编译器未检测到明显未初始化的变量

Question

BOOL NearEqual (int tauxprecis, int max, int value)
{
  int tauxtrouve;      // Not initialized at this point
  int totaldiff;       // Not initialized at this point

  for (int i = 0; i < max; i++)
  {
    if (2 < totaldiff)  // At this point totaldiff is not initialized
    {
      totaldiff = 2;
      tauxtrouve = value;  // Commenting this line out will produce warning
    }
  }

  return tauxtrouve == tauxprecis ;  // At this point tauxtrouve is potentially
                                     // not initialized.
}

我尝试了这些编译器：

• gcc 4.9.2 with-wall-wextra
• 启用所有警告的Microsoft Visual C++2013

Answer 1

这个变量没有初始化的明显性被夸大了。路径分析会花费时间，编译器供应商要么不想实现该特性，要么认为它会花费您太多时间--或者您只是没有明确地选择加入。

例如，使用clang:

$ clang -Wall -Wextra -c obvious.c 
$ clang -Wall -Wextra --analyze -c obvious.c 
obvious.c:9:11: warning: The right operand of '<' is a garbage value
    if (2 < totaldiff)  // at this point totaldiff is not initialized
          ^ ~~~~~~~~~
obvious.c:16:21: warning: The left operand of '==' is a garbage value
  return tauxtrouve == tauxprecis ;  // at this point tauxtrouve is potentially
         ~~~~~~~~~~ ^
2 warnings generated.

对于这些幼稚的示例，在执行时间上的差异可以忽略不计。但是想象一下，一个翻译单元有几千行，几十个函数，每个函数都有循环和大量嵌套。路径的数量很快就变得复杂，并成为分析循环的第一次迭代是否会在比较之前进行分配的一个很大的负担。

; Function Attrs: nounwind uwtable
define i32 @NearEqual(i32 %tauxprecis, i32 %max, i32 %value) #0 {
  br label %1

; <label>:1                                       ; preds = %7, %0
  %tauxtrouve.0 = phi i32 [ undef, %0 ], [ %tauxtrouve.1, %7 ]
  %i.0 = phi i32 [ 0, %0 ], [ %8, %7 ]
  %2 = icmp slt i32 %i.0, %max
  br i1 %2, label %3, label %9

; <label>:3                                       ; preds = %1
  %4 = icmp slt i32 2, 2
  br i1 %4, label %5, label %6

; <label>:5                                       ; preds = %3
  br label %6

; <label>:6                                       ; preds = %5, %3
  %tauxtrouve.1 = phi i32 [ %value, %5 ], [ %tauxtrouve.0, %3 ]
  br label %7

; <label>:7                                       ; preds = %6
  %8 = add nsw i32 %i.0, 1
  br label %1

; <label>:9                                       ; preds = %1
  %10 = icmp eq i32 %tauxtrouve.0, %tauxprecis
  %11 = zext i1 %10 to i32
  ret i32 %11
}