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

[Phil Tomson via llvm-dev]

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
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