commit 16048332f149a1b8d2d34913230f79c8de66fa00
author Akira Hatanaka <ahatanaka@mips.com> Mon Oct 07 18:49:46 2013 +0000
committer Akira Hatanaka <ahatanaka@mips.com> Mon Oct 07 18:49:46 2013 +0000
tree 56c8a30a42f4829d7c433b43c2282b2dd655a15c
parent 0aafb58acadba8b7de3cc923d01203e0cf48b03b

[mips] Fix definition of mfhi and mflo instructions to read from the whole
accumulator instead of its sub-registers, $hi and $lo. 

We need this change to prevent a mflo following a mtlo from reading an
unpredictable/undefined value, as shown in the following example:

mult $6, $7 // result of $6 * $7 is written to $lo and $hi.
mflo $2     // read lower 32-bit result from $lo.
mtlo $4     // write to $lo. the content of $hi becomes unpredictable.
mfhi $3     // read higher 32-bit from $hi, which has an unpredictable value.

I don't have a test case for this change that reliably reproduces the problem.

llvm-svn: 192119

llvm/lib/Target/Mips/MipsSEISelLowering.cpp

diff --git a/llvm/lib/Target/Mips/MipsSEISelLowering.cpp b/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
index 82cb7a8..7fe0f4b 100644
--- a/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
+++ b/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
@@ -332,15 +332,11 @@
 
   // replace uses of adde and addc here
   if (!SDValue(ADDCNode, 0).use_empty()) {
-    SDValue LoIdx = CurDAG->getConstant(Mips::sub_lo, MVT::i32);
-    SDValue LoOut = CurDAG->getNode(MipsISD::ExtractLOHI, DL, MVT::i32, MAdd,
-                                    LoIdx);
+    SDValue LoOut = CurDAG->getNode(MipsISD::ExtractLO, DL, MVT::i32, MAdd);
     CurDAG->ReplaceAllUsesOfValueWith(SDValue(ADDCNode, 0), LoOut);
   }
   if (!SDValue(ADDENode, 0).use_empty()) {
-    SDValue HiIdx = CurDAG->getConstant(Mips::sub_hi, MVT::i32);
-    SDValue HiOut = CurDAG->getNode(MipsISD::ExtractLOHI, DL, MVT::i32, MAdd,
-                                    HiIdx);
+    SDValue HiOut = CurDAG->getNode(MipsISD::ExtractHI, DL, MVT::i32, MAdd);
     CurDAG->ReplaceAllUsesOfValueWith(SDValue(ADDENode, 0), HiOut);
   }
 
@@ -408,15 +404,11 @@
 
   // replace uses of sube and subc here
   if (!SDValue(SUBCNode, 0).use_empty()) {
-    SDValue LoIdx = CurDAG->getConstant(Mips::sub_lo, MVT::i32);
-    SDValue LoOut = CurDAG->getNode(MipsISD::ExtractLOHI, DL, MVT::i32, MSub,
-                                    LoIdx);
+    SDValue LoOut = CurDAG->getNode(MipsISD::ExtractLO, DL, MVT::i32, MSub);
     CurDAG->ReplaceAllUsesOfValueWith(SDValue(SUBCNode, 0), LoOut);
   }
   if (!SDValue(SUBENode, 0).use_empty()) {
-    SDValue HiIdx = CurDAG->getConstant(Mips::sub_hi, MVT::i32);
-    SDValue HiOut = CurDAG->getNode(MipsISD::ExtractLOHI, DL, MVT::i32, MSub,
-                                    HiIdx);
+    SDValue HiOut = CurDAG->getNode(MipsISD::ExtractHI, DL, MVT::i32, MSub);
     CurDAG->ReplaceAllUsesOfValueWith(SDValue(SUBENode, 0), HiOut);
   }
 
@@ -946,11 +938,9 @@
   SDValue Lo, Hi;
 
   if (HasLo)
-    Lo = DAG.getNode(MipsISD::ExtractLOHI, DL, Ty, Mult,
-                     DAG.getConstant(Mips::sub_lo, MVT::i32));
+    Lo = DAG.getNode(MipsISD::ExtractLO, DL, Ty, Mult);
   if (HasHi)
-    Hi = DAG.getNode(MipsISD::ExtractLOHI, DL, Ty, Mult,
-                     DAG.getConstant(Mips::sub_hi, MVT::i32));
+    Hi = DAG.getNode(MipsISD::ExtractHI, DL, Ty, Mult);
 
   if (!HasLo || !HasHi)
     return HasLo ? Lo : Hi;
@@ -969,10 +959,8 @@
 }
 
 static SDValue extractLOHI(SDValue Op, SDLoc DL, SelectionDAG &DAG) {
-  SDValue Lo = DAG.getNode(MipsISD::ExtractLOHI, DL, MVT::i32, Op,
-                           DAG.getConstant(Mips::sub_lo, MVT::i32));
-  SDValue Hi = DAG.getNode(MipsISD::ExtractLOHI, DL, MVT::i32, Op,
-                           DAG.getConstant(Mips::sub_hi, MVT::i32));
+  SDValue Lo = DAG.getNode(MipsISD::ExtractLO, DL, MVT::i32, Op);
+  SDValue Hi = DAG.getNode(MipsISD::ExtractHI, DL, MVT::i32, Op);
   return DAG.getNode(ISD::BUILD_PAIR, DL, MVT::i64, Lo, Hi);
 }