From 0c4dbf9ecd32b2719beff4490b1624ef13705a09 Mon Sep 17 00:00:00 2001
From: David Blaikie <dblaikie@gmail.com>
Date: Thu, 25 Apr 2019 20:09:00 +0000
Subject: [PATCH] Assigning to a local object in a return statement prevents
 copy elision. NFC.

I added a diagnostic along the lines of `-Wpessimizing-move` to detect `return x = y` suppressing copy elision, but I don't know if the diagnostic is really worth it. Anyway, here are the places where my diagnostic reported that copy elision would have been possible if not for the assignment.

P1155R1 in the post-San-Diego WG21 (C++ committee) mailing discusses whether WG21 should fix this pitfall by just changing the core language to permit copy elision in cases like these.

(Kona update: The bulk of P1155 is proceeding to CWG review, but specifically *not* the parts that explored the notion of permitting copy-elision in these specific cases.)

Reviewed By: dblaikie

Author: Arthur O'Dwyer

Differential Revision: https://reviews.llvm.org/D54885

llvm-svn: 359236
---
 llvm/include/llvm/CodeGen/MachineInstrBundle.h          |  6 ++++--
 llvm/include/llvm/Support/BranchProbability.h           | 15 ++++++++++-----
 llvm/lib/Support/Path.cpp                               |  3 ++-
 llvm/lib/Target/X86/X86MCInstLower.cpp                  |  3 ++-
 llvm/lib/Transforms/Instrumentation/Instrumentation.cpp | 10 ++++++----
 5 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/llvm/include/llvm/CodeGen/MachineInstrBundle.h b/llvm/include/llvm/CodeGen/MachineInstrBundle.h
index 9bb5b63..1810d23 100644
--- a/llvm/include/llvm/CodeGen/MachineInstrBundle.h
+++ b/llvm/include/llvm/CodeGen/MachineInstrBundle.h
@@ -61,7 +61,8 @@ inline MachineBasicBlock::instr_iterator getBundleEnd(
     MachineBasicBlock::instr_iterator I) {
   while (I->isBundledWithSucc())
     ++I;
-  return ++I;
+  ++I;
+  return I;
 }
 
 /// Returns an iterator pointing beyond the bundle containing \p I.
@@ -69,7 +70,8 @@ inline MachineBasicBlock::const_instr_iterator getBundleEnd(
     MachineBasicBlock::const_instr_iterator I) {
   while (I->isBundledWithSucc())
     ++I;
-  return ++I;
+  ++I;
+  return I;
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/llvm/include/llvm/Support/BranchProbability.h b/llvm/include/llvm/Support/BranchProbability.h
index dd0aba0..b7dddd5 100644
--- a/llvm/include/llvm/Support/BranchProbability.h
+++ b/llvm/include/llvm/Support/BranchProbability.h
@@ -128,27 +128,32 @@ public:
 
   BranchProbability operator+(BranchProbability RHS) const {
     BranchProbability Prob(*this);
-    return Prob += RHS;
+    Prob += RHS;
+    return Prob;
   }
 
   BranchProbability operator-(BranchProbability RHS) const {
     BranchProbability Prob(*this);
-    return Prob -= RHS;
+    Prob -= RHS;
+    return Prob;
   }
 
   BranchProbability operator*(BranchProbability RHS) const {
     BranchProbability Prob(*this);
-    return Prob *= RHS;
+    Prob *= RHS;
+    return Prob;
   }
 
   BranchProbability operator*(uint32_t RHS) const {
     BranchProbability Prob(*this);
-    return Prob *= RHS;
+    Prob *= RHS;
+    return Prob;
   }
 
   BranchProbability operator/(uint32_t RHS) const {
     BranchProbability Prob(*this);
-    return Prob /= RHS;
+    Prob /= RHS;
+    return Prob;
   }
 
   bool operator==(BranchProbability RHS) const { return N == RHS.N; }
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
index d60c135..5312e1d 100644
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -297,7 +297,8 @@ reverse_iterator rbegin(StringRef Path, Style style) {
   I.Path = Path;
   I.Position = Path.size();
   I.S = style;
-  return ++I;
+  ++I;
+  return I;
 }
 
 reverse_iterator rend(StringRef Path) {
diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index b147cbf..11492f7 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1364,7 +1364,8 @@ PrevCrossBBInst(MachineBasicBlock::const_iterator MBBI) {
     MBB = MBB->getPrevNode();
     MBBI = MBB->end();
   }
-  return --MBBI;
+  --MBBI;
+  return MBBI;
 }
 
 static const Constant *getConstantFromPool(const MachineInstr &MI,
diff --git a/llvm/lib/Transforms/Instrumentation/Instrumentation.cpp b/llvm/lib/Transforms/Instrumentation/Instrumentation.cpp
index 1ed0927..22fa76a 100644
--- a/llvm/lib/Transforms/Instrumentation/Instrumentation.cpp
+++ b/llvm/lib/Transforms/Instrumentation/Instrumentation.cpp
@@ -24,10 +24,12 @@ using namespace llvm;
 /// Moves I before IP. Returns new insert point.
 static BasicBlock::iterator moveBeforeInsertPoint(BasicBlock::iterator I, BasicBlock::iterator IP) {
   // If I is IP, move the insert point down.
-  if (I == IP)
-    return ++IP;
-  // Otherwise, move I before IP and return IP.
-  I->moveBefore(&*IP);
+  if (I == IP) {
+    ++IP;
+  } else {
+    // Otherwise, move I before IP and return IP.
+    I->moveBefore(&*IP);
+  }
   return IP;
 }
 
-- 
2.7.4