From: Dan Gohman <gohman@apple.com>
Date: Wed, 2 Jun 2010 19:13:40 +0000 (+0000)
Subject: Use comments to document non-obvious code rather than
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=71c62a2977c405fb3f7b6dc5dcc59f63ca08be0f;p=oota-llvm.git

Use comments to document non-obvious code rather than
mailing list archives.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105341 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index bb80ace3c24..a322ad2aa25 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -980,6 +980,17 @@ X86TargetLowering::X86TargetLowering(X86TargetMachine &TM)
   setOperationAction(ISD::SSUBO, MVT::i32, Custom);
   setOperationAction(ISD::USUBO, MVT::i32, Custom);
   setOperationAction(ISD::SMULO, MVT::i32, Custom);
+
+  // Don't try to custom-lower 64-bit add-with-overflow and friends
+  // on x86-32; the x86 backend currently doesn't know how to handle them.
+  //
+  // This doesn't really fix anything because LegalizeTypes doesn't know
+  // how to handle them either.  We do get a better error message, though.
+  //
+  // This may not be hard to implement though.
+  // In fact you could even cheat, and turn the 64 bit add-with-overflow
+  // into a 65 bit add, with the top bit being used to compute the overflow
+  // flag.  That should then all get expanded out automagically.
   if (Subtarget->is64Bit()) {
     setOperationAction(ISD::SADDO, MVT::i64, Custom);
     setOperationAction(ISD::UADDO, MVT::i64, Custom);