Question

我有一段代码，我试图变成LLVM bitcode：

int main() {
    volatile double n = 0.45;
    for (int j = 0; j < 32; j++) {
        n *= j;
    }
    return 0;
}

我在其上运行以下命令：

clang -O0 -S -emit-llvm TrainingCode/trainingCode.cpp -o TrainingCode/trainingCode.ll

生成以下LLVM bitcode（记下第6行，＆＃34;功能Atters＆＃34;）

; ModuleID = 'TrainingCode/trainingCode.cpp'
source_filename = "TrainingCode/trainingCode.cpp"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: noinline norecurse nounwind optnone uwtable
define i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  %n = alloca double, align 8
  %j = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  store double 4.500000e-01, double* %n, align 8
  store i32 0, i32* %j, align 4
  br label %for.cond

for.cond:                                         ; preds = %for.inc, %entry
  %0 = load i32, i32* %j, align 4
  %cmp = icmp slt i32 %0, 32
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %1 = load i32, i32* %j, align 4
  %conv = sitofp i32 %1 to double
  %2 = load double, double* %n, align 8
  %mul = fmul double %2, %conv
  store double %mul, double* %n, align 8
  br label %for.inc

for.inc:                                          ; preds = %for.body
  %3 = load i32, i32* %j, align 4
  %inc = add nsw i32 %3, 1
  store i32 %inc, i32* %j, align 4
  br label %for.cond

for.end:                                          ; preds = %for.cond
  ret i32 0
}

attributes #0 = { noinline norecurse nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "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-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 5.0.0 (tags/RELEASE_500/final)"}

为什么clang会将optnone属性添加到main？我需要LLVM在bitcode上运行各种转换传递，optnone属性导致LLVM跳过main ...我需要不添加此属性。

使用-O1进行编译似乎解决了这个问题，但这是不可接受的，因为我需要Clang给我未经优化的代码。我希望LLVM优化Clang给我的未经优化的代码，但optnone属性的存在导致LLVM不执行任何优化。

Answer 1

抱歉不是解决方案，但也许是一个线索。

但是要发布clang版本或环境变量。

使用XCODE 9在OS X上获取代码：

$ clang -O0 -S -emit-llvm test.cpp  -o test2.ll

$ more test2.ll
; ModuleID = 'test.cpp'
source_filename = "test.cpp"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.12.0"

; Function Attrs: norecurse nounwind ssp uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca double, align 8

...

; <label>:15:                                     ; preds = %4
  ret i32 0
}

attributes #0 = { norecurse nounwind ssp uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "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-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"Apple LLVM version 8.1.0 (clang-802.0.42)"}

为什么Clang会自动为我的函数添加属性？

1 个答案: