[llvm-commits] [llvm] r105341 - /llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

[Dan Gohman]

Author: djg
Date: Wed Jun  2 14:13:40 2010
New Revision: 105341

URL: http://llvm.org/viewvc/llvm-project?rev=105341&view=rev
Log:
Use comments to document non-obvious code rather than
mailing list archives.

Modified:
    llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

Modified: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=105341&r1=105340&r2=105341&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp Wed Jun  2 14:13:40 2010
@@ -980,6 +980,17 @@
   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);