[llvm] 9b4b9de - [MRI] Avoid walking past all defs in hasOneUse() (#201249)

[via llvm-commits]

Author: Mingjie Xu
Date: 2026-06-08T09:08:07+08:00
New Revision: 9b4b9de0319b63ff56dd157b1d12465f206681f6

URL: https://github.com/llvm/llvm-project/commit/9b4b9de0319b63ff56dd157b1d12465f206681f6
DIFF: https://github.com/llvm/llvm-project/commit/9b4b9de0319b63ff56dd157b1d12465f206681f6.diff

LOG: [MRI] Avoid walking past all defs in hasOneUse() (#201249)

The use-def list is circular in the Prev direction (Head->Prev == Tail)
and defs always precede uses, see
MachineRegisterInfo::addRegOperandToUseList().

We can implement hasOneUse() by checking only the Tail and its Prev,
instead of
walking past all defs from the head via use_iterator.

Added: 
    

Modified: 
    llvm/include/llvm/CodeGen/MachineRegisterInfo.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/CodeGen/MachineRegisterInfo.h b/llvm/include/llvm/CodeGen/MachineRegisterInfo.h
index 737b74ef3f761..2c29c8bbd0b31 100644
--- a/llvm/include/llvm/CodeGen/MachineRegisterInfo.h
+++ b/llvm/include/llvm/CodeGen/MachineRegisterInfo.h
@@ -511,7 +511,16 @@ class MachineRegisterInfo {
   /// hasOneUse - Return true if there is exactly one instruction using the
   /// specified register.
   bool hasOneUse(Register RegNo) const {
-    return hasSingleElement(use_operands(RegNo));
+    MachineOperand *Head = getRegUseDefListHead(RegNo);
+    if (!Head)
+      return false;
+    // Prev links are circular, and defs always precede uses.
+    MachineOperand *Tail = Head->Contents.Reg.Prev;
+    if (!Tail->isUse())
+      return false;
+    if (Tail == Head)
+      return true;
+    return Tail->Contents.Reg.Prev->isDef();
   }
 
   /// use_nodbg_iterator/use_nodbg_begin/use_nodbg_end - Walk all uses of the