Index: llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
+++ llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
@@ -437,6 +437,13 @@
 
   def UseBTI : Predicate<[{ MF->getFunction().hasFnAttribute("branch-target-enforcement") }]>;
   def NotUseBTI : Predicate<[{ !MF->getFunction().hasFnAttribute("branch-target-enforcement") }]>;
+
+  // Toggles patterns which aren't beneficial in GlobalISel when we aren't
+  // optimizing. This allows us to selectively use patterns without impacting
+  // SelectionDAG's behaviour.
+  // FIXME: One day there will probably be a nicer way to check for this, but
+  // today is not that day.
+  def OptimizedGISelOrOtherSelector : Predicate<"!MF->getFunction().hasOptNone() || MF->getProperties().hasProperty(MachineFunctionProperties::Property::FailedISel) || !MF->getProperties().hasProperty(MachineFunctionProperties::Property::Legalized)">;
 }
 
 include "AArch64InstrFormats.td"
@@ -940,8 +947,12 @@
 def gi_trunc_imm : GICustomOperandRenderer<"renderTruncImm">,
   GISDNodeXFormEquiv<trunc_imm>;
 
+let Predicates = [OptimizedGISelOrOtherSelector] in {
+// The SUBREG_TO_REG isn't eliminated at -O0, which can result in pointless
+// copies.
 def : Pat<(i64 i64imm_32bit:$src),
           (SUBREG_TO_REG (i64 0), (MOVi32imm (trunc_imm imm:$src)), sub_32)>;
+}
 
 // Materialize FP constants via MOVi32imm/MOVi64imm (MachO large code model).
 def bitcast_fpimm_to_i32 : SDNodeXForm<fpimm, [{
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-constant.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-constant.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-constant.mir
@@ -13,6 +13,9 @@
   define i64 @fconst_s64() { ret i64 1234567890123 }
   define float @fconst_s32_0() { ret float 0.0 }
   define double @fconst_s64_0() { ret double 0.0 }
+
+  define void @optnone_i64() optnone noinline { ret void }
+  define void @opt_i64() { ret void }
 ...
 
 ---
@@ -140,3 +143,32 @@
     %0(s64) = G_FCONSTANT double 0.0
     $x0 = COPY %0(s64)
 ...
+---
+name:            optnone_i64
+legalized:       true
+regBankSelected: true
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: optnone_i64
+    ; CHECK: [[MOVi64imm:%[0-9]+]]:gpr64 = MOVi64imm 42
+    ; CHECK: $x0 = COPY [[MOVi64imm]]
+    ; CHECK: RET_ReallyLR implicit $x0
+    %0:gpr(s64) = G_CONSTANT i64 42
+    $x0 = COPY %0(s64)
+    RET_ReallyLR implicit $x0
+...
+---
+name:            opt_i64
+legalized:       true
+regBankSelected: true
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: opt_i64
+    ; CHECK: [[MOVi32imm:%[0-9]+]]:gpr32 = MOVi32imm 42
+    ; CHECK: [[SUBREG_TO_REG:%[0-9]+]]:gpr64all = SUBREG_TO_REG 0, [[MOVi32imm]], %subreg.sub_32
+    ; CHECK: $x0 = COPY [[SUBREG_TO_REG]]
+    ; CHECK: RET_ReallyLR implicit $x0
+    %0:gpr(s64) = G_CONSTANT i64 42
+    $x0 = COPY %0(s64)
+    RET_ReallyLR implicit $x0
+...