用于LLVM库的C++ API生成太多警告。

Question

我不知道是否我是唯一的人，或者这是这么多人的问题。

令我痛苦的是，对于一个甚至包含LLVM头的简单代码，无论我在主函数中写了什么，当我用额外的警告编译代码时，我会收到太多的警告：

#include <llvm/IR/IRBuilder.h>
#include <llvm/IR/Verifier.h>
#include <llvm/Support/TargetSelect.h>
#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/IR/Instructions.h>

int main()
{
    return 0;
}

我使用gcc或clang来获取这些警告

g++-9 -std=c++17 main.cpp -Wall -Wextra -i/usr/lib/llvm-9/ -c -o main.o clang+-9 -std=c++17 main.cpp -Wall -Wextra -i/usr/lib/llvm-9/包含-c -o main.o

它们主要是未使用的参数：

In file included from /usr/lib/llvm-9/include/llvm/IR/ConstantFolder.h:20,
                 from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:24,
                 from main.cpp:1:
/usr/lib/llvm-9/include/llvm/IR/Constants.h: In member function ‘llvm::Value* llvm::ConstantData::handleOperandChangeImpl(llvm::Value*, llvm::Value*)’:
/usr/lib/llvm-9/include/llvm/IR/Constants.h:60:41: warning: unused parameter ‘From’ [-Wunused-parameter]
   60 |   Value *handleOperandChangeImpl(Value *From, Value *To) {
      |                                  ~~~~~~~^~~~
/usr/lib/llvm-9/include/llvm/IR/Constants.h:60:54: warning: unused parameter ‘To’ [-Wunused-parameter]
   60 |   Value *handleOperandChangeImpl(Value *From, Value *To) {
      |                                               ~~~~~~~^~
In file included from /usr/lib/llvm-9/include/llvm/IR/ConstantFolder.h:21,
                 from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:24,
                 from main.cpp:1:
/usr/lib/llvm-9/include/llvm/IR/InstrTypes.h: In member function ‘bool llvm::CallBase::isFnAttrDisallowedByOpBundle(llvm::StringRef) const’:
/usr/lib/llvm-9/include/llvm/IR/InstrTypes.h:1913:47: warning: unused parameter ‘S’ [-Wunused-parameter]
 1913 |   bool isFnAttrDisallowedByOpBundle(StringRef S) const {
      |                                     ~~~~~~~~~~^
In file included from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:33,
                 from main.cpp:1:
/usr/lib/llvm-9/include/llvm/IR/Instructions.h: In member function ‘llvm::BasicBlock* llvm::ReturnInst::getSuccessor(unsigned int) const’:
/usr/lib/llvm-9/include/llvm/IR/Instructions.h:2943:37: warning: unused parameter ‘idx’ [-Wunused-parameter]
 2943 |   BasicBlock *getSuccessor(unsigned idx) const {
      |                            ~~~~~~~~~^~~
...

我只是删掉了这些警告。他们太多了。当我打开-Wconversion时，问题变得更糟了：

In file included from /usr/lib/llvm-9/include/llvm/Support/MathExtras.h:17,
                 from /usr/lib/llvm-9/include/llvm/ADT/SmallVector.h:19,
                 from /usr/lib/llvm-9/include/llvm/ADT/STLExtras.h:20,
                 from /usr/lib/llvm-9/include/llvm/ADT/StringRef.h:12,
                 from /usr/lib/llvm-9/include/llvm/ADT/StringMap.h:16,
                 from /usr/lib/llvm-9/include/llvm/Support/Host.h:16,
                 from /usr/lib/llvm-9/include/llvm/ADT/Hashing.h:48,
                 from /usr/lib/llvm-9/include/llvm/ADT/ArrayRef.h:12,
                 from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:18,
                 from main.cpp:1:
/usr/lib/llvm-9/include/llvm/Support/SwapByteOrder.h: In function ‘uint16_t llvm::sys::SwapByteOrder_16(uint16_t)’:
/usr/lib/llvm-9/include/llvm/Support/SwapByteOrder.h:36:23: warning: conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} may change value [-Wconversion]
   36 |   uint16_t Hi = value << 8;
      |                 ~~~~~~^~~~
/usr/lib/llvm-9/include/llvm/Support/SwapByteOrder.h:37:23: warning: conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} may change value [-Wconversion]
   37 |   uint16_t Lo = value >> 8;
      |                 ~~~~~~^~~~
In file included from /usr/lib/llvm-9/include/llvm/ADT/STLExtras.h:20,
                 from /usr/lib/llvm-9/include/llvm/ADT/StringRef.h:12,
                 from /usr/lib/llvm-9/include/llvm/ADT/StringMap.h:16,
                 from /usr/lib/llvm-9/include/llvm/Support/Host.h:16,
                 from /usr/lib/llvm-9/include/llvm/ADT/Hashing.h:48,
                 from /usr/lib/llvm-9/include/llvm/ADT/ArrayRef.h:12,
                 from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:18,
                 from main.cpp:1:
/usr/lib/llvm-9/include/llvm/ADT/SmallVector.h: In constructor ‘llvm::SmallVectorBase::SmallVectorBase(void*, size_t)’:
/usr/lib/llvm-9/include/llvm/ADT/SmallVector.h:45:35: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘unsigned int’ may change value [-Wconversion]
   45 |       : BeginX(FirstEl), Capacity(TotalCapacity) {}
      |                                   ^~~~~~~~~~~~~
...

我也收到了clang的警告：

...
In file included from main.cpp:1:
In file included from /usr/lib/llvm-9/include/llvm/IR/IRBuilder.h:18:
In file included from /usr/lib/llvm-9/include/llvm/ADT/ArrayRef.h:12:
/usr/lib/llvm-9/include/llvm/ADT/Hashing.h:190:15: warning: implicit conversion changes signedness: 'const char' to 'uint8_t' (aka 'unsigned char') [-Wsign-conversion]
  uint8_t a = s[0];
          ~   ^~~~
/usr/lib/llvm-9/include/llvm/ADT/Hashing.h:191:15: warning: implicit conversion changes signedness: 'const char' to 'uint8_t' (aka 'unsigned char') [-Wsign-conversion]
  uint8_t b = s[len >> 1];
          ~   ^~~~~~~~~~~
/usr/lib/llvm-9/include/llvm/ADT/Hashing.h:192:15: warning: implicit conversion changes signedness: 'const char' to 'uint8_t' (aka 'unsigned char') [-Wsign-conversion]
  uint8_t c = s[len - 1];
          ~   ^~~~~~~~~~
...

这只是一辆MWE。添加更多的标题，我可以获得其他类型的警告，如

/usr/lib/llvm-9/include/llvm/ADT/Twine.h:232:16: warning: ‘<anonymous>.llvm::Twine::RHS.llvm::Twine::Child::twine’ may be used uninitialized in this function [-Wmaybe-uninitialized]
           !RHS.twine->isBinary())
            ~~~~^~~~~

现在，我想知道LLVM是否写得不小心？我总是打开所有警告，并确保我的应用程序使用0警告进行编译。在使用LLVM时，当我这样做时，我的代码的所有警告都会被埋在LLVM警告下。我很难看到他们。通常，人们的第一个建议是关闭这些警告开关。我坚决抵制。我想为我的申请找到它们。我想到的另一个解决方案是，只对LLVM关闭这些警告，而不对我的应用程序关闭这些警告。这有可能吗，特别是当使用CMake时

Answer 1

导致警告的系统头文件问题是常见的问题。这些标题的作者是否“粗心”并不是真的由我来判断，但是有一种相对简单的方法可以在包含‘违犯’标题之前禁用特定的警告，然后在包含了‘完全’警告之后恢复它们。

对于clang，您可以使用各种#pragma diagnostic ...行来实现这一点，如下面的代码片段所示：

#if defined (__clang__)
#pragma clang diagnostic push // Saves current diagnostic settings
#pragma clang diagnostic ignored "-Wsign-conversion" // Ignore this warning
#pragma clang diagnostic ignored "-Wunused-parameter" // and this one...
    // ... Add similar lines for other warnings you wish to disable
#endif

// Now include the 'offending' headers ...
#include <llvm/IR/IRBuilder.h>
#include <llvm/IR/Verifier.h>
#include <llvm/Support/TargetSelect.h>
#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/IR/Instructions.h>

#if defined(__clang__)
#pragma clang diagnostic pop // Restores the saved settings
#endif

我没有访问GCC编译器的权限，但我相信可以使用非常类似的指令:在发生这种情况的行中用GCC代替clang。关于这类#pragma指令( GCC版本)的更多信息可以找到这里。

Answer 2

它依赖于您的GCC/clang/OS安装。在Fedora 32上，它与系统gcc-10.2.1-1.fc32.x86_64 + system clang-10.0.0-2.fc32.x86_64一起开箱即用。

#include <llvm/IR/IRBuilder.h>
#include <llvm/IR/Verifier.h>
#include <llvm/Support/TargetSelect.h>
#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/IR/Instructions.h>

static void func(int val) {}

int main()
{
    func(42);
    return 0;
}

GCC说：

$ g++ -o stackoverflowllvmwarnings stackoverflowllvmwarnings.C $(llvm-config --cflags --libs) -std=c++17 -Wall -Wextra
stackoverflowllvmwarnings.C: In function ‘void func(int)’:
stackoverflowllvmwarnings.C:7:22: warning: unused parameter ‘val’ [-Wunused-parameter]
    7 | static void func(int val) {}
      |                  ~~~~^~~
$ _

对于clang：

$ clang++ -o stackoverflowllvmwarnings stackoverflowllvmwarnings.C $(llvm-config --cflags --libs) -std=c++17 -Wall -Wextra
stackoverflowllvmwarnings.C:7:22: warning: unused parameter 'val' [-Wunused-parameter]
static void func(int val) {}
                     ^
1 warning generated.
$ _

如果您真的希望(您不需要)警告，即使对于系统头使用Wsystem-headers (它的工作方式类似于gcc和clang)：

$ clang++ -o stackoverflowllvmwarnings stackoverflowllvmwarnings.C $(llvm-config --cflags --libs) -std=c++17 -Wall -Wextra -Wsystem-headers
In file included from stackoverflowllvmwarnings.C:1:
In file included from /usr/include/llvm/IR/IRBuilder.h:17:
In file included from /usr/include/llvm-c/Types.h:17:
In file included from /usr/include/llvm-c/DataTypes.h:28:
In file included from /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/cmath:42:
/usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/bits/cpp_type_traits.h:110:12: warning: keyword '__is_void' will be made available as an identifier for the remainder of the translation unit [-Wkeyword-compat]
    struct __is_void
           ^
...