[ARM] GlobalISel: Add support for G_TRUNC

Select them as copies. We only select if both the source and the
destination are on the same register bank, so this shouldn't cause any
trouble.

llvm-svn: 300971

llvm/lib/Target/ARM/ARMInstructionSelector.cpp

@@ -47,12 +47,9 @@ static bool selectCopy(MachineInstr &I, const TargetInstrInfo &TII,
   unsigned SrcReg = I.getOperand(1).getReg();
   const unsigned SrcSize = RBI.getSizeInBits(SrcReg, MRI, TRI);
   (void)SrcSize;
-  assert((DstSize == SrcSize ||
-          // Copies are a means to setup initial types, the number of
-          // bits may not exactly match.
-          (TargetRegisterInfo::isPhysicalRegister(SrcReg) &&
-           DstSize <= SrcSize)) &&
-         "Copy with different width?!");
+  // We use copies for trunc, so it's ok for the size of the destination to be
+  // smaller (the higher bits will just be undefined).
+  assert(DstSize <= SrcSize && "Copy with different width?!");
 
   assert((RegBank->getID() == ARM::GPRRegBankID ||
           RegBank->getID() == ARM::FPRRegBankID) &&
@@ -294,6 +291,28 @@ bool ARMInstructionSelector::select(MachineInstr &I) const {
     }
     break;
   }
+  case G_TRUNC: {
+    // The high bits are undefined, so there's nothing special to do, just
+    // treat it as a copy.
+    auto SrcReg = I.getOperand(1).getReg();
+    auto DstReg = I.getOperand(0).getReg();
+
+    const auto &SrcRegBank = *RBI.getRegBank(SrcReg, MRI, TRI);
+    const auto &DstRegBank = *RBI.getRegBank(DstReg, MRI, TRI);
+
+    if (SrcRegBank.getID() != DstRegBank.getID()) {
+      DEBUG(dbgs() << "G_TRUNC operands on different register banks\n");
+      return false;
+    }
+
+    if (SrcRegBank.getID() != ARM::GPRRegBankID) {
+      DEBUG(dbgs() << "G_TRUNC on non-GPR not supported yet\n");
+      return false;
+    }
+
+    I.setDesc(TII.get(COPY));
+    return selectCopy(I, TII, MRI, TRI, RBI);
+  }
   case G_ADD:
   case G_GEP:
     I.setDesc(TII.get(ARM::ADDrr));

llvm/lib/Target/ARM/ARMRegisterBankInfo.cpp

@@ -223,6 +223,7 @@ ARMRegisterBankInfo::getInstrMapping(const MachineInstr &MI) const {
   case G_MUL:
   case G_SEXT:
   case G_ZEXT:
+  case G_TRUNC:
   case G_GEP:
     // FIXME: We're abusing the fact that everything lives in a GPR for now; in
     // the real world we would use different mappings.

llvm/test/CodeGen/ARM/GlobalISel/arm-instruction-select.mir

@@ -5,6 +5,8 @@
   define void @test_sext_s8() { ret void }
   define void @test_zext_s16() { ret void }
 
+  define void @test_trunc_s32_16() { ret void }
+
   define void @test_add_s8() { ret void }
   define void @test_add_s16() { ret void }
   define void @test_add_s32() { ret void }
@@ -142,6 +144,34 @@ body:             |
     ; CHECK: BX_RET 14, _, implicit %r0
 ...
 ---
+name:            test_trunc_s32_16
+# CHECK-LABEL: name: test_trunc_s32_16
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK: selected: true
+registers:
+  - { id: 0, class: gprb }
+  - { id: 1, class: gprb }
+# CHECK-DAG: id: 0, class: gpr
+# CHECK-DAG: id: 1, class: gpr
+body:             |
+  bb.0:
+    liveins: %r0
+
+    %0(s32) = COPY %r0
+    ; CHECK: [[VREGX:%[0-9]+]] = COPY %r0
+
+    %1(s16) = G_TRUNC %0(s32)
+    ; CHECK: [[VREGTRUNC:%[0-9]+]] = COPY [[VREGX]]
+
+    %r0 = COPY %1(s16)
+    ; CHECK: %r0 = COPY [[VREGTRUNC]]
+
+    BX_RET 14, _, implicit %r0
+    ; CHECK: BX_RET 14, _, implicit %r0
+...
+---
 name:            test_add_s8
 # CHECK-LABEL: name: test_add_s8
 legalized:       true

llvm/test/CodeGen/ARM/GlobalISel/arm-isel.ll

@@ -40,6 +40,30 @@ entry:
   ret i16 %x
 }
 
+define void @test_trunc_i32_i16(i32 %v, i16 *%p) {
+; CHECK-LABEL: test_trunc_i32_i16:
+; The trunc doesn't result in any instructions, but we
+; expect the store to be explicitly 16-bit.
+; CHECK: strh r0, [r1]
+; CHECK: bx lr
+entry:
+  %v16 = trunc i32 %v to i16
+  store i16 %v16, i16 *%p
+  ret void
+}
+
+define void @test_trunc_i32_i8(i32 %v, i8 *%p) {
+; CHECK-LABEL: test_trunc_i32_i8:
+; The trunc doesn't result in any instructions, but we
+; expect the store to be explicitly 8-bit.
+; CHECK: strb r0, [r1]
+; CHECK: bx lr
+entry:
+  %v8 = trunc i32 %v to i8
+  store i8 %v8, i8 *%p
+  ret void
+}
+
 define i8 @test_add_i8(i8 %x, i8 %y) {
 ; CHECK-LABEL: test_add_i8:
 ; CHECK: add r0, r0, r1

llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir

@@ -22,6 +22,8 @@
 
   define void @test_constants() { ret void }
 
+  define void @test_trunc_s32_16() { ret void }
+
   define void @test_fadd_s32() #0 { ret void }
   define void @test_fadd_s64() #0 { ret void }
 
@@ -442,6 +444,27 @@ body:             |
     BX_RET 14, _, implicit %r0
 ...
 ---
+name:            test_trunc_s32_16
+# CHECK-LABEL: name: test_trunc_s32_16
+legalized:       true
+regBankSelected: false
+selected:        false
+# CHECK: registers:
+# CHECK: - { id: 0, class: gprb }
+# CHECK: - { id: 1, class: gprb }
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+body:             |
+  bb.0:
+    liveins: %r0
+
+    %0(s32) = COPY %r0
+    %1(s16) = G_TRUNC %0(s32)
+    %r0 = COPY %1(s16)
+    BX_RET 14, _, implicit %r0
+...
+---
 name:            test_fadd_s32
 # CHECK-LABEL: name: test_fadd_s32
 legalized:       true