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, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index 31a4f6fe2e7..cc30bfe0639 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -28,6 +28,7 @@ bool X86InstrInfo::isMoveInstr(const MachineInstr& MI,
                                unsigned& destReg) const {
   MachineOpCode oc = MI.getOpcode();
   if (oc == X86::MOV8rr || oc == X86::MOV16rr || oc == X86::MOV32rr ||
+      oc == X86::MOV16to16_ || oc == X86::MOV32to32_ ||
       oc == X86::FpMOV  || oc == X86::MOVSSrr || oc == X86::MOVSDrr ||
       oc == X86::FsMOVAPSrr || oc == X86::FsMOVAPDrr ||
       oc == X86::MOVAPSrr || oc == X86::MOVAPDrr ||