Index: bolt/lib/Passes/RegReAssign.cpp
===================================================================
--- bolt/lib/Passes/RegReAssign.cpp
+++ bolt/lib/Passes/RegReAssign.cpp
@@ -175,9 +175,25 @@
           continue;
 
         // Disallow substituitions involving regs in instrs that cannot use REX
+        // The relationship of X86 registers is shown in the diagram. BL and BH
+        // do not have a direct alias relationship. However, if the BH register
+        // cannot be swapped, then the BX/EBX/RBX registers cannot be swapped as
+        // well, which means that BL register also cannot be swapped. Therefore,
+        // in the presence of BX/EBX/RBX registers, BL and BH have an alias
+        // relationship.
+        // ┌─────────────────┐
+        // │  RBX            │
+        // ├─────┬───────────┤
+        // │     │  EBX      │
+        // ├─────┴──┬────────┤
+        // │        │   BX   │
+        // ├────────┼───┬────┤
+        // │        │BH │BL  │
+        // └────────┴───┴────┘
         if (CannotUseREX) {
           RegScore[RegEC] =
               std::numeric_limits<decltype(RegScore)::value_type>::min();
+          RegScore[BC.MIB->getAliasSized(Reg, 1)] = RegScore[RegEC];
           continue;
         }
 
@@ -185,6 +201,7 @@
         if (BC.MIB->isUpper8BitReg(Reg) && ClassicCSR.test(Reg)) {
           RegScore[RegEC] =
               std::numeric_limits<decltype(RegScore)::value_type>::min();
+          RegScore[BC.MIB->getAliasSized(Reg, 1)] = RegScore[RegEC];
           continue;
         }
 
@@ -363,6 +380,9 @@
     if (ScoreRBX <= 0)
       continue;
 
+    // The high 8 bits of the register will never be swapped.
+    if(BC.MIB->isUpper8BitReg(I))
+      continue;
     if (RegScore[Candidate] > (ScoreRBX + 10))
       RBX = I;
   }
Index: bolt/test/runtime/X86/reg-reassign-no-swap-bl.s
===================================================================
--- /dev/null
+++ bolt/test/runtime/X86/reg-reassign-no-swap-bl.s
@@ -0,0 +1,59 @@
+# This test case is used to reproduce an issue found in the mongod database.
+# In function rankRegisters, if there is a BH Reg in the basic block, then the BL Reg
+# also cannot be swap.
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
+# RUN: link_fdata %s %t.o %t.fdata
+# RUN: llvm-strip --strip-unneeded %t.o
+# RUN: %clang -no-pie %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.out -data=%t.fdata --reg-reassign | FileCheck %s
+# RUN: %t.out
+
+# CHECK: Reg Reassignment Pass: no changes were made
+  .text
+  .globl  main
+  .globl  main.cold
+  .p2align  4, 0x90
+  .type  main,@function
+  .type  main.cold,@function
+main.cold:
+bb1:
+  mov    $0x2,  %bh
+bb2:
+  jmp    bb5
+main:                                   # @main
+  .cfi_startproc
+# %bb.0:                                # %entry
+  pushq  %rax
+  pushq   %r12
+  pushq   %rbx
+  .cfi_def_cfa_offset 16
+  mov     $0x1,   %r12
+  shr     $0x14,  %r12
+  add     $0x14,  %r12
+  mov     $0x11,  %rbx
+  mov    $0x1,  %bh
+  mov    $0x1,  %bl
+bb3:
+  add    $0x1,   %r12
+bb4:
+  jmp    bb1
+bb5:
+  cmp     $0x201,  %rbx
+  jne     0x0
+bb6:
+  xorl    %eax, %eax
+  popq    %rcx
+  popq    %rbx
+  popq    %r12
+  .cfi_def_cfa_offset 8
+  retq
+# FDATA: 1 main.cold #bb2# 1 main 0 0 100
+# FDATA: 1 main #bb3# 1 main #bb4# 0 100
+# FDATA: 1 main #bb4# 1 main.cold 0 0 100
+
+.Lfunc_end0:
+  .size  main, .Lfunc_end0-main
+  .cfi_endproc