divb / mulb outputs to ah. Under x86-64 it's not legal to read ah if the instruction requires a rex prefix (i.e. outputs to r8b, etc.). So issue shift right by 8 on AX and then truncate it to 8 bits instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40972 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 46 insertions, 4 deletions

diff --git a/lib/Target/X86/README-X86-64.txt b/lib/Target/X86/README-X86-64.txt
index 191904ac51..bc19986eba 100644
--- a/lib/Target/X86/README-X86-64.txt
+++ b/lib/Target/X86/README-X86-64.txt
@@ -221,3 +221,16 @@ we were to support medium or larger code models, we need to use the movabs
 instruction. We should probably introduce something like AbsoluteAddress to
 distinguish it from GlobalAddress so the asm printer and JIT code emitter can
 do the right thing.
+
+//===---------------------------------------------------------------------===//
+
+It's not possible to reference AH, BH, CH, and DH registers in an instruction
+requiring REX prefix. However, divb and mulb both produce results in AH. If isel
+emits a CopyFromReg which gets turned into a movb and that can be allocated a
+r8b - r15b.
+
+To get around this, isel emits a CopyFromReg from AX and then right shift it
+down by 8 and truncate it. It's not pretty but it works. We need some register
+allocation magic to make the hack go away (e.g. putting additional constraints
+on the result of the movb).
+
diff --git a/lib/Target/X86/X86ISelDAGToDAG.cpp b/lib/Target/X86/X86ISelDAGToDAG.cpp
index 0caf7a5bd3..a851670800 100644
--- a/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -1159,7 +1159,21 @@ SDNode *X86DAGToDAGISel::Select(SDOperand N) {
           SDOperand(CurDAG->getTargetNode(Opc, MVT::Flag, N1, InFlag), 0);
       }
 
-      SDOperand Result = CurDAG->getCopyFromReg(Chain, HiReg, NVT, InFlag);
+      SDOperand Result;
+      if (HiReg == X86::AH && Subtarget->is64Bit()) {
+        // Prevent use of AH in a REX instruction by referencing AX instead.
+        // Shift it down 8 bits.
+        Result = CurDAG->getCopyFromReg(Chain, X86::AX, MVT::i16, InFlag);
+        Chain = Result.getValue(1);
+        Result = SDOperand(CurDAG->getTargetNode(X86::SHR16ri, MVT::i16, Result,
+                                     CurDAG->getTargetConstant(8, MVT::i8)), 0);
+        // Then truncate it down to i8.
+        SDOperand SRIdx = CurDAG->getTargetConstant(1, MVT::i32); // SubRegSet 1
+        Result = SDOperand(CurDAG->getTargetNode(X86::EXTRACT_SUBREG,
+                                                 MVT::i8, Result, SRIdx), 0);
+      } else {
+        Result = CurDAG->getCopyFromReg(Chain, HiReg, NVT, InFlag);
+      }
       ReplaceUses(N.getValue(0), Result);
       if (foldedLoad)
         ReplaceUses(N1.getValue(1), Result.getValue(1));
@@ -1286,11 +1300,26 @@ SDNode *X86DAGToDAGISel::Select(SDOperand N) {
           SDOperand(CurDAG->getTargetNode(Opc, MVT::Flag, N1, InFlag), 0);
       }
 
-      SDOperand Result =
-        CurDAG->getCopyFromReg(Chain, isDiv ? LoReg : HiReg, NVT, InFlag);
+      unsigned Reg = isDiv ? LoReg : HiReg;
+      SDOperand Result;
+      if (Reg == X86::AH && Subtarget->is64Bit()) {
+        // Prevent use of AH in a REX instruction by referencing AX instead.
+        // Shift it down 8 bits.
+        Result = CurDAG->getCopyFromReg(Chain, X86::AX, MVT::i16, InFlag);
+        Chain = Result.getValue(1);
+        Result = SDOperand(CurDAG->getTargetNode(X86::SHR16ri, MVT::i16, Result,
+                                     CurDAG->getTargetConstant(8, MVT::i8)), 0);
+        // Then truncate it down to i8.
+        SDOperand SRIdx = CurDAG->getTargetConstant(1, MVT::i32); // SubRegSet 1
+        Result = SDOperand(CurDAG->getTargetNode(X86::EXTRACT_SUBREG,
+                                                 MVT::i8, Result, SRIdx), 0);
+      } else {
+        Result = CurDAG->getCopyFromReg(Chain, Reg, NVT, InFlag);
+        Chain = Result.getValue(1);
+      }
       ReplaceUses(N.getValue(0), Result);
       if (foldedLoad)
-        ReplaceUses(N1.getValue(1), Result.getValue(1));
+        ReplaceUses(N1.getValue(1), Chain);
 
 #ifndef NDEBUG
       DOUT << std::string(Indent-2, ' ') << "=> ";