[MCP] Enhance MCP copy Instruction removal for special case (#70778)

Machine Copy Propagation Pass may lose some opportunities to further
remove the redundant copy instructions during the ForwardCopyPropagateBlock
procedure. When we Clobber a "Def" register, we also need to remove the record 
from the copy maps that indicates "Src" defined "Def" to ensure the correct semantics
of the ClobberRegister function.

For more information, please see the C++ test case generated code in 
"vector.body" after the MCP Pass: https://gcc.godbolt.org/z/nK4oMaWv5.

1 files changed, 38 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index a032b31..abd6e64 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -175,8 +175,43 @@ public:
         if (MachineInstr *MI = I->second.MI) {
           std::optional<DestSourcePair> CopyOperands =
               isCopyInstr(*MI, TII, UseCopyInstr);
-          markRegsUnavailable({CopyOperands->Destination->getReg().asMCReg()},
-                              TRI);
+
+          MCRegister Def = CopyOperands->Destination->getReg().asMCReg();
+          MCRegister Src = CopyOperands->Source->getReg().asMCReg();
+
+          markRegsUnavailable(Def, TRI);
+
+          // Since we clobber the destination of a copy, the semantic of Src's
+          // "DefRegs" to contain Def is no longer effectual. We will also need
+          // to remove the record from the copy maps that indicates Src defined
+          // Def. Failing to do so might cause the target to miss some
+          // opportunities to further eliminate redundant copy instructions.
+          // Consider the following sequence during the
+          // ForwardCopyPropagateBlock procedure:
+          // L1: r0 = COPY r9     <- TrackMI
+          // L2: r0 = COPY r8     <- TrackMI (Remove r9 defined r0 from tracker)
+          // L3: use r0           <- Remove L2 from MaybeDeadCopies
+          // L4: early-clobber r9 <- Clobber r9 (L2 is still valid in tracker)
+          // L5: r0 = COPY r8     <- Remove NopCopy
+          for (MCRegUnit SrcUnit : TRI.regunits(Src)) {
+            auto SrcCopy = Copies.find(SrcUnit);
+            if (SrcCopy != Copies.end() && SrcCopy->second.LastSeenUseInCopy) {
+              // If SrcCopy defines multiple values, we only need
+              // to erase the record for Def in DefRegs.
+              for (auto itr = SrcCopy->second.DefRegs.begin();
+                   itr != SrcCopy->second.DefRegs.end(); itr++) {
+                if (*itr == Def) {
+                  SrcCopy->second.DefRegs.erase(itr);
+                  // If DefReg becomes empty after removal, we can directly
+                  // remove SrcCopy from the tracker's copy maps.
+                  if (SrcCopy->second.DefRegs.empty()) {
+                    Copies.erase(SrcCopy);
+                  }
+                  break;
+                }
+              }
+            }
+          }
         }
         // Now we can erase the copy.
         Copies.erase(I);
@@ -785,6 +820,7 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
         // ...
         // %xmm2 = copy %xmm9
         Tracker.clobberRegister(Def, *TRI, *TII, UseCopyInstr);
+
         for (const MachineOperand &MO : MI.implicit_operands()) {
           if (!MO.isReg() || !MO.isDef())
             continue;
@@ -795,7 +831,6 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
         }
 
         Tracker.trackCopy(&MI, *TRI, *TII, UseCopyInstr);
-
         continue;
       }
     }