[LLVMdev] clang optimizer does not remove unused/uneeded variables(and accesses) from global scope (original) (raw)

Robinson, Paul Paul.Robinson at am.sony.com
Fri Jul 6 11:20:09 PDT 2012

Duncan Sands wrote:

On 05/07/12 13:30, Dimitry Andric wrote:

On 2012-07-05 12:53, Dennis Luehring wrote:

addition to my last post: http://groups.google.com/group/llvm-dev/browsethread/thread/afd85c3f303ec2a9

i know that using static on my globals will remove them - no other compilation unit can then extern the vars but my question is - in this closed szenario clang is "directly" producing the exe - isn't clang aware of that the vars are not in use? It can't know that, unless you use link-time optimization. Which optimization level did you use? it can know: if you looked at his command line you can see that it is producing a final executable from the file, not just compiling it to object code. As far as I know clang relies on an LLVM aware linker to do the link-time optimization, though I'm not sure why: in this kind of case it could just do it itself. On linux you can use the gold linker with the LLVM gold plugin, maybe that can be used on windows too, I wouldn't know.

It can't know. It can make the assumption that all possible runtime environments would be unaffected by this externally-available definition, but it's an assumption not a known fact, even if you see that a single translation unit is producing an executable image.

Just to postulate an example, suppose I have a runtime environment that makes a weak reference to some symbol. If the program defines that symbol, then the behavior of my runtime is affected. If clang/LLVM arrogantly assumes that can't possibly happen, then the behavior of my program is affected when it should not be.

Link-time optimization can figure this out, but the compiler by itself cannot and must not. --paulr

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