assembly - LLVM 程序集 : assign integer constant to register

Question

我在写 a compiler使用 LLVM 作为后端，我的编译器生成以下 LLVM 汇编代码

@0 = private constant [25 x i8] c"Hello World to %dntegers\00"

declare void @printf (i8*, i32)

define void @main () {
  %1 = getelementptr [25 x i8]* @0, i32 0, i32 0
  %2 = 1
  tail call void @printf(i8* %1, i32 %2)
  ret void
}

但我收到以下错误:

c.ll:8:8: error: expected instruction opcode
  %2 = 1
       ^

文档显示 examples like that尽管。

你知道如何让这个工作吗？目前，我正在使用:

  %2 = add i32 0, 1 ; FIXME

Answer 1

参见 LLVMdev 讨论 "How to assign a constant to a register?"正如 Duncan Sands 指出的那样:

let me say that in general doing this is pointless. Due to SSA form, if %x is set to 5 you can't set it to something else later. Thus everywhere that you use %x you might as well just directly use 5 there instead. A common situation is that you have a register %x, and due to performing optimizations you discover that in fact %x will always have the value 5. You can use RAUW (aka the replaceAllUsesWith method) to replace %x with 5 everywhere.

该线程确实生成了几个直接使用常量的替代方法:

使用 alloca获取一个指针，存储到它，然后根据需要加载该值。

创建一个指向常量的全局变量，然后根据需要加载该值。

有人指出 alloca方法最终将在优化后使用寄存器。

无论如何，按照@SK-logic 的建议直接使用常量似乎是最干净的解决方案。