Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -1910,6 +1910,13 @@
     }
     LI.removeEmptySubRanges();
   }
+
+  // CP.getSrcReg()'s live interval has been merged into CP.getDstReg's live
+  // interval. Since CP.getSrcReg() is in ToBeUpdated set and its live interval
+  // is not up-to-date, need to update the merged live interval here.
+  if (ToBeUpdated.count(CP.getSrcReg()))
+    ShrinkMainRange = true;
+
   if (ShrinkMainRange) {
     LiveInterval &LI = LIS->getInterval(CP.getDstReg());
     shrinkToUses(&LI);
Index: test/CodeGen/X86/late-remat-update-2.mir
===================================================================
--- test/CodeGen/X86/late-remat-update-2.mir
+++ test/CodeGen/X86/late-remat-update-2.mir
@@ -0,0 +1,61 @@
+# REQUIRES: asserts
+# RUN: llc -mtriple=x86_64-- -run-pass=simple-register-coalescing -late-remat-update-threshold=0 -debug-only=regalloc %s -o /dev/null 2>&1 | FileCheck %s
+#
+# PR40061: %t2 = %t1 is rematerialized and %t1 is added into toBeUpdated set
+# to postpone its live interval update. After the rematerialization, the live
+# interval of %t1 is larger than necessary. Then %t1 is merged into %t3 and %t1
+# gets removed. After the merge, %t3 contains live interval larger than
+# necessary. Because %t3 is not in toBeUpdated set so its live interval is not
+# updated after register coalescing, and it will break some assumption in
+# regalloc. This test checks the live interval is up-to-date after register
+# coalescing.
+#
+# Check the interval of %t3 after %t1 is merged into %t3 in register coalescing.
+# Make sure the interval of %t3 doesn't cover the entry of bb.1 because %t1 is
+# rematerialized in bb.1.
+# CHECK: INTERVALS
+# CHECK: %2 [16r,80B:1)[240B,272r:1)[272r,304r:0)  0@272r 1@16r
+#
+# CHECK: MACHINEINSTRS
+# Check %t1 has been merged with %t3
+# CHECK: 0B	bb.0.entry:
+# CHECK: 16B	  %t3:gr64 = MOV64ri32 -11
+# CHECK: 32B	  CMP64ri8 %t3:gr64, 1
+# Check %t1 here has been rematerialized.
+# CHECK: 80B	bb.1:
+# CHECK: 96B	  %t2:gr64 = MOV64ri32 -11
+
+---
+name:            _Z3fooi
+body:             |
+  bb.0.entry:
+    successors: %bb.1(0x15555555), %bb.2(0x6aaaaaab)
+
+    %t1:gr64 = MOV64ri32 -11
+    CMP64ri8 %t1, 1, implicit-def $eflags
+    JE_1 %bb.2, implicit killed $eflags
+    JMP_1 %bb.1
+
+  bb.1:
+    successors: %bb.1(0x80000000)
+
+    %t2:gr64 = COPY %t1
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %t2:gr64 = ADD64ri8 %t2, 5, implicit-def $eflags
+    $rdi = COPY %t2
+    CALL64pcrel32 @_Z3fooi, csr_64, implicit $rsp, implicit $ssp, implicit killed $rdi, implicit-def $rsp, implicit-def $ssp
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CMP64ri8 %t2, 1, implicit-def $eflags
+    JE_1 %bb.1, implicit killed $eflags
+    RET -1
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+    %t3:gr64 = COPY %t1
+    %t3:gr64 = ADD64ri8 %t3, 10, implicit-def $eflags
+
+  bb.3:
+    $rax = COPY %t3
+    RET 0, $rax
+
+...