[cfe-dev] Elimination of dead globals (functions/variables)

[Chris Lattner]

On Nov 26, 2007, at 8:41 AM, Devang Patel wrote:

>
> On Nov 26, 2007, at 12:02 AM, Christopher Lamb wrote:
>
>> With llvm-gcc the following test case
>>
>> static int alive, dead;
>> void foo(int v) { glob_alive = v; }
>> int FOO() { return glob_alive; }
>> static void bar(int v) { gdead = v; }
>> static int BAR() { return dead; }
>>
>> is correctly pruned down to only foo, FOO, and alive, but with the
>> following warnings:
>>
>> warning: 'bar' defined but not used
>> warning: 'BAR' defined but not used
>>
>> clang with -emit-llvm currently emits everything (with no warnings)
>> and opt isn't able to reduce it. What needs to be done by clang to
>> get the non-visible parts of the module stripped out? In addition to
>> diagnostics is clang responsible for not generating code for unused
>> functions, or is it simply responsible for passing some sort of
>> information so that opt can do the chopping?
>
> clang should mark static functions as internal functions to help
> optimizer identify dead code.
>
> llvm-gcc emits
> 	define internal void @bar(i32 %v) { ... }
> where as clang emits
> 	define void @bar(i32 %v) { ... }

I just implemented this, we now get:

$ clang t.c -emit-llvm | llvm-as | opt -std-compile-opts | llvm-dis

; ModuleID = '<stdin>'
target triple = "i686-apple-darwin9"
@glob_alive = internal global i32 0		; <i32*> [#uses=2]

define void @foo(i32 %v) {
entry:
	store i32 %v, i32* @glob_alive, align 4
	ret void
}

define i32 @FOO() {
entry:
	%tmp = load i32* @glob_alive, align 4		; <i32> [#uses=1]
	ret i32 %tmp
}



However, it would be even better to emit functions on demand, starting  
from ones that are intrinsically alive.  Basically, only emit LLVM  
code for functions/globals if they have external linkage.  Once those  
are emitted, emit all static functions/globals that are referenced  
from these.  At the end, do a walk over static functions and emit the  
warning above for any that weren't emitted.

-Chris