[llvm-dev] cmp instruction form (original) (raw)

zbh via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 10 04:54:02 PDT 2021

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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