[llvm] r359236 - Assigning to a local object in a return statement prevents copy elision. NFC.

[David Blaikie via llvm-commits]

Author: dblaikie
Date: Thu Apr 25 13:09:00 2019
New Revision: 359236

URL: http://llvm.org/viewvc/llvm-project?rev=359236&view=rev
Log:
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

Modified:
    llvm/trunk/include/llvm/CodeGen/MachineInstrBundle.h
    llvm/trunk/include/llvm/Support/BranchProbability.h
    llvm/trunk/lib/Support/Path.cpp
    llvm/trunk/lib/Target/X86/X86MCInstLower.cpp
    llvm/trunk/lib/Transforms/Instrumentation/Instrumentation.cpp

Modified: llvm/trunk/include/llvm/CodeGen/MachineInstrBundle.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/MachineInstrBundle.h?rev=359236&r1=359235&r2=359236&view=diff
==============================================================================
--- llvm/trunk/include/llvm/CodeGen/MachineInstrBundle.h (original)
+++ llvm/trunk/include/llvm/CodeGen/MachineInstrBundle.h Thu Apr 25 13:09:00 2019
@@ -61,7 +61,8 @@ inline MachineBasicBlock::instr_iterator
     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_it
     MachineBasicBlock::const_instr_iterator I) {
   while (I->isBundledWithSucc())
     ++I;
-  return ++I;
+  ++I;
+  return I;
 }
 
 //===----------------------------------------------------------------------===//

Modified: llvm/trunk/include/llvm/Support/BranchProbability.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/BranchProbability.h?rev=359236&r1=359235&r2=359236&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Support/BranchProbability.h (original)
+++ llvm/trunk/include/llvm/Support/BranchProbability.h Thu Apr 25 13:09:00 2019
@@ -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; }

Modified: llvm/trunk/lib/Support/Path.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Path.cpp?rev=359236&r1=359235&r2=359236&view=diff
==============================================================================
--- llvm/trunk/lib/Support/Path.cpp (original)
+++ llvm/trunk/lib/Support/Path.cpp Thu Apr 25 13:09:00 2019
@@ -297,7 +297,8 @@ reverse_iterator rbegin(StringRef Path,
   I.Path = Path;
   I.Position = Path.size();
   I.S = style;
-  return ++I;
+  ++I;
+  return I;
 }
 
 reverse_iterator rend(StringRef Path) {

Modified: llvm/trunk/lib/Target/X86/X86MCInstLower.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86MCInstLower.cpp?rev=359236&r1=359235&r2=359236&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86MCInstLower.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86MCInstLower.cpp Thu Apr 25 13:09:00 2019
@@ -1364,7 +1364,8 @@ PrevCrossBBInst(MachineBasicBlock::const
     MBB = MBB->getPrevNode();
     MBBI = MBB->end();
   }
-  return --MBBI;
+  --MBBI;
+  return MBBI;
 }
 
 static const Constant *getConstantFromPool(const MachineInstr &MI,

Modified: llvm/trunk/lib/Transforms/Instrumentation/Instrumentation.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Instrumentation/Instrumentation.cpp?rev=359236&r1=359235&r2=359236&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Instrumentation/Instrumentation.cpp (original)
+++ llvm/trunk/lib/Transforms/Instrumentation/Instrumentation.cpp Thu Apr 25 13:09:00 2019
@@ -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;
 }