SimplifyCFG: correctly handle borrowed-from values when removing a redundant phi argument

Fixes a verifier crash

Author: eeckstein

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -3850,11 +3850,21 @@ static void tryToReplaceArgWithIncomingValue(SILBasicBlock *BB, unsigned i,
   // An argument has one result value. We need to replace this with the *value*
   // of the incoming block(s).
   LLVM_DEBUG(llvm::dbgs() << "replace arg with incoming value:" << *A);
-  if (auto *bfi = getBorrowedFromUser(A)) {
-    bfi->replaceAllUsesWith(A);
-    bfi->eraseFromParent();
+  while (!A->use_empty()) {
+    Operand *op = *A->use_begin();
+    if (auto *bfi = dyn_cast<BorrowedFromInst>(op->getUser())) {
+      if (op->getOperandNumber() == 0) {
+        bfi->replaceAllUsesWith(V);
+        bfi->eraseFromParent();
+        continue;
+      }
+      if (auto *prevBfi = dyn_cast<BorrowedFromInst>(V)) {
+        op->set(prevBfi->getBorrowedValue());
+        continue;
+      }
+    }
+    op->set(V);
   }
-  A->replaceAllUsesWith(V);
 }
 
 bool SimplifyCFG::simplifyArgs(SILBasicBlock *BB) {

test/SILOptimizer/simplify_cfg_ossa.sil

@@ -1939,3 +1939,35 @@ bb3:
   return %t : $()
 }
 
+// CHECK-LABEL: sil [ossa] @replace_phi_arg_with_borrowed_from_use :
+// CHECK:       bb3([[R:%.*]] : @reborrow $B):
+// CHECK:       bb6([[G:%.*]] : @guaranteed $E):
+// CHECK-NEXT:    [[X:%.*]] = borrowed [[G]] : $E from ([[R]] : $B)
+// CHECK-NEXT:    fix_lifetime [[X]]
+// CHECK:       } // end sil function 'replace_phi_arg_with_borrowed_from_use'
+sil [ossa] @replace_phi_arg_with_borrowed_from_use : $@convention(thin) (@in_guaranteed B) -> () {
+bb0(%0 : $*B):
+  cond_br undef, bb1, bb2
+bb1:
+  %4 = load_borrow %0
+  br bb3(%4)
+bb2:
+  %6 = load_borrow %0
+  br bb3(%6)
+bb3(%8 : @reborrow $B):
+  %9 = borrowed %8 from ()
+  cond_br undef, bb4, bb5
+bb4:
+  %11 = unchecked_ref_cast %9 to $E
+  br bb6(%11, %9)
+bb5:
+  %13 = unchecked_ref_cast %9 to $E
+  br bb6(%13, %9)
+bb6(%15: @guaranteed $E, %16 : @reborrow $B):
+  %17 = borrowed %15 from (%16)
+  %18 = borrowed %16 from ()
+  fix_lifetime %17
+  end_borrow %18
+  %r = tuple()
+  return %r
+}