Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -743,8 +743,17 @@
   case TargetOpcode::G_CONSTANT: {
     MachineOperand &SrcMO = MI.getOperand(1);
     LLVMContext &Ctx = MIRBuilder.getMF().getFunction().getContext();
-    const APInt &Val = SrcMO.getCImm()->getValue().sext(WideTy.getSizeInBits());
-    SrcMO.setCImm(ConstantInt::get(Ctx, Val));
+    APInt Val = SrcMO.getCImm()->getValue();
+    APInt WideVal;
+    const auto &TLI = *MIRBuilder.getMF().getSubtarget().getTargetLowering();
+    if (Val.getBitWidth() == 1 &&
+        (TLI.getBooleanContents(false, false) ==
+         TargetLoweringBase::ZeroOrOneBooleanContent)) {
+      WideVal = Val.zext(WideTy.getSizeInBits());
+    } else {
+      WideVal = Val.sext(WideTy.getSizeInBits());
+    }
+    SrcMO.setCImm(ConstantInt::get(Ctx, WideVal));
 
     widenScalarDst(MI, WideTy);
     MIRBuilder.recordInsertion(&MI);
Index: test/CodeGen/AArch64/GlobalISel/legalize-bool.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/GlobalISel/legalize-bool.mir
@@ -0,0 +1,33 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -O0 -mtriple=aarch64-linux-gnu -run-pass=legalizer %s -o - | FileCheck %s
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64-linux-gnu"
+
+  define i1 @test() {
+  entry:
+    ret i1 true
+  }
+
+
+...
+---
+name:            test
+alignment:       2
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+body:             |
+  bb.1.entry:
+    ; CHECK-LABEL: name: test
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY [[C]](s32)
+    ; CHECK: $w0 = COPY [[COPY]](s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:_(s1) = G_CONSTANT i1 true
+    %1:_(s32) = G_ANYEXT %0(s1)
+    $w0 = COPY %1(s32)
+    RET_ReallyLR implicit $w0
+
+...