[llvm-dev] cmp instruction form

[zbh via llvm-dev]

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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