[llvm-dev] InstrEmitter::CreateVirtualRegisters handling of CopyToReg

[Alexey Zhikhartsev via llvm-dev]

Hi Jonas,

This patch helps with a problem on our out-of-tree backend; we would love
to get some feedback about this patch from the community and potentially
upstream it. Would you mind submitting this patch to phabricator? I
understand that you might be busy; if so, I could submit it myself (and
give credit to you, of course). Thanks!

Best,
Alex


-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Jonas
Paulsson via llvm-dev
Sent: Wednesday, May 30, 2018 9:03 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: Ulrich Weigand <ulrich.weigand at de.ibm.com>
Subject: [llvm-dev] InstrEmitter::CreateVirtualRegisters handling of
CopyToReg

Hi,

I wonder if anyone has any comment on a patch like:

diff --git a/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
b/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
index 65ee3816f84..4780f6f0e59 100644
--- a/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
+++ b/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
@@ -243,18 +243,21 @@ void InstrEmitter::CreateVirtualRegisters(SDNode
*Node,

      if (!VRBase && !IsClone && !IsCloned)
        for (SDNode *User : Node->uses()) {
          if (User->getOpcode() == ISD::CopyToReg &&
              User->getOperand(2).getNode() == Node &&
              User->getOperand(2).getResNo() == i) {
            unsigned Reg =
cast<RegisterSDNode>(User->getOperand(1))->getReg();
            if (TargetRegisterInfo::isVirtualRegister(Reg)) {
-            const TargetRegisterClass *RegRC = MRI->getRegClass(Reg);
-            if (RegRC == RC) {
+            // Allow constraining the virtual register's class within
reason,
+            // just like what AddRegisterOperand will allow.
+            const TargetRegisterClass *ConstrainedRC
+              = MRI->constrainRegClass(Reg, RC, MinRCSize);
+            if (ConstrainedRC) {
                VRBase = Reg;
                MIB.addReg(VRBase, RegState::Define);
                break;
              }
            }
          }
        }

Why do the register classes currently have to match exactly in this case?

It seems that these COPYs that now remain may end up in the same register
class, if the users require it. So why not constrain also here directly, if
this is done generally when the register is used as input?

/Jonas


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