[llvm-dev] Disabling select instructions

[Charitha Saumya via llvm-dev]

I highly appreciate your suggestions*. *can someone explain how to add this
type of pass to the optimization pipeline (probably point to code where
optimization pipeline is built). Or can I do this using LLVM opt?


On Fri, Jan 31, 2020 at 8:57 AM Alexey Zhikhartsev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I agree with John; also, if you decide to go this route, you can reuse the
> code from CodeGenPrepare::optimizeSelectInst:
>
>
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/CodeGen/CodeGenPrepare.cpp#L6065
>
> Alexey
>
> On Thu, Jan 30, 2020 at 9:00 PM John Regehr via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Several different passes introduce select instructions, such as
>> InstCombine, SimplifyCFG, and SROA. Searching for calls to
>> "CreateSelect" in the source tree should give you a fairly complete list.
>>
>> It may be better to let the optimization pipeline proceed undisturbed,
>> but then run a custom pass at the end that turns selects back into
>> control flow. This would be an easy pass to write.
>>
>> John
>>
>>
>> On 1/30/20 3:31 PM, Charitha Saumya via llvm-dev wrote:
>> > Hi,
>> >
>> > I would like to know if there's a way to avoid select instructions
>> > during the IR generation. What are the optimization passes that can
>> > result in a select instruction?
>> > i.e. I want to preserve branches in my code without disabling any other
>> > optimizations applicable.
>> >
>> > For example,
>> > void foo(int* x, int* y){
>> >      if(*x > 0){
>> >          *y = *x + 10;
>> >      }
>> >      else{
>> >          *y = *x + 20;
>> >      }
>> > }
>> >
>> > with O1 I get,
>> >
>> > define void @foo(i32* nocapture readonly, i32* nocapture)
>> > local_unnamed_addr #0 {
>> >    %3 = load i32, i32* %0, align 4, !tbaa !2
>> >    %4 = icmp sgt i32 %3, 0
>> >    %5 = select i1 %4, i32 10, i32 20
>> >    %6 = add nsw i32 %5, %3
>> >    store i32 %6, i32* %1, align 4, !tbaa !2
>> >    ret void
>> > }
>> >
>> > But with O0,
>> >
>> > define void @foo(i32*, i32*) #0 {
>> >    %3 = alloca i32*, align 8
>> >    %4 = alloca i32*, align 8
>> >    store i32* %0, i32** %3, align 8
>> >    store i32* %1, i32** %4, align 8
>> >    %5 = load i32*, i32** %3, align 8
>> >    %6 = load i32, i32* %5, align 4
>> >    %7 = icmp sgt i32 %6, 0
>> >    br i1 %7, label %8, label %13
>> >
>> > ; <label>:8:                                      ; preds = %2
>> >    %9 = load i32*, i32** %3, align 8
>> >    %10 = load i32, i32* %9, align 4
>> >    %11 = add nsw i32 %10, 10
>> >    %12 = load i32*, i32** %4, align 8
>> >    store i32 %11, i32* %12, align 4
>> >    br label %18
>> >
>> > ; <label>:13:                                     ; preds = %2
>> >    %14 = load i32*, i32** %3, align 8
>> >    %15 = load i32, i32* %14, align 4
>> >    %16 = add nsw i32 %15, 20
>> >    %17 = load i32*, i32** %4, align 8
>> >    store i32 %16, i32* %17, align 4
>> >    br label %18
>> >
>> > ; <label>:18:                                     ; preds = %13, %8
>> >    ret void
>> > }
>> >
>> > Thanks,
>> > Charitha
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
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