[GlobalISel] Add lowering support for G_ABS and use for AArch64

[paquette]

LGTM

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

case G_SSUBSAT: {
     // preferred boolean type for the carry out for the query.
     if (LI.isLegalOrCustom({G_SMIN, Ty}) && LI.isLegalOrCustom({G_SMAX, Ty}))
       return lowerAddSubSatToMinMax(MI);
     return lowerAddSubSatToAddoSubo(MI);
   }
   case G_SSHLSAT:
   case G_USHLSAT:
     return lowerShlSat(MI);
+  case G_ABS: {
+    // Expand %res = G_ABS %a into:
+    // %v1 = G_ASHR %a, scalar_size-1
+    // %v2 = G_ADD %a, %v1
+    // %res = G_XOR %v2, %v1
+    LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
+    Register OpReg = MI.getOperand(1).getReg();
+    auto ShiftAmt =
+        MIRBuilder.buildConstant(DstTy, DstTy.getScalarSizeInBits() - 1);
+    auto Shift =
+        MIRBuilder.buildAShr(DstTy, OpReg, ShiftAmt);
+    auto Add = MIRBuilder.buildAdd(DstTy, OpReg, Shift);
+    MIRBuilder.buildXor(MI.getOperand(0).getReg(), Add, Shift);
+    MI.eraseFromParent();
+    return Legalized;
+  }
   }
 }
 
 Align LegalizerHelper::getStackTemporaryAlignment(LLT Ty,
                                                   Align MinAlign) const {
   // FIXME: We're missing a way to go back from LLT to llvm::Type to query the
   // datalayout for the preferred alignment. Also there should be a target hook
   // for this to allow targets to reduce the alignment and ignore the

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
   getActionDefinitionsBuilder(G_BRJT).legalIf([=](const LegalityQuery &Query) {
     return Query.Types[0] == p0 && Query.Types[1] == s64;
   });
 
   getActionDefinitionsBuilder(G_DYN_STACKALLOC).lower();
 
   getActionDefinitionsBuilder({G_MEMCPY, G_MEMMOVE, G_MEMSET}).libcall();
 
+  getActionDefinitionsBuilder(G_ABS).lowerIf(
+      [=](const LegalityQuery &Query) { return Query.Types[0].isScalar(); });
+
   computeTables();
   verify(*ST.getInstrInfo());
 }
 
 bool AArch64LegalizerInfo::legalizeCustom(LegalizerHelper &Helper,
                                           MachineInstr &MI) const {
   MachineIRBuilder &MIRBuilder = Helper.MIRBuilder;
   MachineRegisterInfo &MRI = *MIRBuilder.getMRI();

llvm/test/CodeGen/AArch64/GlobalISel/legalize-abs.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=aarch64 -run-pass=legalizer -global-isel-abort=1 %s -o - | FileCheck %s
+---
+name:            abs_s32
+liveins:
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: abs_s32
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 31
+    ; CHECK: [[ASHR:%[0-9]+]]:_(s32) = G_ASHR [[COPY]], [[C]](s64)
+    ; CHECK: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[ASHR]]
+    ; CHECK: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[ADD]], [[ASHR]]
+    ; CHECK: $w0 = COPY [[XOR]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = G_ABS %0(s32)
+    $w0 = COPY %1(s32)
+...
+---
+name:            abs_s64
+liveins:
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: abs_s64
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY $x0
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 63
+    ; CHECK: [[ASHR:%[0-9]+]]:_(s64) = G_ASHR [[COPY]], [[C]](s64)
+    ; CHECK: [[ADD:%[0-9]+]]:_(s64) = G_ADD [[COPY]], [[ASHR]]
+    ; CHECK: [[XOR:%[0-9]+]]:_(s64) = G_XOR [[ADD]], [[ASHR]]
+    ; CHECK: $x0 = COPY [[XOR]](s64)
+    %0:_(s64) = COPY $x0
+    %1:_(s64) = G_ABS %0(s64)
+    $x0 = COPY %1(s64)
+...