[llvm-dev] Understanding LLVM Optimization (Rpass=inline) (original) (raw)

praveen via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 3 11:20:03 PDT 2018

Dear LLVM community :-)

I'm a novice in compiler principles and optimization, I'm currently working with some example code to see what && how LLVM optimization is doing to generate good machine code. While working  I find some functions are being inlined by llvm and some are not, but I don't know the internals of optimization so I can't able to figure  the reasons and rationale behind them.

For example:

struct value{

int set(){} // Function does nothing!

};

void foo () {} // Function does nothing!

void func( ) {

value obj;

obj.set();

foo();

}

Compiling with command  "clang++ -O2 -std=c++11 -stdlib=libc++/_//-Rpass=inline_/ name.cpp prints

only obj.set() was inlined but not foo(). I don't understand the rationale behind this (both are empty functions right ?). Why LLVM not inlining the function "foo" ?

Could you please share some pointers (videos or blogs) which explains the design of llvm optimizations :-). And also what is mean by /cost and threshold in inlining functions?//like (cost=always and cost=never). // /

Next, //

Clang documentation (here <https://clang.llvm.org/docs/UsersManual.html#cmdoption-wambiguous-member-template>) mentions that -/Wambiguous-member-template/ warning is defaulted to on. But when I actually compile the same code as in documentation it doesn't throw up any warnings. Is the documentation is out-dated for clang 8.0.0 ?

Software details :

Clang 8.0.0 , Linux mint.

Hardware details:

X86-64 bit.

Could you please share your knowledge for the above questions ? it is a great help to me

Thank you very much for your time,

PreeJackie

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