[MachineCopyPropagation] More robust isForwardableRegClassCopy

Change the implementation of isForwardableRegClassCopy so that it
does not rely on getMinimalPhysRegClass. Instead, iterate over all
classes looking for any that satisfy a required property.

NFCI on current upstream targets, but this copes better with
downstream AMDGPU changes where some new smaller classes have been
introduced, which was breaking regclass equality tests in the old
code like:
    if (UseDstRC != CrossCopyRC && CopyDstRC == CrossCopyRC)

Differential Revision: https://reviews.llvm.org/D121903

1 files changed, 27 insertions, 30 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index a15523f..0535332 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -413,31 +413,6 @@ bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
   if (!UseI.isCopy())
     return false;
 
-  const TargetRegisterClass *CopySrcRC =
-      TRI->getMinimalPhysRegClass(CopySrcReg);
-  const TargetRegisterClass *UseDstRC =
-      TRI->getMinimalPhysRegClass(UseI.getOperand(0).getReg());
-  const TargetRegisterClass *CrossCopyRC = TRI->getCrossCopyRegClass(CopySrcRC);
-
-  // If cross copy register class is not the same as copy source register class
-  // then it is not possible to copy the register directly and requires a cross
-  // register class copy. Fowarding this copy without checking register class of
-  // UseDst may create additional cross register copies when expanding the copy
-  // instruction in later passes.
-  if (CopySrcRC != CrossCopyRC) {
-    const TargetRegisterClass *CopyDstRC =
-        TRI->getMinimalPhysRegClass(Copy.getOperand(0).getReg());
-
-    // Check if UseDstRC matches the necessary register class to copy from
-    // CopySrc's register class. If so then forwarding the copy will not
-    // introduce any cross-class copys. Else if CopyDstRC matches then keep the
-    // copy and do not forward. If neither UseDstRC or CopyDstRC matches then
-    // we may need a cross register copy later but we do not worry about it
-    // here.
-    if (UseDstRC != CrossCopyRC && CopyDstRC == CrossCopyRC)
-      return false;
-  }
-
   /// COPYs don't have register class constraints, so if the user instruction
   /// is a COPY, we just try to avoid introducing additional cross-class
   /// COPYs.  For example:
@@ -454,12 +429,34 @@ bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
   ///
   /// so we have reduced the number of cross-class COPYs and potentially
   /// introduced a nop COPY that can be removed.
-  const TargetRegisterClass *SuperRC = UseDstRC;
-  for (TargetRegisterClass::sc_iterator SuperRCI = UseDstRC->getSuperClasses();
-       SuperRC; SuperRC = *SuperRCI++)
-    if (SuperRC->contains(CopySrcReg))
-      return true;
 
+  // Allow forwarding if src and dst belong to any common class, so long as they
+  // don't belong to any (possibly smaller) common class that requires copies to
+  // go via a different class.
+  Register UseDstReg = UseI.getOperand(0).getReg();
+  bool Found = false;
+  bool IsCrossClass = false;
+  for (const TargetRegisterClass *RC : TRI->regclasses()) {
+    if (RC->contains(CopySrcReg) && RC->contains(UseDstReg)) {
+      Found = true;
+      if (TRI->getCrossCopyRegClass(RC) != RC) {
+        IsCrossClass = true;
+        break;
+      }
+    }
+  }
+  if (!Found)
+    return false;
+  if (!IsCrossClass)
+    return true;
+  // The forwarded copy would be cross-class. Only do this if the original copy
+  // was also cross-class.
+  Register CopyDstReg = Copy.getOperand(0).getReg();
+  for (const TargetRegisterClass *RC : TRI->regclasses()) {
+    if (RC->contains(CopySrcReg) && RC->contains(CopyDstReg) &&
+        TRI->getCrossCopyRegClass(RC) != RC)
+      return true;
+  }
   return false;
 }