[PATCH] Thumb2SizeReduction: Check the correct set of registers for LDMIA.

[Peter Collingbourne]

Hi rengolin,

The register set for LDMIA begins at offset 3, not 4. We were previously
missing the short encoding of this instruction in the case where the base
register was the first register in the register set.

Also clean up some dead code:

- The isARMLowRegister check is redundant with what VerifyLowRegs does.
- Remove handling of LDMDB instruction, which has no short encoding (and
  does not appear in ReduceTable).

http://reviews.llvm.org/D9485

Files:
  lib/Target/ARM/Thumb2SizeReduction.cpp
  test/CodeGen/Thumb2/float-ops.ll

Index: lib/Target/ARM/Thumb2SizeReduction.cpp
===================================================================
--- lib/Target/ARM/Thumb2SizeReduction.cpp
+++ lib/Target/ARM/Thumb2SizeReduction.cpp
@@ -334,8 +334,7 @@
 static bool VerifyLowRegs(MachineInstr *MI) {
   unsigned Opc = MI->getOpcode();
   bool isPCOk = (Opc == ARM::t2LDMIA_RET || Opc == ARM::t2LDMIA     ||
-                 Opc == ARM::t2LDMDB     || Opc == ARM::t2LDMIA_UPD ||
-                 Opc == ARM::t2LDMDB_UPD);
+                 Opc == ARM::t2LDMIA_UPD);
   bool isLROk = (Opc == ARM::t2STMDB_UPD);
   bool isSPOk = isPCOk || isLROk;
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
@@ -413,16 +412,13 @@
     HasShift = true;
     OpNum = 4;
     break;
-  case ARM::t2LDMIA:
-  case ARM::t2LDMDB: {
+  case ARM::t2LDMIA: {
     unsigned BaseReg = MI->getOperand(0).getReg();
-    if (!isARMLowRegister(BaseReg) || Entry.WideOpc != ARM::t2LDMIA)
-      return false;
 
     // For the non-writeback version (this one), the base register must be
     // one of the registers being loaded.
     bool isOK = false;
-    for (unsigned i = 4; i < MI->getNumOperands(); ++i) {
+    for (unsigned i = 3; i < MI->getNumOperands(); ++i) {
       if (MI->getOperand(i).getReg() == BaseReg) {
         isOK = true;
         break;
@@ -446,7 +442,6 @@
     break;
   }
   case ARM::t2LDMIA_UPD:
-  case ARM::t2LDMDB_UPD:
   case ARM::t2STMIA_UPD:
   case ARM::t2STMDB_UPD: {
     OpNum = 0;
Index: test/CodeGen/Thumb2/float-ops.ll
===================================================================
--- test/CodeGen/Thumb2/float-ops.ll
+++ test/CodeGen/Thumb2/float-ops.ll
@@ -109,7 +109,7 @@
 define double @load_d(double* %a) {
 entry:
 ; CHECK-LABEL: load_d:
-; NONE: ldm.w r0, {r0, r1}
+; NONE: ldm r0, {r0, r1}
 ; HARD: vldr d0, [r0]
   %0 = load double, double* %a, align 8
   ret double %0

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