[LLVMdev] LICM promoting memory to scalar
Robin Morisset
morisset at google.com
Wed Sep 3 13:19:51 PDT 2014
I do not know the loop infrastructure in LLVM well, so I cannot really give
advice about how to proceed.
It would be fine in this case to do the optimization as long as the loop is
entered, but the real condition is "would there be at least one load/store
to this variable ?".
For example, if instead of "i < n/2" there was "i % 10 == 5" in your code,
the optimization would have to be guarded by a check for n >= 6.
Robin
On Wed, Sep 3, 2014 at 9:59 AM, Balaram Makam <bmakam at codeaurora.org> wrote:
> Thanks for the background on the concurrent memory model.
>
> So, is it sufficient that the loop entry is guarded by condition (cbz at
> top) for preventing the race?
> The loop entry will be guarded by condition if loop has been rotated by
> loop
> rotate pass.
>
> Since LICM runs after loop rotate, we can use
> ScalarEvolution::isLoopEntryGuardedByCond to check if we can speculatively
> execute load without causing a race.
> Is it heavy-handed solution for this problem? I can create a bug if you
> would like to.
>
> Thanks,
> Balaram
>
> From: Robin Morisset [mailto:morisset at google.com]
> Sent: Tuesday, September 02, 2014 7:40 PM
> To: Filip Pizlo
> Cc: Balaram Makam; LLVM Developers Mailing List
> Subject: Re: [LLVMdev] LICM promoting memory to scalar
>
> Ah right, I had missed the cbz, my bad. It is indeed sound under that
> condition.
> My point was that just hoisting/sinking unconditionally the memory accesses
> is unsound. It is good news that gcc learnt how to do it carefully :)
>
> On Tue, Sep 2, 2014 at 4:36 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> I think gcc is right.
>
> It inserted a branch for n == 0 (the cbz at the top), so that's not a
> problem.
>
> In all other regards, this is safe: if you examine the sequence of loads
> and
> stores, it eliminated all but the first load and all but the last store.
> How's that unsafe?
>
> If I had to guess, the bug here is that LLVM doesn't want to hoist the load
> over the condition (which it is right to want to avoid) but it fails to
> realize that the condition is true for the first iteration.
>
> -Filip
>
> On Sep 2, 2014, at 4:23 PM, Robin Morisset <morisset at google.com> wrote:
> The problem here is that doing this can introduce a race which is undefined
> at the IR level.
> In the example you gave above I suspect that this is a bug in GCC. I may
> have missed something in the assembly, but it appears that gcc loads a copy
> of globalvar in w3, does stuff with w3 and then stores w3 in globalvar.
> This
> is unsound in the case where the loop is run with n = 0.
> For an example, if you have a thread run foo(0,0) in a loop (so not doing
> anything) and another thread doing:
> for (int i = 0 ; i <1000000 ; ++i) {
> globalvar = i;
> assert(globalvar == i);
> }
> the code should never assert. But if you host the load/store out of the
> loop
> (as GCC appears to do), then occasionaly there will be something like
> thread 2: globalvar = i (= 42)
> thread 1: w3 = globalvar (= 42)
> thread 2: ++i
> thread 2: globalvar = i (= 43)
> thread 1: globalvar =w3 (= 42)
> thread 2: assert(globalvar == i)
> And it will assert (42 == 43) and crash.
>
> Shorter answer:
> - speculatively executing stores is unsound because the value may have been
> modified behind your back by another thread.
> - speculatively executing loads might not be observable in some specific
> case but may always introduce races, thus introducing undefined behaviour
> and breaking assumptions that other passes may rely on.
>
> Best regards,
> Robin
>
> On Tue, Sep 2, 2014 at 3:46 PM, Balaram Makam <bmakam at codeaurora.org>
> wrote:
> All,
>
> If we can speculatively execute a load instruction, why isn’t it safe to
> hoist it out by promoting it to a scalar in LICM pass?
>
>
> There is a comment in LICM pass that if a load/store is conditional then it
> is not safe because it would break the LLVM concurrency model (See commit
> 73bfa4a).
> It has an IR test for checking this in
> test/Transforms/LICM/scalar-promote-memmodel.ll
>
> However, I have a sample code where GCC is able to promote the memory to
> scalar and hoist/sink load/store out of loop but LLVM cannot.
> Is GCC being aggressive here or LLVM missing out an opportunity?
>
> Here is my sample code:
>
> extern int globalvar;
> void foo(int n , int incr) {
> unsigned int i;
> for (i = 0 ; i < n; i += incr ) {
> if (i < n/2)
> globalvar += incr;
> }
> return;
> }
>
> GCC output:
>
> $ aarch64-linux-gnu-g++ -S -o - -O3 -ffast-math -march=armv8-a+simd
> test.cpp
> .arch armv8-a+fp+simd
> .file "test.cpp"
> .text
> .align 2
> .global _Z3fooii
> .type _Z3fooii, %function
> _Z3fooii:
> .LFB0:
> .cfi_startproc
> cbz w0, .L1
> adrp x6, globalvar
> add w5, w0, w0, lsr 31
> ldr w3, [x6,#:lo12:globalvar] <== hoist
> load of globalvar
> mov w2, 0
> asr w5, w5, 1
> .L4:
> cmp w5, w2
> add w2, w2, w1
> add w4, w3, w1
> csel w3, w4, w3, hi
> cmp w2, w0
> bcc .L4
> str w3, [x6,#:lo12:globalvar] <== sink
> store of globalvar
> .L1:
> ret
> .cfi_endproc
> .LFE0:
> .size _Z3fooii, .-_Z3fooii
> .ident "GCC: (crosstool-NG linaro-1.13.1-4.8-2014.01 - Linaro GCC
> 2013.11) 4.9.0"
>
> LLVM output:
>
> $ clang-aarch64-x++ -S -o - -O3 -ffast-math -fslp-vectorize test.cpp
> .text
> .file "test.cpp"
> .globl _Z3fooii
> .align 2
> .type _Z3fooii, at function
> _Z3fooii: // @_Z3fooii
> // BB#0: // %entry
> cbz w0, .LBB0_5
> // BB#1: // %for.body.lr.ph
> mov w8, wzr
> cmp w0, #0 // =0
> cinc w9, w0, lt
> asr w9, w9, #1
> adrp x10, globalvar
> .LBB0_2: // %for.body
> // =>This Inner Loop Header:
> Depth=1
> cmp w8, w9
> b.hs .LBB0_4
> // BB#3: // %if.then
> // in Loop: Header=BB0_2 Depth=1
> ldr w11, [x10, :lo12:globalvar] <===== load
> inside loop
> add w11, w11, w1
> str w11, [x10, :lo12:globalvar] <====
> store
> inside loop
> .LBB0_4: // %for.inc
> // in Loop: Header=BB0_2 Depth=1
> add w8, w8, w1
> cmp w8, w0
> b.lo .LBB0_2
> .LBB0_5: // %for.end
> ret
> .Ltmp1:
> .size _Z3fooii, .Ltmp1-_Z3fooii
>
>
> .ident "clang version 3.6.0 "
>
> Thanks,
> Balaram
>
>
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