[gvn] Precisely propagate equalities to phi operands

[reames]

Summary

The code used for propagating equalities (e.g. assume facts) was conservative in two ways - one of which this patch fixes. Specifically, it shifts the code reasoning about whether a use is dominated by the end of the assume block to consider phi uses to exist on the predecessor edge. This matches the dominator tree handling for dominates(X, Use), and simply extends it to properlyDominates.

Note that the decision to use the end of the block is itself a conservative choice. The more precise option would be to use the later of the assume and the value, and replace all uses after that. GVN handles that case separately (with the replace operand mechanism) because it used to be expensive to ask dominator questions within blocks. With the new instruction ordering support, we should probably rewrite this code at some point to simplify.

[reames]

Rebase over landed tests.

[jdoerfert]

I don't understand how second commit message paragraph in this context but the logic seems fine. Second pair of eyes won't hurt though.

[nikic]

llvm/lib/Transforms/Utils/Local.cpp

2698

I'd suggest to write it like this:

Instruction *UserInst = cast<Instruction>(U.getUser());
if (auto *PN = dyn_cast<PHINode>(UserInst))
  return DT.dominates(BB, PN->getIncomingBlock());
else
  return DT.properlyDominates(BB, UserInst->getParent());

Though ideally this should be an API on DominatorTree. We currently have dominates(BasicBlockEdge, Use), but we're missing the dominates(BasicBlock, Use) variant.

[reames]

address review comments + rebase over landed change which causes a further test diff

[nikic]

LGTM

llvm/include/llvm/IR/Dominators.h

public:
 
   /// Handle invalidation explicitly.
   bool invalidate(Function &F, const PreservedAnalyses &PA,
                   FunctionAnalysisManager::Invalidator &);
 
   // Ensure base-class overloads are visible.
   using Base::dominates;
 
+  /// Return true if the (end of the) basic block BB dominates the use U.
+  bool dominates(const BasicBlock *BB, const Use &U) const;
+
   /// Return true if value Def dominates use U, in the sense that Def is
   /// available at U, and could be substituted as the used value without
   /// violating the SSA dominance requirement.
   ///
   /// In particular, it is worth noting that:
   ///  * Non-instruction Defs dominate everything.
   ///  * Def does not dominate a use in Def itself (outside of degenerate cases
   ///    like unreachable code or trivial phi cycles).

llvm/lib/IR/Dominators.cpp

bool DominatorTree::invalidate(Function &F, const PreservedAnalyses &PA,
                                FunctionAnalysisManager::Invalidator &) {
   // Check whether the analysis, all analyses on functions, or the function's
   // CFG have been preserved.
   auto PAC = PA.getChecker<DominatorTreeAnalysis>();
   return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
            PAC.preservedSet<CFGAnalyses>());
 }
 
+bool DominatorTree::dominates(const BasicBlock *BB, const Use &U) const {
+  Instruction *UserInst = cast<Instruction>(U.getUser());
+  if (auto *PN = dyn_cast<PHINode>(UserInst))
+    // A phi use using a value from a block is dominated by the end of that
+    // block.  Note that the phi's parent block may not be.
+    return dominates(BB, PN->getIncomingBlock(U));
+  else
+    return properlyDominates(BB, UserInst->getParent());
+}
+
 // dominates - Return true if Def dominates a use in User. This performs
 // the special checks necessary if Def and User are in the same basic block.
 // Note that Def doesn't dominate a use in Def itself!
 bool DominatorTree::dominates(const Value *DefV,
                               const Instruction *User) const {
   const Instruction *Def = dyn_cast<Instruction>(DefV);
   if (!Def) {
     assert((isa<Argument>(DefV) || isa<Constant>(DefV)) &&

llvm/lib/Transforms/Utils/Local.cpp

auto Dominates = [&DT](const BasicBlockEdge &Root, const Use &U) {
     return DT.dominates(Root, U);
   };
   return ::replaceDominatedUsesWith(From, To, Root, Dominates);
 }
 
 unsigned llvm::replaceDominatedUsesWith(Value *From, Value *To,
                                         DominatorTree &DT,
                                         const BasicBlock *BB) {
-  auto ProperlyDominates = [&DT](const BasicBlock *BB, const Use &U) {
-    auto *I = cast<Instruction>(U.getUser())->getParent();
-    return DT.properlyDominates(BB, I);
+  auto Dominates = [&DT](const BasicBlock *BB, const Use &U) {
+    return DT.dominates(BB, U);
   };
-  return ::replaceDominatedUsesWith(From, To, BB, ProperlyDominates);
+  return ::replaceDominatedUsesWith(From, To, BB, Dominates);
 }
 
 bool llvm::callsGCLeafFunction(const CallBase *Call,
                                const TargetLibraryInfo &TLI) {
   // Check if the function is specifically marked as a gc leaf function.
   if (Call->hasFnAttr("gc-leaf-function"))
     return true;
   if (const Function *F = Call->getCalledFunction()) {
     if (F->hasFnAttribute("gc-leaf-function"))
       return true;
 
     if (auto IID = F->getIntrinsicID()) {
       // Most LLVM intrinsics do not take safepoints.
       return IID != Intrinsic::experimental_gc_statepoint &&
              IID != Intrinsic::experimental_deoptimize &&
              IID != Intrinsic::memcpy_element_unordered_atomic &&
              IID != Intrinsic::memmove_element_unordered_atomic;

llvm/test/Transforms/GVN/phi.ll

 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
 ; CHECK:       taken:
 ; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
 ; CHECK-NEXT:    br label [[MERGE:%.*]]
 ; CHECK:       untaken:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ [[A]], [[TAKEN]] ], [ [[B:%.*]], [[UNTAKEN]] ]
-; CHECK-NEXT:    [[PHI2:%.*]] = phi i64 [ 0, [[TAKEN]] ], [ [[B]], [[UNTAKEN]] ]
-; CHECK-NEXT:    [[RET:%.*]] = sub i64 [[PHI1]], [[PHI2]]
-; CHECK-NEXT:    ret i64 [[RET]]
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ 0, [[TAKEN]] ], [ [[B:%.*]], [[UNTAKEN]] ]
+; CHECK-NEXT:    ret i64 0
 ;
   br i1 %c, label %taken, label %untaken
 taken:
   %assumption = icmp eq i64 %a, 0
   call void @llvm.assume(i1 %assumption)
   br label %merge
 untaken:
   br label %merge

 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
 ; CHECK:       taken:
 ; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
 ; CHECK-NEXT:    br label [[MERGE:%.*]]
 ; CHECK:       untaken:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ [[A]], [[TAKEN]] ], [ 0, [[UNTAKEN]] ]
-; CHECK-NEXT:    ret i64 [[PHI]]
+; CHECK-NEXT:    ret i64 0
 ;
   br i1 %c, label %taken, label %untaken
 taken:
   %assumption = icmp eq i64 %a, 0
   call void @llvm.assume(i1 %assumption)
   br label %merge
 untaken:
   br label %merge
 merge:
   %phi = phi i64 [%a, %taken], [0, %untaken]
   ret i64 %phi
 }
 
 define i64 @test6(i1 %c, i64 %a) {
 ; CHECK-LABEL: @test6(
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
 ; CHECK:       taken:
 ; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
 ; CHECK-NEXT:    br label [[MERGE:%.*]]
 ; CHECK:       untaken:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ [[A]], [[TAKEN]] ], [ 0, [[UNTAKEN]] ]
-; CHECK-NEXT:    ret i64 [[PHI]]
+; CHECK-NEXT:    ret i64 0
 ;
   br i1 %c, label %taken, label %untaken
 taken:
   %assumption = icmp eq i64 %a, 0
   call void @llvm.assume(i1 %assumption)
   br label %next
 next:
   br label %merge