Add/update comments

Author: nikic

llvm/include/llvm/Transforms/Utils/Local.h

@@ -452,6 +452,8 @@ LLVM_ABI unsigned replaceDominatedUsesWith(Value *From, Value *To,
 LLVM_ABI unsigned replaceDominatedUsesWith(Value *From, Value *To,
                                            DominatorTree &DT,
                                            const BasicBlock *BB);
+/// Replace each use of 'From' with 'To' if that use is dominated by the
+/// given instruction. Returns the number of replacements made.
 LLVM_ABI unsigned replaceDominatedUsesWith(Value *From, Value *To,
                                            DominatorTree &DT,
                                            const Instruction *I);
@@ -467,7 +469,9 @@ LLVM_ABI unsigned replaceDominatedUsesWithIf(
 LLVM_ABI unsigned replaceDominatedUsesWithIf(
     Value *From, Value *To, DominatorTree &DT, const BasicBlock *BB,
     function_ref<bool(const Use &U, const Value *To)> ShouldReplace);
-
+/// Replace each use of 'From' with 'To' if that use is dominated by
+/// the given instruction and the callback ShouldReplace returns true. Returns
+/// the number of replacements made.
 LLVM_ABI unsigned replaceDominatedUsesWithIf(
     Value *From, Value *To, DominatorTree &DT, const Instruction *I,
     function_ref<bool(const Use &U, const Value *To)> ShouldReplace);

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -2444,8 +2444,8 @@ void GVNPass::assignBlockRPONumber(Function &F) {
 /// The given values are known to be equal in every use
 /// dominated by 'Root'.  Exploit this, for example by replacing 'LHS' with
 /// 'RHS' everywhere in the scope.  Returns whether a change was made.
-/// If DominatesByEdge is false, then it means that we will propagate the RHS
-/// value starting from the end of Root.Start.
+/// The Root may either be a basic block edge (for conditions) or an
+/// instruction (for assumes).
 bool GVNPass::propagateEquality(
     Value *LHS, Value *RHS,
     const std::variant<BasicBlockEdge, Instruction *> &Root) {