Reduce dropTriviallyDeadConstantArrays cumulative time percentage from 17% to 4%

The history of dropTriviallyDeadConstantArrays is like this. Because the appending linkage uses too much memory (http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150105/251381.html), dropTriviallyDeadConstantArrays was introduced (https://reviews.llvm.org/rG81f385b0c6ea37dd7195a65be162c75bbdef29d2) to release unused constant arrays. Recently, dropTriviallyDeadConstantArrays was improved (https://reviews.llvm.org/rG81f385b0c6ea37dd7195a65be162c75bbdef29d2) to reduce its quadratic cost.

Our recent LTO profiling shows that when a target is large, 15-20% of time cost is from the SetVector::insert called by dropTriviallyDeadConstantArrays.

A large application has hundreds or thousands of modules; each module calls dropTriviallyDeadConstantArrays once for cleaning up tens of thousands of ConstantArrays a module has. In those ConstantArrays, usually around 5 can be deleted; a very very few deleted ConstantArrays reference other ConstantArrays: less than 10 out of millions.

Given this, the cost of SetVector::insert is mainly from the construction of WorkList from ArrayConstants. This motivated the fix that iterates ArrayConstants directly, and uses WorkList only when necessary.

Our evaluation shows that
1) The cumulative time percentage of dropTriviallyDeadConstantArrays is reduced from 15-17% to 4-6%.
2) For targets with LTO time > 20min, the time reduction is about 20%.
3) No observable performance impact for build without using LTO.

{F12506218}
{F12506221}

Reviewed By: mehdi_amini, tejohnson, jdoerfert

Differential Revision: https://reviews.llvm.org/D85379

lib/IR/LLVMContextImpl.cpp

@@ -129,8 +129,15 @@ LLVMContextImpl::~LLVMContextImpl() {
 }
 
 void LLVMContextImpl::dropTriviallyDeadConstantArrays() {
-  SmallSetVector<ConstantArray *, 4> WorkList(ArrayConstants.begin(),
-                                              ArrayConstants.end());
+  SmallSetVector<ConstantArray *, 4> WorkList;
+
+  // When ArrayConstants are of substantial size and only a few in them are
+  // dead, starting WorkList with all elements of ArrayConstants can be
+  // wasteful. Instead, starting WorkList with only elements that have empty
+  // uses.
+  for (ConstantArray *C : ArrayConstants)
+    if (C->use_empty())
+      WorkList.insert(C);
 
   while (!WorkList.empty()) {
     ConstantArray *C = WorkList.pop_back_val();