Index: llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
+++ llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
@@ -470,6 +470,13 @@
     unsigned Tmp = DstTy.getScalarSizeInBits() - SrcTy.getScalarSizeInBits();
     return computeNumSignBits(Src, DemandedElts, Depth + 1) + Tmp;
   }
+  case TargetOpcode::G_SEXT_INREG: {
+    // Max of the input and what this extends.
+    Register Src = MI.getOperand(1).getReg();
+    unsigned SrcBits = MI.getOperand(2).getImm();
+    unsigned InRegBits = TyBits - SrcBits + 1;
+    return std::max(computeNumSignBits(Src, DemandedElts, Depth + 1), InRegBits);
+  }
   case TargetOpcode::G_TRUNC: {
     Register Src = MI.getOperand(1).getReg();
     LLT SrcTy = MRI.getType(Src);
Index: llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
===================================================================
--- llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
+++ llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
@@ -373,6 +373,70 @@
   EXPECT_EQ(32u, Info.computeNumSignBits(CopySextNeg1));
 }
 
+TEST_F(AArch64GISelMITest, TestNumSignBitsSextInReg) {
+  StringRef MIRString = R"(
+   %ptr:_(p0) = G_IMPLICIT_DEF
+   %load4:_(s32) = G_LOAD %ptr :: (load 4)
+
+   %inreg7:_(s32) = G_SEXT_INREG %load4, 7
+   %copy_inreg7:_(s32) = COPY %inreg7
+
+   %inreg8:_(s32) = G_SEXT_INREG %load4, 8
+   %copy_inreg8:_(s32) = COPY %inreg8
+
+   %inreg9:_(s32) = G_SEXT_INREG %load4, 9
+   %copy_inreg9:_(s32) = COPY %inreg9
+
+   %inreg31:_(s32) = G_SEXT_INREG %load4, 31
+   %copy_inreg31:_(s32) = COPY %inreg31
+
+   %load1:_(s8) = G_LOAD %ptr :: (load 1)
+   %sext_load1:_(s32) = G_SEXT %load1
+
+   %inreg6_sext:_(s32) = G_SEXT_INREG %sext_load1, 6
+   %copy_inreg6_sext:_(s32) = COPY %inreg6_sext
+
+   %inreg7_sext:_(s32) = G_SEXT_INREG %sext_load1, 7
+   %copy_inreg7_sext:_(s32) = COPY %inreg7_sext
+
+   %inreg8_sext:_(s32) = G_SEXT_INREG %sext_load1, 8
+   %copy_inreg8_sext:_(s32) = COPY %inreg8_sext
+
+   %inreg9_sext:_(s32) = G_SEXT_INREG %sext_load1, 9
+   %copy_inreg9_sext:_(s32) = COPY %inreg9_sext
+
+   %inreg31_sext:_(s32) = G_SEXT_INREG %sext_load1, 31
+   %copy_inreg31_sext:_(s32) = COPY %inreg31_sext
+)";
+
+  setUp(MIRString);
+  if (!TM)
+    return;
+
+  Register CopyInReg7 = Copies[Copies.size() - 9];
+  Register CopyInReg8 = Copies[Copies.size() - 8];
+  Register CopyInReg9 = Copies[Copies.size() - 7];
+  Register CopyInReg31 = Copies[Copies.size() - 6];
+
+  Register CopyInReg6Sext = Copies[Copies.size() - 5];
+  Register CopyInReg7Sext = Copies[Copies.size() - 4];
+  Register CopyInReg8Sext = Copies[Copies.size() - 3];
+  Register CopyInReg9Sext = Copies[Copies.size() - 2];
+  Register CopyInReg31Sext = Copies[Copies.size() - 1];
+
+  GISelKnownBits Info(*MF);
+  EXPECT_EQ(26u, Info.computeNumSignBits(CopyInReg7));
+  EXPECT_EQ(25u, Info.computeNumSignBits(CopyInReg8));
+  EXPECT_EQ(24u, Info.computeNumSignBits(CopyInReg9));
+  EXPECT_EQ(2u, Info.computeNumSignBits(CopyInReg31));
+
+  EXPECT_EQ(27u, Info.computeNumSignBits(CopyInReg6Sext));
+  EXPECT_EQ(26u, Info.computeNumSignBits(CopyInReg7Sext));
+  EXPECT_EQ(25u, Info.computeNumSignBits(CopyInReg8Sext));
+  EXPECT_EQ(25u, Info.computeNumSignBits(CopyInReg9Sext));
+  EXPECT_EQ(25u, Info.computeNumSignBits(CopyInReg31Sext));
+}
+
 TEST_F(AArch64GISelMITest, TestNumSignBitsTrunc) {
   StringRef MIRString = "  %3:_(p0) = G_IMPLICIT_DEF\n"
                         "  %4:_(s32) = G_LOAD %3 :: (load 4)\n"