[llvm-dev] cmp instruction form

[Craig Topper via llvm-dev]

There are a couple things going on.

InstCombine prefers to canonicalize ule/uge/sle/sge compares with
constants to ult/ugt/slt/sgt by adjusting the constant. This allows
transforms in InstCombine and other passes to not need to check for two
forms of equivalent operations. The backends sometimes have to reverse this
to get better code.

There's a second transform that's turning the signed compare into an
unsigned compare because the loop counter starts at zero and counts up to a
positive number so the value will never be negative. Unsigned compares are
easier for some optimizations to reason about so the compiler tries to make
unsigned compares when it can.

~Craig


On Tue, Aug 10, 2021 at 4:54 AM zbh via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

>
> I understand it's  equivalent.But I don't know the benefit. And I don't
> understand why this statement changed to
> not eq in the last either.
> ···
> %exitcond = icmp ne i32 %inc, 129
>
> ···
>
>
> Thanks
> Rock
>
>
>
> At 2021-08-10 19:23:59, "Dimitry Andric" <dimitry at andric.com> wrote:
> >On 10 Aug 2021, at 13:16, zbh via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >>
> >> I find the value 128 chaned to 129. So I want to know what's the purpose to changed the form?Can you give some example?
> >
> >(icmp reference: <https://llvm.org/docs/LangRef.html#id303f>)
> >
> >Before, it uses "icmp sle" which is "signed <=", so effectively "<= 128".
> >
> >After, it uses "icmp ult" which is "unsigned <", so effectively "< 129".
> >
> >These are equivalent.
> >
> >-Dimitry
>
> -------------------------------------------------------------
>
> Hi all:
>
> I write C code like this:
>
> ```
> for (int i =0; i< 128; i++) {
>  printf("hello");
> }
> ```
> I use this command to compile the above code.
> ``
> clang -Xclang -disable-O0-optnone -O0  -emit-llvm -S main.c
> opt -O2  --print-after-all main.ll -S -o  main.mid.ll
> ``
> And I find the LLVM IR have this changed after instCombine optimization.
>
> ```
> *** IR Dump After Dead Argument Elimination ***
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @main(i32 %argc, i8** %argv) local_unnamed_addr #0 {
> entry:
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.body, %entry
>   %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
>   %cmp =* icmp sle i32 %i.0, 128*
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @.str, i64 0, i64 0))
>   %inc = add nsw i32 %i.0, 1
>   br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>   ret i32 0
> }
>
> *** IR Dump After Combine redundant instructions ***
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @main(i32 %argc, i8** %argv) local_unnamed_addr #0 {
> entry:
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.body, %entry
>   %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
>   %cmp = *icmp ult i32 %i.0, 129*
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %call = call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @.str, i64 0, i64 0))
>   %inc = add nuw nsw i32 %i.0, 1
>   br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>   ret i32 0
> }
>
> ···
> I find the value 128 chaned to 129. So I want to know what's the purpose to changed the form?Can you give some example?
>
> Thanks
> Rock
>
>
>
>
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