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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359236 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/CodeGen/MachineInstrBundle.h b/include/llvm/CodeGen/MachineInstrBundle.h
index 9bb5b633566551186538dbc73d74134abf302e84..1810d23072d00ce34f8b4bd78882506773791126 100644 (file)
--- a/include/llvm/CodeGen/MachineInstrBundle.h
+++ b/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/include/llvm/Support/BranchProbability.h b/include/llvm/Support/BranchProbability.h
index dd0aba025abfb6188241823e1718c4e6e677459c..b7dddd56af78dd96121a6a37fda5131f2d07f607 100644 (file)
--- a/include/llvm/Support/BranchProbability.h
+++ b/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/lib/Support/Path.cpp b/lib/Support/Path.cpp
index d60c1359b180b312226cb0fb601e41e6b768a35a..5312e1df3b6a1d65095246e07cf0619108f3b938 100644 (file)
--- a/lib/Support/Path.cpp
+++ b/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/lib/Target/X86/X86MCInstLower.cpp b/lib/Target/X86/X86MCInstLower.cpp
index b147cbff002dbd3fa2d458e7f2673fe68587b33f..11492f7096848ffe0d2f9d8dde917b43133f4a01 100644 (file)
--- a/lib/Target/X86/X86MCInstLower.cpp
+++ b/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/lib/Transforms/Instrumentation/Instrumentation.cpp b/lib/Transforms/Instrumentation/Instrumentation.cpp
index 1ed0927cd73ad3c98e9ca357a86a08bda0a7ec1d..22fa76a6c1fa90aece6d24a0bf273bc74e506a97 100644 (file)
--- a/lib/Transforms/Instrumentation/Instrumentation.cpp
+++ b/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;
 }