[FuncSpec]Save compilation time by caching uses of replaced value

[bin.cheng]

Summary

Hi,
This patch saves compilation time by caching uses of replace value.

We only need to do propagation on use instructions of the original value, rather than the replacing const value which might have lots of irrelavant uses. As an example, it improves LTO compilation time by 3~4% for one example case in speccpu2017 with Function Specialization enabled.

Thanks,
bin

[ChuanqiXu]

LGTM. It would better add [NFC] tag in the title.

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

347

// Cache uses of V so that we could avoid visiting unwanted users of Const.

[SjoerdMeijer]

Yeah, nice patch, thanks.

I was wondering if this was really caching and if it should be called something else, but I guess it is, it's cached very shortly/briefly, so it's alright. :)

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

348

Nit, quoting from the programmers manual:

it is recommended to use SmallVector<T> (that is, omitting the N)

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

bool tryToReplaceWithConstant(Value *V) {
     if (isOverdefined(IV))
       return false;
     auto *Const =
         isConstant(IV) ? Solver.getConstant(IV) : UndefValue::get(V->getType());
 
     LLVM_DEBUG(dbgs() << "FnSpecialization: Replacing " << *V
                       << "\nFnSpecialization: with " << *Const << "\n");
 
-    V->replaceAllUsesWith(Const);
-
-    for (auto *U : Const->users())
+    // Record uses of V to avoid visiting irrelevant uses of const later.
+    SmallVector<Instruction *> UseInsts;
+    for (auto *U : V->users())
       if (auto *I = dyn_cast<Instruction>(U))
         if (Solver.isBlockExecutable(I->getParent()))
-          Solver.visit(I);
+          UseInsts.push_back(I);
+
+    V->replaceAllUsesWith(Const);
+
+    for (auto *I : UseInsts)
 
     // Remove the instruction from Block and Solver.
     if (auto *I = dyn_cast<Instruction>(V)) {
       if (I->isSafeToRemove()) {
         ReplacedWithConstant.push_back(I);
         Solver.removeLatticeValueFor(I);
       }
     }