X87: Simplify string copy in SubStringStub.

port r21756.

original commit message:
  The optimizations are unnecessary since the maximum
  string length they operate on is currently 12.

R=yangguo@chromium.org, bmeurer@chromium.org
  BUG=352155
  LOG=N

Review URL: https://codereview.chromium.org/326353002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21772 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x87/code-stubs-x87.cc

@@ -2855,18 +2855,12 @@ void StringCharFromCodeGenerator::GenerateSlow(
 }
 
 
-void StringHelper::GenerateCopyCharactersREP(MacroAssembler* masm,
-                                             Register dest,
-                                             Register src,
-                                             Register count,
-                                             Register scratch,
-                                             bool ascii) {
-  // Copy characters using rep movs of doublewords.
-  // The destination is aligned on a 4 byte boundary because we are
-  // copying to the beginning of a newly allocated string.
-  ASSERT(dest.is(edi));  // rep movs destination
-  ASSERT(src.is(esi));  // rep movs source
-  ASSERT(count.is(ecx));  // rep movs count
+void StringHelper::GenerateCopyCharacters(MacroAssembler* masm,
+                                          Register dest,
+                                          Register src,
+                                          Register count,
+                                          Register scratch,
+                                          String::Encoding encoding) {
   ASSERT(!scratch.is(dest));
   ASSERT(!scratch.is(src));
   ASSERT(!scratch.is(count));
@@ -2877,38 +2871,17 @@ void StringHelper::GenerateCopyCharactersREP(MacroAssembler* masm,
   __ j(zero, &done);
 
   // Make count the number of bytes to copy.
-  if (!ascii) {
+  if (encoding == String::TWO_BYTE_ENCODING) {
     __ shl(count, 1);
   }
 
-  // Don't enter the rep movs if there are less than 4 bytes to copy.
-  Label last_bytes;
-  __ test(count, Immediate(~3));
-  __ j(zero, &last_bytes, Label::kNear);
-
-  // Copy from edi to esi using rep movs instruction.
-  __ mov(scratch, count);
-  __ sar(count, 2);  // Number of doublewords to copy.
-  __ cld();
-  __ rep_movs();
-
-  // Find number of bytes left.
-  __ mov(count, scratch);
-  __ and_(count, 3);
-
-  // Check if there are more bytes to copy.
-  __ bind(&last_bytes);
-  __ test(count, count);
-  __ j(zero, &done);
-
-  // Copy remaining characters.
   Label loop;
   __ bind(&loop);
   __ mov_b(scratch, Operand(src, 0));
   __ mov_b(Operand(dest, 0), scratch);
-  __ add(src, Immediate(1));
-  __ add(dest, Immediate(1));
-  __ sub(count, Immediate(1));
+  __ inc(src);
+  __ inc(dest);
+  __ dec(count);
   __ j(not_zero, &loop);
 
   __ bind(&done);
@@ -3137,23 +3110,21 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
   // eax: result string
   // ecx: result string length
-  __ mov(edx, esi);  // esi used by following code.
   // Locate first character of result.
   __ mov(edi, eax);
   __ add(edi, Immediate(SeqOneByteString::kHeaderSize - kHeapObjectTag));
   // Load string argument and locate character of sub string start.
-  __ pop(esi);
+  __ pop(edx);
   __ pop(ebx);
   __ SmiUntag(ebx);
-  __ lea(esi, FieldOperand(esi, ebx, times_1, SeqOneByteString::kHeaderSize));
+  __ lea(edx, FieldOperand(edx, ebx, times_1, SeqOneByteString::kHeaderSize));
 
   // eax: result string
   // ecx: result length
-  // edx: original value of esi
   // edi: first character of result
-  // esi: character of sub string start
-  StringHelper::GenerateCopyCharactersREP(masm, edi, esi, ecx, ebx, true);
-  __ mov(esi, edx);  // Restore esi.
+  // edx: character of sub string start
+  StringHelper::GenerateCopyCharacters(
+      masm, edi, edx, ecx, ebx, String::ONE_BYTE_ENCODING);
   __ IncrementCounter(counters->sub_string_native(), 1);
   __ ret(3 * kPointerSize);
 
@@ -3163,27 +3134,25 @@ void SubStringStub::Generate(MacroAssembler* masm) {
 
   // eax: result string
   // ecx: result string length
-  __ mov(edx, esi);  // esi used by following code.
   // Locate first character of result.
   __ mov(edi, eax);
   __ add(edi,
          Immediate(SeqTwoByteString::kHeaderSize - kHeapObjectTag));
   // Load string argument and locate character of sub string start.
-  __ pop(esi);
+  __ pop(edx);
   __ pop(ebx);
   // As from is a smi it is 2 times the value which matches the size of a two
   // byte character.
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize + kSmiShiftSize == 1);
-  __ lea(esi, FieldOperand(esi, ebx, times_1, SeqTwoByteString::kHeaderSize));
+  __ lea(edx, FieldOperand(edx, ebx, times_1, SeqTwoByteString::kHeaderSize));
 
   // eax: result string
   // ecx: result length
-  // edx: original value of esi
   // edi: first character of result
-  // esi: character of sub string start
-  StringHelper::GenerateCopyCharactersREP(masm, edi, esi, ecx, ebx, false);
-  __ mov(esi, edx);  // Restore esi.
+  // edx: character of sub string start
+  StringHelper::GenerateCopyCharacters(
+      masm, edi, edx, ecx, ebx, String::TWO_BYTE_ENCODING);
   __ IncrementCounter(counters->sub_string_native(), 1);
   __ ret(3 * kPointerSize);
 

src/x87/code-stubs-x87.h

@@ -38,12 +38,12 @@ class StringHelper : public AllStatic {
   // Generate code for copying characters using the rep movs instruction.
   // Copies ecx characters from esi to edi. Copying of overlapping regions is
   // not supported.
-  static void GenerateCopyCharactersREP(MacroAssembler* masm,
-                                        Register dest,     // Must be edi.
-                                        Register src,      // Must be esi.
-                                        Register count,    // Must be ecx.
-                                        Register scratch,  // Neither of above.
-                                        bool ascii);
+  static void GenerateCopyCharacters(MacroAssembler* masm,
+                                     Register dest,
+                                     Register src,
+                                     Register count,
+                                     Register scratch,
+                                     String::Encoding encoding);
 
   // Generate string hash.
   static void GenerateHashInit(MacroAssembler* masm,