[llvm-dev] Instruction itineraries and fence/barrier instructions

[Phil Tomson via llvm-dev]

On Mon, Aug 22, 2016 at 11:40 AM, Matt Arsenault <arsenm2 at gmail.com> wrote:

>
> > On Aug 22, 2016, at 11:20, Phil Tomson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > We improved our instruction itineraries and now we're seeing our
> testcases for fence instructions break.
> >
> > For example, we have this testcase:
> >
> > @write_me = external global i32
> > @read_me = external global i32
> >
> > ; Function Attrs: nounwind
> > define i32 @xstg_intrinsic(i32 %foo) #0 {
> > entry:
> > ; CHECK: store        r0, r1, 0, 32
> > ; CHECK-NEXT: fence 2
> >   %foo.addr = alloca i32, align 4
> >   store i32 %foo, i32* %foo.addr, align 4
> >   %0 = load i32* %foo.addr, align 4
> >   store volatile i32 %0, i32* @write_me, align 4
> >   call void @llvm.xstg.memory.barrier(i32 2, i8 0)
> >   %1 = load volatile i32* @read_me, align 4
> >   ret i32 %1
> > }
> >
> > Prior to adding our instruction itineraries the code generated was:
> >
> > xstg_intrinsic:                         # @xstg_intrinsic
> > # BB#0:                                 # %entry
> >     subI    r509, r509, 16, 64
> >     store        r510, r509, 0, 64
> >     bitop1        r510, r509, 0, OR, 64
> >     store        r0, r510, 12, 32
> >     movimm        r1, %hi(write_me), 64
> >     movimmshf32    r1, r1, %lo(write_me)
> >     store        r0, r1, 0, 32
> >     fence 2
> >     movimm        r0, %hi(read_me), 64
> >     movimmshf32    r0, r0, %lo(read_me)
> >     load        r1, r0, 0, 32
> >     bitop1        r509, r510, 0, OR, 64
> >     load        r510, r509, 0, 64
> >     addI    r509, r509, 16, 64
> >     jabs        r511
> >
> > Note the separation between the store prior to the fence and the code
> that comes after.
> >
> > Now that we've got itineraries in place we see:
> >
> >     subI    r509, r509, 16, 64
> >     store        r510, r509, 0, 64
> >     bitop1        r510, r509, 0, OR, 64
> >     movimm        r1, %hi(write_me), 64
> >     store        r0, r510, 12, 32
> >     movimmshf32    r1, r1, %lo(write_me)
> >     movimm        r2, %hi(read_me), 64
> >     store        r0, r1, 0, 32
> >     movimmshf32    r2, r2, %lo(read_me)
> >     fence 2
> >     load        r1, r2, 0, 32
> >     bitop1        r509, r510, 0, OR, 64
> >     load        r510, r509, 0, 64
> >     addI    r509, r509, 16, 64
> >     jabs        r511
> >
> > the movimm which sets up the address for the load has been moved up
> prior to the fence.
> >
> > Is there a way to indicate in the itinerary that position of the fence
> should be fixed - no instruction reordering "through" the fence/barrier?
> >
> > Phil
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> I don’t see a change relative to the memory instructions.


True, we may be being a bit too skiddish about this... perhaps the solution
is to change the testcase so that we can ensure that the relative order
between the store and the fence has been preserved.


> Do you mean you want this to avoid scheduling of any instruction around
> any other? Does the instruction have isSideEffects set on it?


Where can I find information about isSideEffects? Googling "LLVm
isSideEffects" didnt' reveal anything that looked relevant.


> I think the fallback if that isn’t enough is to override TargetInstrInfo::
> isSchedulingBoundary
>
>
Thanks, I'll look at that in other targets.

Phil

> -Matt
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