Replace 2 ARM ldr instructions with one ldrd in the code generated

for a SubStringStub and StringCompareStub in the ARM backend.  This
is a commit of http://codereview.chromium.org/3341012 for Andreas
Anyuru.
Review URL: http://codereview.chromium.org/3387003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@5457 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

AUTHORS

@@ -9,6 +9,7 @@ ARM Ltd.
 
 Alexander Botero-Lowry <alexbl@FreeBSD.org>
 Alexandre Vassalotti <avassalotti@gmail.com>
+Andreas Anyuru <andreas.anyuru@gmail.com>
 Burcu Dogan <burcujdogan@gmail.com>
 Craig Schlenter <craig.schlenter@gmail.com>
 Daniel Andersson <kodandersson@gmail.com>

src/arm/code-stubs-arm.cc

@@ -4187,17 +4187,21 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
 
   // Check bounds and smi-ness.
-  __ ldr(r7, MemOperand(sp, kToOffset));
-  __ ldr(r6, MemOperand(sp, kFromOffset));
+  Register to = r6;
+  Register from = r7;
+  __ Ldrd(to, from, MemOperand(sp, kToOffset));
+  STATIC_ASSERT(kFromOffset == kToOffset + 4);
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize + kSmiShiftSize == 1);
   // I.e., arithmetic shift right by one un-smi-tags.
-  __ mov(r2, Operand(r7, ASR, 1), SetCC);
-  __ mov(r3, Operand(r6, ASR, 1), SetCC, cc);
-  // If either r2 or r6 had the smi tag bit set, then carry is set now.
+  __ mov(r2, Operand(to, ASR, 1), SetCC);
+  __ mov(r3, Operand(from, ASR, 1), SetCC, cc);
+  // If either to or from had the smi tag bit set, then carry is set now.
   __ b(cs, &runtime);  // Either "from" or "to" is not a smi.
   __ b(mi, &runtime);  // From is negative.
 
+  // Both to and from are smis.
+
   __ sub(r2, r2, Operand(r3), SetCC);
   __ b(mi, &runtime);  // Fail if from > to.
   // Special handling of sub-strings of length 1 and 2. One character strings
@@ -4208,8 +4212,8 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
   // r2: length
   // r3: from index (untaged smi)
-  // r6: from (smi)
-  // r7: to (smi)
+  // r6 (a.k.a. to): to (smi)
+  // r7 (a.k.a. from): from offset (smi)
 
   // Make sure first argument is a sequential (or flat) string.
   __ ldr(r5, MemOperand(sp, kStringOffset));
@@ -4221,10 +4225,10 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
   // r1: instance type
   // r2: length
-  // r3: from index (untaged smi)
+  // r3: from index (untagged smi)
   // r5: string
-  // r6: from (smi)
-  // r7: to (smi)
+  // r6 (a.k.a. to): to (smi)
+  // r7 (a.k.a. from): from offset (smi)
   Label seq_string;
   __ and_(r4, r1, Operand(kStringRepresentationMask));
   STATIC_ASSERT(kSeqStringTag < kConsStringTag);
@@ -4250,17 +4254,18 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // r2: length
   // r3: from index (untaged smi)
   // r5: string
-  // r6: from (smi)
-  // r7: to (smi)
+  // r6 (a.k.a. to): to (smi)
+  // r7 (a.k.a. from): from offset (smi)
   __ ldr(r4, FieldMemOperand(r5, String::kLengthOffset));
-  __ cmp(r4, Operand(r7));
+  __ cmp(r4, Operand(to));
   __ b(lt, &runtime);  // Fail if to > length.
+  to = no_reg;
 
   // r1: instance type.
   // r2: result string length.
   // r3: from index (untaged smi)
   // r5: string.
-  // r6: from offset (smi)
+  // r7 (a.k.a. from): from offset (smi)
   // Check for flat ascii string.
   Label non_ascii_flat;
   __ tst(r1, Operand(kStringEncodingMask));
@@ -4302,12 +4307,12 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // r0: result string.
   // r2: result string length.
   // r5: string.
-  // r6: from offset (smi)
+  // r7 (a.k.a. from): from offset (smi)
   // Locate first character of result.
   __ add(r1, r0, Operand(SeqAsciiString::kHeaderSize - kHeapObjectTag));
   // Locate 'from' character of string.
   __ add(r5, r5, Operand(SeqAsciiString::kHeaderSize - kHeapObjectTag));
-  __ add(r5, r5, Operand(r6, ASR, 1));
+  __ add(r5, r5, Operand(from, ASR, 1));
 
   // r0: result string.
   // r1: first character of result string.
@@ -4323,7 +4328,7 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   __ bind(&non_ascii_flat);
   // r2: result string length.
   // r5: string.
-  // r6: from offset (smi)
+  // r7 (a.k.a. from): from offset (smi)
   // Check for flat two byte string.
 
   // Allocate the result.
@@ -4335,18 +4340,19 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // Locate first character of result.
   __ add(r1, r0, Operand(SeqTwoByteString::kHeaderSize - kHeapObjectTag));
   // Locate 'from' character of string.
-    __ add(r5, r5, Operand(SeqTwoByteString::kHeaderSize - kHeapObjectTag));
+  __ add(r5, r5, Operand(SeqTwoByteString::kHeaderSize - kHeapObjectTag));
   // As "from" is a smi it is 2 times the value which matches the size of a two
   // byte character.
-  __ add(r5, r5, Operand(r6));
+  __ add(r5, r5, Operand(from));
+  from = no_reg;
 
   // r0: result string.
   // r1: first character of result.
   // r2: result length.
   // r5: first character of string to copy.
   STATIC_ASSERT((SeqTwoByteString::kHeaderSize & kObjectAlignmentMask) == 0);
-  StringHelper::GenerateCopyCharactersLong(masm, r1, r5, r2, r3, r4, r6, r7, r9,
-                                           DEST_ALWAYS_ALIGNED);
+  StringHelper::GenerateCopyCharactersLong(
+      masm, r1, r5, r2, r3, r4, r6, r7, r9, DEST_ALWAYS_ALIGNED);
   __ IncrementCounter(&Counters::sub_string_native, 1, r3, r4);
   __ add(sp, sp, Operand(3 * kPointerSize));
   __ Ret();
@@ -4422,8 +4428,7 @@ void StringCompareStub::Generate(MacroAssembler* masm) {
   // Stack frame on entry.
   //  sp[0]: right string
   //  sp[4]: left string
-  __ ldr(r0, MemOperand(sp, 1 * kPointerSize));  // left
-  __ ldr(r1, MemOperand(sp, 0 * kPointerSize));  // right
+  __ Ldrd(r0 , r1, MemOperand(sp));  // Load right in r0, left in r1.
 
   Label not_same;
   __ cmp(r0, r1);
@@ -4438,12 +4443,12 @@ void StringCompareStub::Generate(MacroAssembler* masm) {
   __ bind(&not_same);
 
   // Check that both objects are sequential ascii strings.
-  __ JumpIfNotBothSequentialAsciiStrings(r0, r1, r2, r3, &runtime);
+  __ JumpIfNotBothSequentialAsciiStrings(r1, r0, r2, r3, &runtime);
 
   // Compare flat ascii strings natively. Remove arguments from stack first.
   __ IncrementCounter(&Counters::string_compare_native, 1, r2, r3);
   __ add(sp, sp, Operand(2 * kPointerSize));
-  GenerateCompareFlatAsciiStrings(masm, r0, r1, r2, r3, r4, r5);
+  GenerateCompareFlatAsciiStrings(masm, r1, r0, r2, r3, r4, r5);
 
   // Call the runtime; it returns -1 (less), 0 (equal), or 1 (greater)
   // tagged as a small integer.