Optimization llvm代码优化选项不起作用

我正在阅读有关LLVM代码优化的文章。我尝试在许多示例上应用opt命令选项，但它们没有任何效果。例如，这里有一个C++代码，叫做死代码。CPP:< /P>

 #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);

}

我使用的优化命令：

opt-S-adce deadcode之前-o deadcode之后1

正如我所读到的，它应该删除对square函数的调用以及c变量的声明，因为它们没有任何效果。但结果是一样的。这里是deadCodeAfter1，它与deadCodeBefore相同：

 ; 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)"}

---

因为它做的正是它应该做的。它在IR中检查一条指令是否被其他指令使用。如果不仅如此，它还会将其删除。例如，在您的代码中，变量

%a

（

%a=alloca i32，align 4

）的声明正在

存储
指令
存储i32 2，i32*%a，align 4

如果您刚刚声明了一个变量，并且没有给它分配任何值，那么
adce
pass就会消除它。您只需声明一个变量，如
inte
并在其上运行优化

为了有效，LLVM中的过程通常依赖于其他过程的输出。单个传递本身可能不会给您预期的结果

 ; 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)"}