llvm code optimization options do not work - optimization

Question

I am reading about LLVM Code optimization.I tried to apply opt command options on a number of examples but they do not have any effect. For example.Here is a c++ code called deadCode.cpp:
#include<stdio.h>
int square(int x){
return x*x;
}
int main(){
int a=2;
int b=3;
int c=4;
int result =square(a);
printf("%d\n",b);
}
I generated the LLVM IR with clang like this:
clang -emit-llvm -S deadCode.cpp -o deadCodeBefore
and the result file deadCodeBefore content is :
; ModuleID = 'deadCode.cpp'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
#.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
; Function Attrs: nounwind uwtable
define i32 #_Z6squarei(i32 %x) #0 {
%1 = alloca i32, align 4
store i32 %x, i32* %1, align 4
%2 = load i32, i32* %1, align 4
%3 = load i32, i32* %1, align 4
%4 = mul nsw i32 %2, %3
ret i32 %4
}
; Function Attrs: norecurse uwtable
define i32 #main() #1 {
%a = alloca i32, align 4
%b = alloca i32, align 4
%c = alloca i32, align 4
%result = alloca i32, align 4
store i32 2, i32* %a, align 4
store i32 3, i32* %b, align 4
store i32 4, i32* %c, align 4
%1 = load i32, i32* %a, align 4
%2 = call i32 #_Z6squarei(i32 %1)
store i32 %2, i32* %result, align 4
%3 = load i32, i32* %b, align 4
%4 = call i32 (i8*, ...) #printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* #.str, i32 0, i32 0), i32 %3)
ret i32 0
}
declare i32 #printf(i8*, ...) #2
attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { norecurse uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
!llvm.ident = !{!0}
!0 = !{!"clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)"}
the optimization command I used:
opt -S -adce deadCodeBefore -o deadCodeAfter1
As I read it should remove the call to square function and also the declaration of c variable because they have no effect. But the result is the same. Here is deadCodeAfter1 which is the same as deadCodeBefore:
; ModuleID = 'deadCodeBefore'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
#.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
; Function Attrs: nounwind uwtable
define i32 #_Z6squarei(i32 %x) #0 {
%1 = alloca i32, align 4
store i32 %x, i32* %1, align 4
%2 = load i32, i32* %1, align 4
%3 = load i32, i32* %1, align 4
%4 = mul nsw i32 %2, %3
ret i32 %4
}
; Function Attrs: norecurse uwtable
define i32 #main() #1 {
%a = alloca i32, align 4
%b = alloca i32, align 4
%c = alloca i32, align 4
%result = alloca i32, align 4
store i32 2, i32* %a, align 4
store i32 3, i32* %b, align 4
store i32 4, i32* %c, align 4
%1 = load i32, i32* %a, align 4
%2 = call i32 #_Z6squarei(i32 %1)
store i32 %2, i32* %result, align 4
%3 = load i32, i32* %b, align 4
%4 = call i32 (i8*, ...) #printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* #.str, i32 0, i32 0), i32 %3)
ret i32 0
}
declare i32 #printf(i8*, ...) #2
attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { norecurse uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
!llvm.ident = !{!0}
!0 = !{!"clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)"}

Answer 1

Because it's doing exactly as it is supposed to do. It checks in the IR if an instruction is being used by some other instruction or not. If not only then it removes it. For example in your code, declaration of variable %a (%a = alloca i32, align 4) is being used in the store instruction store i32 2, i32* %a, align 4
If you had just declare a variable and not assigned any value to it, then adce pass would have eliminated it. You can see that by just declaring a variable like int e; and run the optimization on it.
Usually passes in LLVM are dependent on the output of some other pass in order to be effective. An individual pass on itself might not give you the result that you might have expected it to provide.