Lower TAGPstack with negative offset to SUBG.

[eugenis]

Summary

This never really occurs in the current codegen, so only a MIR test is
possible.

[merge_guards_bot]

Unit tests: pass. 61176 tests passed, 0 failed and 729 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[ostannard]

LGTM

llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp

case AArch64::IRGstack: {
      BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(AArch64::IRG))
          .add(MI.getOperand(0))
          .addUse(SrcReg)
          .add(MI.getOperand(2));
      MI.eraseFromParent();
      return true;
    }
    case AArch64::TAGPstack: {
-     BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(AArch64::ADDG))
+     int64_t Offset = MI.getOperand(2).getImm();
+     BuildMI(MBB, MBBI, MI.getDebugLoc(),
+             TII->get(Offset >= 0 ? AArch64::ADDG : AArch64::SUBG))
          .add(MI.getOperand(0))
          .add(MI.getOperand(1))
-         .add(MI.getOperand(2))
+         .addImm(std::abs(Offset))
          .add(MI.getOperand(4));
      MI.eraseFromParent();
      return true;
    }
    case AArch64::STGloop:
    case AArch64::STZGloop:
      return expandSetTagLoop(MBB, MBBI, NextMBBI);
   }

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

bool AArch64InstrInfo::getMemOpInfo(unsigned Opcode, unsigned &Scale,
   case AArch64::LDRSBXui:
   case AArch64::STRBui:
   case AArch64::STRBBui:
     Scale = Width = 1;
     MinOffset = 0;
     MaxOffset = 4095;
     break;
   case AArch64::ADDG:
-  case AArch64::TAGPstack:
     Scale = 16;
     Width = 0;
     MinOffset = 0;
     MaxOffset = 63;
     break;
+  case AArch64::TAGPstack:
+    Scale = 16;
+    Width = 0;
+    // TAGP with a negative offset turns into SUBP, which has a maximum offset
+    // of 63 (not 64!).
+    MinOffset = -63;
+    MaxOffset = 63;
+    break;
   case AArch64::LDG:
   case AArch64::STGOffset:
   case AArch64::STZGOffset:
     Scale = Width = 16;
     MinOffset = -256;
     MaxOffset = 255;
     break;
   case AArch64::LDR_PXI:

llvm/test/CodeGen/AArch64/addg_subg.mir

+# RUN: llc -mtriple=aarch64 -run-pass=prologepilog,aarch64-expand-pseudo %s -o - | FileCheck %s
+
+# CHECK: renamable $x8 = IRG $sp, $xzr
+
+# CHECK: renamable $x0 = ADDG $x8, 0, 0
+# CHECK: renamable $x0 = ADDG $x8, 5, 0
+# CHECK: renamable $x0 = ADDG $x8, 63, 0
+# CHECK: $[[R:x[0-9]+]] = ADDXri $x8, 16, 0
+# CHECK: renamable $x0 = ADDG killed $[[R]], 63, 0
+
+# CHECK: renamable $x0 = SUBG $x8, 5, 0
+# CHECK: renamable $x0 = SUBG $x8, 63, 0
+# CHECK: $[[R:x[0-9]+]] = SUBXri $x8, 16, 0
+# CHECK: renamable $x0 = SUBG killed $[[R]], 63, 0
+
+---
+name: subg
+stack:
+  - { id: 0, type: default, offset: 0, size: 16, alignment: 16,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      local-offset: -16, debug-info-variable: '', debug-info-expression: '',
+      debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    renamable $x8 = IRGstack $sp, $xzr
+
+    renamable $x0 = TAGPstack %stack.0, 0, killed renamable $x8, 0
+    renamable $x0 = TAGPstack %stack.0, 5, killed renamable $x8, 0
+    renamable $x0 = TAGPstack %stack.0, 63, killed renamable $x8, 0
+    renamable $x0 = TAGPstack %stack.0, 64, killed renamable $x8, 0
+
+    renamable $x0 = TAGPstack %stack.0, -5, killed renamable $x8, 0
+    renamable $x0 = TAGPstack %stack.0, -63, killed renamable $x8, 0
+    renamable $x0 = TAGPstack %stack.0, -64, killed renamable $x8, 0
+    RET_ReallyLR
+
+...