[LLVMdev] Behaviour of NVPTX intrinsic

[RAVI KORSA]

I have written test.ll as below and ran 'opt' on it  as
" opt -std-compile-opts test.ll -S -o -" . But the output shows that there
is code motion around the barrier intrinsics.

test.ll
-------
; ModuleID = 'test.bc'

define void @test(i16* %I_0, i16* %I_1, i16* %I_2, i16* %I_3, i16* %O_0) {
entry:
  %T_0 = load volatile i16* %I_0
  %T_1 = load volatile i16* %I_1
  %T_2 = load volatile i16* %I_2
  %T_3 = load volatile i16* %I_3
  call void @llvm.nvvm.barrier0()
  %T_5 = add i16 %T_1, %T_3
  call void @llvm.nvvm.barrier0()
  %T_7 = mul i16 %T_0, %T_2
  %T_8 = xor i16 %T_2, %T_0
  %T_9 = mul i16 %T_0, %T_1
  call void @llvm.nvvm.barrier0()
  %T_11 = sub i16 %T_7, %T_5
  %T_12 = add i16 %T_8, %T_9
  %T_13 = add i16 %T_11, %T_12
  store volatile i16 %T_13, i16* %O_0
  ret void
}

;declare i32 @llvm.nvvm.read.ptx.sreg.tid.x()
declare void @llvm.nvvm.barrier0()

output
---------
define void @test(i16* nocapture %I_0, i16* nocapture %I_1, i16* nocapture
%I_2, i16* nocapture %I_3, i16* nocapture %O_0) nounwind {
entry:
  %T_0 = load volatile i16* %I_0
  %T_1 = load volatile i16* %I_1
  %T_2 = load volatile i16* %I_2
  %T_3 = load volatile i16* %I_3
  tail call void @llvm.nvvm.barrier0()
  tail call void @llvm.nvvm.barrier0()
  %T_8 = xor i16 %T_2, %T_0
  tail call void @llvm.nvvm.barrier0()
  %sum = add i16 %T_3, %T_1
  %tmp = add i16 %T_2, %T_1
  %tmp1 = mul i16 %tmp, %T_0
  %T_11 = sub i16 %T_8, %sum
  %T_13 = add i16 %T_11, %tmp1
  store volatile i16 %T_13, i16* %O_0
  ret void
}

Thanks

On Tue, Sep 30, 2014 at 9:50 PM, Eli Bendersky <eliben at google.com> wrote:

>
>
> On Tue, Sep 30, 2014 at 9:04 AM, RAVI KORSA <ravi.korsa at gmail.com> wrote:
>
>> is there any guarantee that the nvptx intrinsic "llvm.nvvm.barrier0" will
>> not be moved around by opt ?
>> In other words, can I expect all the instructions above
>> "llvm.nvvm.barrier0" to remain above it and those below it to remain below,
>> after all the opt passes are run ?
>>
>
> AFAIU, yes. Here's the definition:
>
>   def int_nvvm_barrier0 : GCCBuiltin<"__nvvm_bar0">,
>       Intrinsic<[], [], [IntrNoDuplicate]>;
>
> Note that IntrNoDuplicate is the only intrinsic attribute. It has no other
> attributes (like IntrNoMem) that would make it permissible for LLVM
> optimizations to reorder things around it. By default, the optimizers would
> not do this for function calls; only if these function calls are marked
> with special attributes that permit this.
>
> Eli
>
>
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