[ARM] Resolve FIXME: Swap adds <-> subs offset is 0

[AtariDreams]

Also erase instruction if offset is 0.

[llvmbot]

@llvm/pr-subscribers-backend-arm

Author: AtariDreams (AtariDreams)

Changes

Also erase instruction if offset is 0

---

Full diff: https://github.com/llvm/llvm-project/pull/78870.diff

1 Files Affected:

• (modified) llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp (+13-7)

diff --git a/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp b/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
index ed9d30c3c3ab90..10478611497a08 100644
--- a/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
@@ -525,13 +525,19 @@ void ARMLoadStoreOpt::UpdateBaseRegUses(MachineBasicBlock &MBB,
         // Merge it with the update; if the merged offset is too large,
         // insert a new sub instead.
         MachineOperand &MO =
-          MBBI->getOperand(MBBI->getDesc().getNumOperands() - 3);
-        Offset = (Opc == ARM::tSUBi8) ?
-          MO.getImm() + WordOffset * 4 :
-          MO.getImm() - WordOffset * 4 ;
-        if (Offset >= 0 && TL->isLegalAddImmediate(Offset)) {
-          // FIXME: Swap ADDS<->SUBS if Offset < 0, erase instruction if
-          // Offset == 0.
+            MBBI->getOperand(MBBI->getDesc().getNumOperands() - 3);
+        Offset = (Opc == ARM::tSUBi8) ? MO.getImm() + WordOffset * 4
+                                      : MO.getImm() - WordOffset * 4;
+        if (TL->isLegalAddImmediate(Offset)) {
+          if (Offset < 0) {
+            // Swap ADDS<->SUBS if Offset < 0
+            Opc = (Opc == ARM::tSUBi8) ? ARM::tADDi8 : ARM::tSUBi8;
+            Offset = -Offset;
+          } else if (Offset == 0) {
+            // Erase instruction if Offset == 0
+            MBBI = MBB.erase(MBBI);
+            return;
+          }
           MO.setImm(Offset);
           // The base register has now been reset, so exit early.
           return;

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[davemgreen]

Do you have tests cases for where this can come up? I'm not sure we should be generating instructions with negative offsets.
Could the add/sub do setting nzcv flags that are needed?