Fixing truncate. Previously we were emitting truncate from r16 to r8 as

movw. That is we promote the destination operand to r16. So
        %CH = TRUNC_R16_R8 %BP
is emitted as
        movw %bp, %cx.

This is incorrect. If %cl is live, it would be clobbered.
Ideally we want to do the opposite, that is emitted it as
        movb ??, %ch
But this is not possible since %bp does not have a r8 sub-register.

We are now defining a new register class R16_ which is a subclass of R16
containing only those 16-bit registers that have r8 sub-registers (i.e.
AX - DX). We isel the truncate to two instructions, a MOV16to16_ to copy the
value to the R16_ class, followed by a TRUNC_R16_R8.

Due to bug 770, the register colaescer is not going to coalesce between R16 and
R16_. That will be fixed later so we can eliminate the MOV16to16_. Right now, it
can only be eliminated if we are lucky that source and destination registers are
the same.

llvm-svn: 28164

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/Target/X86/X86ATTAsmPrinter.cpp b/llvm/lib/Target/X86/X86ATTAsmPrinter.cpp
index 326251ccc2e..7baa8f87178 100755
--- a/llvm/lib/Target/X86/X86ATTAsmPrinter.cpp
+++ b/llvm/lib/Target/X86/X86ATTAsmPrinter.cpp
@@ -115,7 +115,7 @@ void X86ATTAsmPrinter::printOperand(const MachineInstr *MI, unsigned OpNo,
     unsigned Reg = MO.getReg();
     if (Modifier && strncmp(Modifier, "trunc", strlen("trunc")) == 0) {
       MVT::ValueType VT = (strcmp(Modifier,"trunc16") == 0)
-        ? MVT::i16 : MVT::i32;
+        ? MVT::i16 : MVT::i8;
       Reg = getX86SubSuperRegister(Reg, VT);
     }
     for (const char *Name = RI.get(Reg).Name; *Name; ++Name)
@@ -366,12 +366,13 @@ void X86ATTAsmPrinter::printMachineInstruction(const MachineInstr *MI) {
     const MachineOperand &MO1 = MI->getOperand(1);
     unsigned Reg0 = MO0.getReg();
     unsigned Reg1 = MO1.getReg();
-    if (MI->getOpcode() == X86::TRUNC_R16_R8)
-      Reg0 = getX86SubSuperRegister(Reg0, MVT::i16);
+    if (MI->getOpcode() == X86::TRUNC_R32_R16)
+      Reg1 = getX86SubSuperRegister(Reg1, MVT::i16);
     else
-      Reg0 = getX86SubSuperRegister(Reg0, MVT::i32);
-    if (Reg0 == Reg1)
-      O << CommentString << " TRUNCATE ";
+      Reg1 = getX86SubSuperRegister(Reg1, MVT::i8);
+    O << CommentString << " TRUNCATE ";
+    if (Reg0 != Reg1)
+      O << "\n\t";
     break;
   }
   }