[PATCH] D54885: Assigning to a local object in a return statement prevents copy elision. NFC.

[Arthur O'Dwyer via Phabricator via llvm-commits]

Quuxplusone created this revision.
Quuxplusone added a reviewer: mgrang.
Herald added subscribers: llvm-commits, kristina, javed.absar.

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.

P1144R1 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.


Repository:
  rL LLVM

https://reviews.llvm.org/D54885

Files:
  include/llvm/Support/BranchProbability.h
  lib/Target/AArch64/AArch64ISelLowering.cpp


Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -7842,7 +7842,7 @@
   Cmp = DAG.getSExtOrTrunc(Cmp, dl, Op.getValueType());
 
   if (ShouldInvert)
-    return Cmp = DAG.getNOT(dl, Cmp, Cmp.getValueType());
+    Cmp = DAG.getNOT(dl, Cmp, Cmp.getValueType());
 
   return Cmp;
 }
Index: include/llvm/Support/BranchProbability.h
===================================================================
--- include/llvm/Support/BranchProbability.h
+++ include/llvm/Support/BranchProbability.h
@@ -129,27 +129,32 @@
 
   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; }


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