[PATCH] D125564: Teach PeepholeOpt to eliminate redundant copy from constant physreg (e.g VLENB on RISCV)

[Philip Reames via Phabricator via llvm-commits]

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The existing redundant copy elimination required a virtual register source, but the same logic works for any physreg where we don't have to worry about clobbers.

(Aside: The code can be pretty easily generalized to handle the clobber case too, but that'll be a separate patch.)

On RISCV, this helps eliminate redundant CSR reads from VLENB.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125564

Files:
  llvm/lib/CodeGen/PeepholeOptimizer.cpp
  llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
  llvm/test/CodeGen/RISCV/vlenb.ll


Index: llvm/test/CodeGen/RISCV/vlenb.ll
===================================================================
--- llvm/test/CodeGen/RISCV/vlenb.ll
+++ llvm/test/CodeGen/RISCV/vlenb.ll
@@ -16,8 +16,7 @@
 ; CHECK-LABEL: simple_cse:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    csrr a0, vlenb
-; CHECK-NEXT:    csrr a1, vlenb
-; CHECK-NEXT:    sub a0, a0, a1
+; CHECK-NEXT:    sub a0, a0, a0
 ; CHECK-NEXT:    ret
 entry:
   %v1 = call i32 @llvm.read_register.i32(metadata !0)
Index: llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
+++ llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
@@ -118,7 +118,7 @@
 }
 
 bool RISCVRegisterInfo::isConstantPhysReg(MCRegister PhysReg) const {
-  return PhysReg == RISCV::X0;
+  return PhysReg == RISCV::X0 || PhysReg == RISCV::VLENB;
 }
 
 const uint32_t *RISCVRegisterInfo::getNoPreservedMask() const {
Index: llvm/lib/CodeGen/PeepholeOptimizer.cpp
===================================================================
--- llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -213,8 +213,9 @@
                               const SmallSet<Register, 2> &TargetReg,
                               RecurrenceCycle &RC);
 
-    /// If copy instruction \p MI is a virtual register copy, track it in
-    /// the set \p CopyMIs. If this virtual register was previously seen as a
+    /// If copy instruction \p MI is a virtual register copy or a copy of a
+    /// constant physical register to a virtual register, track it in the
+    /// set \p CopyMIs. If this virtual register was previously seen as a
     /// copy, replace the uses of this copy with the previously seen copy's
     /// destination register.
     bool foldRedundantCopy(MachineInstr &MI,
@@ -1411,7 +1412,7 @@
 
   Register SrcReg = MI.getOperand(1).getReg();
   unsigned SrcSubReg = MI.getOperand(1).getSubReg();
-  if (!SrcReg.isVirtual())
+  if (!SrcReg.isVirtual() && !MRI->isConstantPhysReg(SrcReg))
     return false;
 
   Register DstReg = MI.getOperand(0).getReg();
@@ -1642,8 +1643,8 @@
     // without any intervening re-definition of $physreg.
     DenseMap<Register, MachineInstr *> NAPhysToVirtMIs;
 
-    // Set of pairs of virtual registers and their subregs that are copied
-    // from.
+    // Set of copies to virtual registers keyed by source register.  Never
+    // holds any physreg which requires def tracking.
     DenseMap<RegSubRegPair, MachineInstr *> CopySrcMIs;
 
     bool IsLoopHeader = MLI->isLoopHeader(&MBB);


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