compiler-construction - llvm IR 代码中标签的表达

Question

有时我发现 llvm IR 中的标签标识符以逗号“;” ; <label> 6 开头，但是据我所知，逗号用于注释。那么llvm如何检索评论中的标签信息呢？我错过了什么吗？
接下来是一个简单的测试。
C源文件：

#include <stdio.h>

int main()
{
 int a;
 scanf("%d", &a);
 if ( a > 3)
  a *= 2;
 return 0;
}

http://llvm.org/demo/index.cgi生成的 llvm IR 代码（与 clang -c -emit-llvm main.c 相同）如下：

; ModuleID = '/tmp/webcompile/_13654_0.bc'

@.str = private unnamed_addr constant [3 x i8] c"%d\00", align 1

define i32 @main() nounwind uwtable {
  %a = alloca i32, align 4
  %1 = call i32 (i8*, ...)* @__isoc99_scanf(i8* getelementptr inbounds ([3 x i8]* @.str, i64 0, i64 0), i32* %a) nounwind
  %2 = load i32* %a, align 4, !tbaa !0
  %3 = icmp sgt i32 %2, 3
  br i1 %3, label %4, label %6

; <label>:4                                       ; preds = %0
  %5 = shl nsw i32 %2, 1
  store i32 %5, i32* %a, align 4, !tbaa !0
  br label %6

; <label>:6                                       ; preds = %4, %0
  ret i32 0
}

Answer 1

In LLVM IR a block does not need an explicit label. Instructions are the same way which leads to the %1, %2, %3. LLVM assigns numbers to unnamed instructions and blocks in increasing order. The br i1 %3... terminates the first block and the last used number label is 3 so the next block gets labelled as 4. That block ends with the next br instruction and the last used number is 5 so the next and final block is labelled with 6. At first it might seem weird that blocks and instructions share the same namespace, but remember that blocks are values too.

Answer 2

尽管有这样的措辞，%4inlabel %4不是标签，它只是对块的引用。您是对的，这很混乱，请参阅此问题进行讨论。

Answer 3

您可以尝试在您的 IR 上运行 instnamer 传递，这将为所有内容提供显式名称，因此您无需担心查找隐式名称。