[llvm-dev] cmp instruction form

[zbh via llvm-dev]

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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