Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -1610,6 +1610,10 @@
         Known.One |= Known2.One.reverseBits();
         break;
       case Intrinsic::bswap:
+        // APInt::byteSwap doesn't handle these cases
+        if (BitWidth < 16 || BitWidth % 16 != 0)
+          break;
+
         computeKnownBits(I->getOperand(0), Known2, Depth + 1, Q);
         Known.Zero |= Known2.Zero.byteSwap();
         Known.One |= Known2.One.byteSwap();
Index: llvm/test/Analysis/ValueTracking/bswap.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/ValueTracking/bswap.ll
@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instsimplify -S < %s | FileCheck %s
+
+; Make sure there's no assert on weird sized bswaps
+
+declare i8 @llvm.bswap.i8(i8) #1
+declare i24 @llvm.bswap.i24(i24) #1
+
+define i8 @bswap_i8(i8 %src) {
+; CHECK-LABEL: @bswap_i8(
+; CHECK-NEXT:    [[BSWAP:%.*]] = call i8 @llvm.bswap.i8(i8 [[SRC:%.*]])
+; CHECK-NEXT:    ret i8 [[BSWAP]]
+;
+  %bswap = call i8 @llvm.bswap.i8(i8 %src)
+  ret i8 %bswap
+}
+
+define i24 @bswap_i24(i24 %src) {
+; CHECK-LABEL: @bswap_i24(
+; CHECK-NEXT:    [[BSWAP:%.*]] = call i24 @llvm.bswap.i24(i24 [[SRC:%.*]])
+; CHECK-NEXT:    ret i24 [[BSWAP]]
+;
+  %bswap = call i24 @llvm.bswap.i24(i24 %src)
+  ret i24 %bswap
+}
+
+attributes #1 = { nounwind readnone speculatable willreturn }