diff --git a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
index e2ed488e3988f..5bfece010bec2 100644
--- a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
@@ -501,9 +501,13 @@ void ConstantHoistingPass::collectConstantCandidates(
 /// into an instruction itself.
 void ConstantHoistingPass::collectConstantCandidates(Function &Fn) {
   ConstCandMapType ConstCandMap;
-  for (BasicBlock &BB : Fn)
+  for (BasicBlock &BB : Fn) {
+    // Ignore unreachable basic blocks.
+    if (!DT->isReachableFromEntry(&BB))
+      continue;
     for (Instruction &Inst : BB)
       collectConstantCandidates(ConstCandMap, &Inst);
+  }
 }
 
 // This helper function is necessary to deal with values that have different
diff --git a/llvm/test/Transforms/ConstantHoisting/AArch64/consthoist-unreachable.ll b/llvm/test/Transforms/ConstantHoisting/AArch64/consthoist-unreachable.ll
new file mode 100755
index 0000000000000..6f8d00f664e85
--- /dev/null
+++ b/llvm/test/Transforms/ConstantHoisting/AArch64/consthoist-unreachable.ll
@@ -0,0 +1,64 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mtriple aarch64-- -consthoist -S | FileCheck %s
+
+; This used to trigger an assertion failure:
+;
+;    ../lib/Transforms/Scalar/ConstantHoisting.cpp:779: void llvm::ConstantHoistingPass::emitBaseConstants(llvm::Instruction *, llvm::Constant *, llvm::Type *, const llvm::consthoist::ConstantUser &): Assertion `CastInst->isCast() && "Expected an cast instruction!"' failed.
+
+@c.a = external global i32, align 1
+
+define void @c() {
+; CHECK-LABEL: @c(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i16 0, 0
+; CHECK-NEXT:    br i1 undef, label [[LBL1_US:%.*]], label [[ENTRY_ENTRY_SPLIT_CRIT_EDGE:%.*]]
+; CHECK:       entry.entry.split_crit_edge:
+; CHECK-NEXT:    [[CONST:%.*]] = bitcast i32 1232131 to i32
+; CHECK-NEXT:    br label [[LBL1:%.*]]
+; CHECK:       lbl1.us:
+; CHECK-NEXT:    [[CONST1:%.*]] = bitcast i32 1232131 to i32
+; CHECK-NEXT:    store i32 [[CONST1]], i32* @c.a, align 1
+; CHECK-NEXT:    br label [[FOR_COND4:%.*]]
+; CHECK:       lbl1:
+; CHECK-NEXT:    store i32 [[CONST]], i32* @c.a, align 1
+; CHECK-NEXT:    br i1 undef, label [[IF_THEN:%.*]], label [[FOR_END12:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br i1 undef, label [[LBL1]], label [[FOR_COND4]]
+; CHECK:       for.cond4:
+; CHECK-NEXT:    br label [[FOR_COND4]]
+; CHECK:       for.body9:
+; CHECK-NEXT:    store i32 1232131, i32* undef, align 1
+; CHECK-NEXT:    store i32 1232132, i32* undef, align 1
+; CHECK-NEXT:    br label [[FOR_BODY9:%.*]]
+; CHECK:       for.end12:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tobool = icmp ne i16 0, 0
+  br i1 undef, label %lbl1.us, label %entry.entry.split_crit_edge
+
+entry.entry.split_crit_edge:                      ; preds = %entry
+  br label %lbl1
+
+lbl1.us:                                          ; preds = %entry
+  store i32 1232131, i32* @c.a, align 1
+  br label %for.cond4
+
+lbl1:                                             ; preds = %if.then, %entry.entry.split_crit_edge
+  store i32 1232131, i32* @c.a, align 1
+  br i1 undef, label %if.then, label %for.end12
+
+if.then:                                          ; preds = %lbl1
+  br i1 undef, label %lbl1, label %for.cond4
+
+for.cond4:                                        ; preds = %for.cond4, %if.then, %lbl1.us
+  br label %for.cond4
+
+for.body9:                                        ; preds = %for.body9
+  store i32 1232131, i32* undef, align 1
+  store i32 1232132, i32* undef, align 1
+  br label %for.body9
+
+for.end12:                                        ; preds = %lbl1
+  ret void
+}
diff --git a/llvm/test/Transforms/ConstantHoisting/X86/pr43903-not-all-uses-rebased.ll b/llvm/test/Transforms/ConstantHoisting/X86/pr43903-not-all-uses-rebased.ll
new file mode 100644
index 0000000000000..a5ca9b4b859ee
--- /dev/null
+++ b/llvm/test/Transforms/ConstantHoisting/X86/pr43903-not-all-uses-rebased.ll
@@ -0,0 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mtriple=x86_64-unknown-linux-gnu -consthoist -consthoist-gep=1 -S | FileCheck %s
+
+; This is a reproducer for PR43903 where we hit an assertion:
+; opt: ../lib/Transforms/Scalar/ConstantHoisting.cpp:903: bool llvm::ConstantHoistingPass::emitBaseConstants(llvm::GlobalVariable *): Assertion `UsesNum == (ReBasesNum + NotRebasedNum) && "Not all uses are rebased"' failed.
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = external global [2 x i16], align 1
+
+define void @c() {
+; CHECK-LABEL: @c(
+; CHECK-NEXT:  for.cond:
+; CHECK-NEXT:    br i1 undef, label [[FOR_BODY2:%.*]], label [[FOR_END4:%.*]]
+; CHECK:       for.body2:
+; CHECK-NEXT:    br i1 undef, label [[LAND_RHS:%.*]], label [[LAND_END:%.*]]
+; CHECK:       land.rhs:
+; CHECK-NEXT:    unreachable
+; CHECK:       land.end:
+; CHECK-NEXT:    [[CONST1:%.*]] = bitcast i16* getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 0) to i16*
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ule i16* undef, [[CONST1]]
+; CHECK-NEXT:    unreachable
+; CHECK:       for.cond3:
+; CHECK-NEXT:    [[TMP0:%.*]] = load i16, i16* getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 1), align 1
+; CHECK-NEXT:    br label [[FOR_COND3:%.*]]
+; CHECK:       for.end4:
+; CHECK-NEXT:    [[CONST:%.*]] = bitcast i16* getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 0) to i16*
+; CHECK-NEXT:    [[TMP1:%.*]] = load i16, i16* [[CONST]], align 1
+; CHECK-NEXT:    ret void
+;
+for.cond:
+  br i1 undef, label %for.body2, label %for.end4
+
+for.body2:                                        ; preds = %for.cond
+  br i1 undef, label %land.rhs, label %land.end
+
+land.rhs:                                         ; preds = %for.body2
+  unreachable
+
+land.end:                                         ; preds = %for.body2
+  %cmp = icmp ule i16* undef, getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 0)
+  unreachable
+
+for.cond3:                                        ; preds = %for.cond3
+  %tmp0 = load i16, i16* getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 1), align 1
+  br label %for.cond3
+
+for.end4:                                         ; preds = %for.cond
+  %tmp1 = load i16, i16* getelementptr inbounds ([2 x i16], [2 x i16]* @a, i32 0, i32 0), align 1
+  ret void
+}