did happen at one point in the tree's history, but I don't think they happen any more.)

Sorry for the noise.

From: chandlerc at google.com [mailto:chandlerc at google.com] On Behalf Of Chandler Carruth
Sent: 24 September 2012 16:17
To: David Tweed
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Heads up! New SROA implementation is going on-by-default today!

On Mon, Sep 24, 2012 at 3:41 AM, David Tweed <david.tweed at arm.com<mailto:david.tweed at arm.com>> wrote:
Just a note that the following new regressions have started to show up on ARM/Linux:

Thanks for letting me know!

I know that there is one serious bug that would impact in BE system. I should have that fixed today, along with a crasher. I would appreciate help tracking down any issues once the one I know of is fixed though.

If this is cause problems for anyone, you can disable it with a flag, or we can temporarily turn it back off. I'm expecting to have most of the issues folks have reported fixed today though.

    LLVM :: Bitcode/blockaddress.ll
    LLVM :: CodeGen/ARM/fast-isel.ll
    LLVM :: CodeGen/X86/atomic16.ll
    LLVM :: MC/ARM/arm-shift-encoding.s
    LLVM :: MC/ARM/thumb-shift-encoding.s
    LLVM :: MC/COFF/global_ctors_dtors.ll
    LLVM :: Transforms/InstCombine/div-shift.ll
    LLVM :: Transforms/LoopIdiom/non-canonical-loop.ll
    LLVM :: Transforms/SROA/basictest.ll
    LLVM :: Transforms/SROA/phi-and-select.ll

I haven't yet looked at them in detail, and obviously on the last two clearly involve SROA, but given that these tests haven't started to fail (or at least been fixed before I noticed) on my x86 machine I thought it worth reporting.


From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of Chandler Carruth
Sent: 22 September 2012 20:09
To: LLVM Developers Mailing List
Subject: [LLVMdev] Heads up! New SROA implementation is going on-by-default today!

After a lot of testing and help from Duncan, Benjamin, Joerg and others, I think the new SROA is ready for some broader testing. I've fixed all the crashers and miscompiles that Duncan and Joerg have been able to find (although I'm sure there are a few left I'll tackle when there are reports), and the LNT numbers look *really* good. Here is the latest LNT run we got by flipping it on and back off:


Most of this is very, very green. There are three somewhat worrisome regressions in execution time:

1) sse_expandfft -- when I build this, the binaries have no differences before and after
2) sse_stepfft -- ditto
3) matmul_f64_4x4 -- this one is interesting

The last one represents the only real regressions I expect to see with the new pass. There is a helpful indicator about what caused it: the compile time *improved* by 44%!!! This is because the benchmark was tickling the bad behavior of the old SROA pass that inspired a lot of this work, and was building massive bit vectors to store integer arrays. The new pass doesn't do that, produces the exact IR desired from this benchmark, and speeds up compilation by a huge factor.

Unfortunately, sometimes having all the integers be stuffed into a big bitvector and lowered during legalization papered over problems in the register allocator (they look like spill placement to me? Haven't dug too deeply) that are now being exposed. That's responsible for the 7% regression in matmul.

Just ping me if this causes anyone problems! You can also temporarily disable it by passing '-mllvm -use-new-sroa=false' to Clang.

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