[LLVMdev] Removing AvailableExternal values in GlobalDCE (was Re: RFC: ThinLTO Impementation Plan)

Tue May 19 12:41:43 PDT 2015

On Mon, May 18, 2015 at 9:09 PM, Teresa Johnson <tejohnson at google.com>
wrote:

> On Fri, May 15, 2015 at 11:20 PM, Teresa Johnson <tejohnson at google.com>
> wrote:
> > On Fri, May 15, 2015 at 2:52 PM, David Blaikie <dblaikie at gmail.com>
> wrote:
> >>
> >>
> >> On Fri, May 15, 2015 at 1:15 PM, Teresa Johnson <tejohnson at google.com>
> >> wrote:
> >>>
> >>> On Fri, May 15, 2015 at 10:04 AM, David Blaikie <dblaikie at gmail.com>
> >>> wrote:
> >>> >
> >>> >
> >>> > On Fri, May 15, 2015 at 9:53 AM, Teresa Johnson <
> tejohnson at google.com>
> >>> > wrote:
> >>> >>
> >>> >> Looking back through my GlobalDCE changes, it looks like one of the
> >>> >> places I had changed (where we mark defined globals in runOnModule)
> >>> >> already has a guard for !hasAvailableExternallyLinkage and
> >>> >> !isDiscardableIfUnused, so my additional guard against marking
> >>> >> imported functions is unnecessary. But the other place I had to
> change
> >>> >> was in GlobalIsNeeded where it walks through the function and
> >>> >> recursively marks any referenced global as needed. Here there was no
> >>> >> guard against marking a global that is available externally as
> needed
> >>> >> if it is referenced. I had added a check here to not mark imported
> >>> >> functions as needed on reference unless they were discardable (i.e.
> >>> >> link once). Is this a bug - should this have a guard against marking
> >>> >> available externally function refs as needed?
> >>> >
> >>> >
> >>> > Duncan's probably got a better idea of what the right answer is
> here. I
> >>> > suspect "yes".
> >>> >
> >>> > The trick with available_externally is to ensure we keep these around
> >>> > for
> >>> > long enough that their definitions are useful (for inlining, constant
> >>> > prop,
> >>> > all that good stuff) but remove them before we actually do too much
> work
> >>> > on
> >>> > them, or at least before we emit them into the final object file.
> >>>
> >>> Yep, and that is exactly how long I want my imported functions to
> >>> stick around. Currently I have the ThinLTO import pass added at the
> >>> start of addLTOOptimizationPasses, just after the AA passes. So the
> >>> imported functions stick around through global opt, constant merge,
> >>> combining, and inlining. The call to GlobalDCE is just after inlining
> >>> and a second globalopt.
> >>>
> >>> >
> >>> > I imagine if GlobalDCE isn't removing available_externally functions
> >>> > it's
> >>> > because they're still useful in the optimization pipeline and
> something
> >>> > further down the pipe removes them (because they do, ultimately, get
> >>> > removed).
> >>>
> >>> That would be good to know, since if they are useful later on in the
> >>> pipe then presumably my imported functions could be as well. But I
> >>> wonder who is removing them since GlobalDCE isn't (if there are refs)?
> >>
> >>
> >> Yep - seems no one removes them (well, probably if there are no calls
> to the
> >> function at all they get removed as usual (eg: if all the calls are
> inlined
> >> we probably drop the function as we would for a linkonce_odr
> function)). If
> >> they're still around come codegen time, we just skip them:
> >>
> >> lib/CodeGen/MachineFunctionPass.cpp, MachineFunctionPass::runOnFunction:
> >>
> >> 33| bool MachineFunctionPass::runOnFunction(Function &F) {
> >> 34|   // Do not codegen any 'available_externally' functions at all,
> they
> >> have
> >> 35|   // definitions outside the translation unit.
> >> 36|   if (F.hasAvailableExternallyLinkage())
> >> 37|     return false;
> >> 38|
> >> 39|   MachineFunction &MF =
> getAnalysis<MachineFunctionAnalysis>().getMF();
> >> 40|   return runOnMachineFunction(MF);
> >> 41| }
> >
> > Ok, thanks for digging this up. I guess that would work for the
> > imported functions as well, but it seems like a waste of compile time
> > if they are not useful after inlining/global opt. Is there any reason
> > to keep them past GlobalDCE or should I try changing GlobalDCE so that
> > it removes them?
> >
>
> I found allowing GlobalDCE to remove referenced
> AvailableExternallyLinkage values earlier passes all the clang/llvm
> tests (see patch below).

The LLVM regression tests might not catch the sort of regression this
change could cause. Usually we test each pass in isolation and tend towards
white box testing (so we test the cases that are "interesting" according to
the algorithm) - in this case, you're adding a condition (adding an
"interesting" case that wasn't interesting before - so didn't need to be
tested explicitly) that changes the behavior of GlobalDCE. This behavior
may've been depended upon by further down-stream optimizations for overall
performance.

> AFAICT, it shouldn't be necessary to keep
> these functions around past the first call to GlobalDCE (which is
> after inlining), but let me know if I missed something.

It's possible they're still useful for optimizations other than inlining.
We can still do constant propagation through such functions (hey, look,
this function always returns 3, etc).

It might be necessary/worth actually running perf numbers on such a change
(or manually (possibly by asking those who know - not me, unfortunately)
checking the passes that come after GlobalDCE to see if any would do cross
function constant prop, etc. Perhaps someone else knows this off-the-cuff.

> Although they
> will be removed eventually by MachineFunctionPass as David pointed
> out,

Interestingly they're not exactly /removed/, they're just not emitted. I
don't know whether that's important.

> so this is more of a compile-time improvement than anything. It
> will affect ThinLTO more significantly in compile time because there
> will be more functions with this linkage type, depending on how many
> static functions

Hmm - why static functions in particular? I imagine this attribute would be
used for non-static functions too in ThinLTO.

> are imported that we have to promote.

> Also, what about available_externally variable
> definitions/initializations? I'm not sure what source construct
> currently can generate those,

I believe we emit C++ vtables as available_externally when the type has a
key function. This allows devirtualization opportunities without bloating
object file size with duplicate vtable data.

> but are these suppressed/turned into
> declarations at some point?

Maybe? Again, I would guess (and you can check some of this with
-print-after-all to see how the IR changes as it goes through the
optimizations) they might just be omitted from the generated code, not
necessarily removed from the IR at any point.

> Note that ThinLTO imported file static
> variables always need to be promoted, and will be marked
> available_external.
>
> Index: lib/Transforms/IPO/GlobalDCE.cpp
> ===================================================================
> --- lib/Transforms/IPO/GlobalDCE.cpp (revision 237590)
> +++ lib/Transforms/IPO/GlobalDCE.cpp (working copy)
> @@ -162,7 +162,10 @@ bool GlobalDCE::runOnModule(Module &M) {
>      // themselves.
>      for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
>        RemoveUnusedGlobalValue(*DeadFunctions[i]);
> -      M.getFunctionList().erase(DeadFunctions[i]);
> +      // Might have deleted the body of an available externally function
> that
> +      // is still referenced. Leave the declaration.
> +      if (DeadFunctions[i]->use_empty())
> +        M.getFunctionList().erase(DeadFunctions[i]);
>      }
>      NumFunctions += DeadFunctions.size();
>      Changed = true;
> @@ -171,7 +174,10 @@ bool GlobalDCE::runOnModule(Module &M) {
>    if (!DeadGlobalVars.empty()) {
>      for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
>        RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
> -      M.getGlobalList().erase(DeadGlobalVars[i]);
> +      // Might have deleted the definition of an available externally
> function
> +      // that is still referenced. Leave the declaration.
> +      if (DeadGlobalVars[i]->use_empty())
> +        M.getGlobalList().erase(DeadGlobalVars[i]);
>      }
>      NumVariables += DeadGlobalVars.size();
>      Changed = true;
> @@ -231,8 +237,10 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
>      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
>        for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;
> ++I)
>          for (User::op_iterator U = I->op_begin(), E = I->op_end(); U !=
> E; ++U)
> -          if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
> -            GlobalIsNeeded(GV);
> +          if (GlobalValue *GV = dyn_cast<GlobalValue>(*U)) {
> +            if (!GV->hasAvailableExternallyLinkage())
> +              GlobalIsNeeded(GV);
> +          }
>            else if (Constant *C = dyn_cast<Constant>(*U))
>              MarkUsedGlobalsAsNeeded(C);
>    }
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>
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