From 5e5bda74b61b3e8a4434abe2b5fe5d7ac5880ccd Mon Sep 17 00:00:00 2001
From: Jay Foad <jay.foad@amd.com>
Date: Mon, 18 May 2020 11:08:57 +0100
Subject: [PATCH] [IR] Simplify Use::swap. NFCI.

The new implementation makes it clear that there are exactly two
conditional stores (after the initial no-op optimization). By contrast
the old implementation had seven conditionals, some hidden inside other
functions.

This commit can change the order of operands in operand lists, hence the
tweak to one test case.

Differential Revision: https://reviews.llvm.org/D80116
---
 llvm/lib/IR/Use.cpp                            | 29 ++++++++++----------------
 llvm/test/Transforms/LoopReroll/nonconst_lb.ll |  4 ++--
 2 files changed, 13 insertions(+), 20 deletions(-)

diff --git a/llvm/lib/IR/Use.cpp b/llvm/lib/IR/Use.cpp
index dc0716b8..99049c0 100644
--- a/llvm/lib/IR/Use.cpp
+++ b/llvm/lib/IR/Use.cpp
@@ -17,24 +17,17 @@ void Use::swap(Use &RHS) {
   if (Val == RHS.Val)
     return;
 
-  if (Val)
-    removeFromList();
-
-  Value *OldVal = Val;
-  if (RHS.Val) {
-    RHS.removeFromList();
-    Val = RHS.Val;
-    Val->addUse(*this);
-  } else {
-    Val = nullptr;
-  }
-
-  if (OldVal) {
-    RHS.Val = OldVal;
-    RHS.Val->addUse(RHS);
-  } else {
-    RHS.Val = nullptr;
-  }
+  std::swap(Val, RHS.Val);
+  std::swap(Next, RHS.Next);
+  std::swap(Prev, RHS.Prev);
+
+  *Prev = this;
+  if (Next)
+    Next->Prev = &Next;
+
+  *RHS.Prev = &RHS;
+  if (RHS.Next)
+    RHS.Next->Prev = &RHS.Next;
 }
 
 unsigned Use::getOperandNo() const {
diff --git a/llvm/test/Transforms/LoopReroll/nonconst_lb.ll b/llvm/test/Transforms/LoopReroll/nonconst_lb.ll
index 46a5805..92d893f 100644
--- a/llvm/test/Transforms/LoopReroll/nonconst_lb.ll
+++ b/llvm/test/Transforms/LoopReroll/nonconst_lb.ll
@@ -48,7 +48,7 @@ for.end:                                          ; preds = %for.body, %entry
   ret void
 }
 ; CHECK-LABEL: @foo
-; CHECK: for.body.preheader:                               ; preds = %entry
+; CHECK: for.body.preheader:
 ; CHECK:   %0 = add i32 %n, -1
 ; CHECK:   %1 = sub i32 %0, %m
 ; CHECK:   %2 = lshr i32 %1, 2
@@ -56,7 +56,7 @@ for.end:                                          ; preds = %for.body, %entry
 ; CHECK:   %4 = add i32 %3, 3
 ; CHECK:   br label %for.body
 
-; CHECK: for.body:                                         ; preds = %for.body, %for.body.preheader
+; CHECK: for.body:
 ; CHECK:   %indvar = phi i32 [ 0, %for.body.preheader ], [ %indvar.next, %for.body ]
 ; CHECK:   %5 = add i32 %m, %indvar
 ; CHECK:   %arrayidx = getelementptr inbounds i32, i32* %B, i32 %5
-- 
2.7.4