[ARM] Transform LDMs into writeback form to save code size

[jmolloy]

Summary

If we have an LDM that uses only low registers and doesn't write to its base register:

ldm.w r0, {r1, r2, r3}

And that base register is dead after the LDM, then we can convert it to writeback form and use a narrow encoding:

ldm.n r0!, {r1, r2, r3}

Obviously, this introduces a new register write and so can cause WAW hazards, so I've enabled it only in minsize mode. This is a code size trick that ARM Compiler 5 ("armcc") does that we don't.

[mcrosier]

lib/Target/ARM/ARMLoadStoreOptimizer.cpp

1233

Could we just predicate all/most of this code with optForMinSize()?

// We couldn't find an inc/dec to merge. But if the base is dead, we can still
// change to a writeback form as that will save us 2 bytes of code size. It can
// create WAW hazards though, so only do it if we're minimizing code size.
if (MBB.getParent()->getFunction()->optForMinSize()) {
  bool HighRegsUsed = false;
  for (unsigned i = 2, e = MI->getNumOperands(); i != e; ++i)
    if (MI->getOperand(i).getReg() >= ARM::R8) {
      HighRegsUsed = true;
      break;
    }

  if (BaseKill && MBB.getParent()->getFunction()->optForMinSize() &&
      !HighRegsUsed)
    MergeInstr = MBB.end();
} else
  return false;

[jmolloy]

Hi Chad,

That makes sense to me. I'll make that change.

James

[mcrosier]

LGTM with nit addressed.

[mcrosier]

lib/Target/ARM/ARMLoadStoreOptimizer.cpp

1245

'BaseKill' looks invariant in this code as well, so you could put that in the outer check as well, if I'm not mistaken..

if (BaseKill && MBB.getParent()->getFunction()->optForMinSize()) {

} else
  return false;

[jmolloy]

Hi,

Thanks! committed with those changes in r272000.

Cheers,

James

lib/Target/ARM/ARMLoadStoreOptimizer.cpp

bool ARMLoadStoreOpt::MergeBaseUpdateLSMultiple(MachineInstr *MI) {
   ARM_AM::AMSubMode Mode = getLoadStoreMultipleSubMode(Opcode);
   if (Mode == ARM_AM::ia && Offset == -Bytes) {
     Mode = ARM_AM::db;
   } else if (Mode == ARM_AM::ib && Offset == -Bytes) {
     Mode = ARM_AM::da;
   } else {
     MergeInstr = findIncDecAfter(MBBI, Base, Pred, PredReg, Offset);
     if (((Mode != ARM_AM::ia && Mode != ARM_AM::ib) || Offset != Bytes) &&
-        ((Mode != ARM_AM::da && Mode != ARM_AM::db) || Offset != -Bytes))
+        ((Mode != ARM_AM::da && Mode != ARM_AM::db) || Offset != -Bytes)) {
+
+      // We couldn't find an inc/dec to merge. But if the base is dead, we
+      // can still change to a writeback form as that will save us 2 bytes
+      // of code size. It can create WAW hazards though, so only do it if
+      // we're minimizing code size.
+      bool HighRegsUsed = false;
+      for (unsigned i = 2, e = MI->getNumOperands(); i != e; ++i)
+        if (MI->getOperand(i).getReg() >= ARM::R8) {
+          HighRegsUsed = true;
+          break;
+        }
+
+      if (BaseKill && MBB.getParent()->getFunction()->optForMinSize() &&
+          !HighRegsUsed)
+        MergeInstr = MBB.end();
+      else
         return false;
     }
+  }
+  if (MergeInstr != MBB.end())
     MBB.erase(MergeInstr);
 
   unsigned NewOpc = getUpdatingLSMultipleOpcode(Opcode, Mode);
   MachineInstrBuilder MIB = BuildMI(MBB, MBBI, DL, TII->get(NewOpc))
     .addReg(Base, getDefRegState(true)) // WB base register
     .addReg(Base, getKillRegState(BaseKill))
     .addImm(Pred).addReg(PredReg);
 
   // Transfer the rest of operands.

test/CodeGen/ARM/ldm-base-writeback.ll

+; RUN: llc -O3 < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
+target triple = "armv7--linux-gnu"
+
+@a = global i32 0, align 4
+@b = global i32 0, align 4
+@c = global i32 0, align 4
+
+; CHECK-LABEL: bar:
+; CHECK: ldm r{{[0-9]}}!, {r0, r{{[0-9]}}, r{{[0-9]}}}
+define void @bar(i32 %a1, i32 %b1, i32 %c1) minsize optsize {
+  %1 = load i32, i32* @a, align 4
+  %2 = load i32, i32* @b, align 4
+  %3 = load i32, i32* @c, align 4
+  %4 = tail call i32 @baz(i32 %1, i32 %3) minsize optsize
+  %5 = tail call i32 @baz(i32 %2, i32 %3) minsize optsize
+  ret void
+}
+
+declare i32 @baz(i32,i32) minsize optsize