[llvm-dev] Lowering llvm.memset for ARM target

[Evgeny Astigeevich via llvm-dev]

Hi Eli,

Thank you for information. Now it’s clear what to do.
BTW I tried to apply Thumb2 to the test and it didn’t work. It means we have more motivation to fix this (.

Thanks,
Evgeny

-----Original Message-----
From: "Friedman, Eli" <efriedma at codeaurora.org>
Date: Wednesday, 25 October 2017 at 19:48
To: Evgeny Astigeevich <Evgeny.Astigeevich at arm.com>, Bharathi Seshadri <bharathi.seshadri at gmail.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>, nd <nd at arm.com>
Subject: Re: [llvm-dev] Lowering llvm.memset for ARM target

    On 10/25/2017 8:22 AM, Evgeny Astigeevich via llvm-dev wrote:
    > Hi Bharathi,
    >
    > I did some debugging, the current problem is that the same threshold values are used for SIMD and non-SIMD memory instructions.
    > For the test you provided, when the SIMD extension is disabled the current implementation of llvm.memset lowering finds out 9 store instructions will be required. As 9 > 8 llvm.memset is lowered to a call.
    > When the SIMD is enabled, it finds out 3 stores (two vst1.32 + one str) will be enough. As 3 < 8 llvm.memset is lowered to a sequence of stores.
    >
    > So before changing the threshold values we need to figure out:
    >
    > 1. Do we need separate thresholds for SIMD and non-SIMD memory instructions? For example, 8 for SIMD and 10-16 for non-SIMD. Some benchmarking is needed to find proper value.
    > 2. If we keep the single value, how much it should be increased? This might affect performance of SIMD using applications. So benchmarking again.
    > 3. If STRD are used then only 5 instructions are needed and llvm.memset is lowered as expected. I don’t know why the variant with STRD is not considered, maybe to avoid register pressure.
    
    We probably do want to lower memset to strd/stm when we don't have 
    NEON.  Patch welcome. :)  (Note that in Thumb2, we often form strd 
    anyway in ARMLoadStoreOptimizer, but we don't do it reliably, and we 
    probably want to do something similar in ARM, and maybe Thumb1.)
    
    -Eli
    
    -- 
    Employee of Qualcomm Innovation Center, Inc.
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