LLVM结构返回优化 [英] LLVM Struct Return Optimization

问题描述

我想知道为什么LLVM无法优化以下IR代码(使用PassManagerBuilder(优化设置为"3"，还使用LLVM的"opt"工具):

I'm wondering why LLVM fails to optimize the following IR code (using the PassManagerBuilder with optimisation set to '3', and also using LLVM's 'opt' tool):

%GenericStruct = type { i32 }

define void @makeGenericStructOuter(%GenericStruct* noalias nocapture sret) {
entry:
  %1 = alloca %GenericStruct
  call void @makeGenericStructInner(%GenericStruct* %1)
  %2 = load %GenericStruct* %1
  store %GenericStruct %2, %GenericStruct* %0
  ret void
}

declare void @makeGenericStructInner(%GenericStruct* noalias nocapture sret)

期望的代码是:

%GenericStruct = type { i32 }

define void @makeGenericStructOuter(%GenericStruct* noalias nocapture sret) {
entry:
  call void @makeGenericStructInner(%GenericStruct* %0)
  ret void
}

declare void @makeGenericStructInner(%GenericStruct* noalias nocapture sret)

目前是否没有优化可用于处理这种情况?还是我无法生成(该代码是从我正在开发的前端生成的)正确的IR(允许进行优化)?

Are there simply no optimizations currently available to handle this case? Or am I failing to produce (this code is generated from a front-end I'm developing) the right IR that would allow optimization?

在建议之前，我无法生成按值返回的代码，因为这些函数必须可以从其他不知道'GenericStruct'大小或内容的模块/库中调用(它们将在本地声明'TestClass'为结构不透明").

Before it's suggested, I can't produce code that returns by value since these functions must be callable from other modules/libraries that don't know the size or contents of 'GenericStruct' (and they would locally declare 'TestClass' as 'struct opaque').

推荐答案

如果您认为应该进行优化，请在

If you belive that optimization should take place, report a bug in LLVM Bug Tracker. LLVM developers are usually very happy and interested when reporting optimization opportunities missed.

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