[cfe-dev] always_inline and noinline attributes
clattner at apple.com
Mon Jul 21 18:07:40 CDT 2008
On Jul 18, 2008, at 2:09 PM, Matthijs Kooijman wrote:
>> Once the information is encoded in the IR, the remaining issue is
>> how to
>> invoke the LLVM inlining pass when at least one function is marked as
>> always_inline. There are two possible approaches:
>> 1) Teach FE tools (e.g. clang, llvm-gcc) to insert inlining pass in
>> PassManager while requesting (opt + code generation) when it least
>> function with attribute always_inline is seen.
> Perhaps there could be a specific inliner that does only this? This
> surprises when people didn't expect inlining?
I'd strongly prefer to do this in the LLVM IR level instead of on
>> 2) Teach the LLVM PassManager to sniff always_inline property
>> encoded in
>> the LLVM IR and do the right thing.
> This sounds like it might also surprise people, if they only add a
> single pass
> and still things are inlined. Might not be a big problem, though.
GCC runs its inliner, even at -O0. I agree it is ugly, but seems
> Another reason why I think encoding in the IR is necessary: If I
> have an
> always inline function defined in one module and referenced in
> another, I
> think it should still be inlined after I link the two modules
> together, right?
No, it should only happen within a translation unit. The semantics of
the program should not change based on whether you're using LTO or
not, and inlining can sometimes change semantics for (arguably very
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