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

David Tweed david.tweed at arm.com
Mon Sep 24 03:41:51 PDT 2012

Just a note that the following new regressions have started to show up on ARM/Linux:


    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] 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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120924/3f02976a/attachment.html>

More information about the llvm-dev mailing list