Make SubStringStub more robust wrt unsafe arguments.

src/arm/code-stubs-arm.cc

Index: src/arm/code-stubs-arm.cc
diff --git a/src/arm/code-stubs-arm.cc b/src/arm/code-stubs-arm.cc
index f772db9be2078f0ecaa24a633e1af438be6dbe9c..2f14d191b0598d9f27be0eab01fdb90493a76be9 100644
--- a/src/arm/code-stubs-arm.cc
+++ b/src/arm/code-stubs-arm.cc
@@ -5873,36 +5873,12 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // r2: result string length
   __ ldr(r4, FieldMemOperand(r0, String::kLengthOffset));
   __ cmp(r2, Operand(r4, ASR, 1));
+  // Return original string.
   __ b(eq, &return_r0);
+  // Longer than original string's length or negative: unsafe arguments.
+  __ b(hi, &runtime);
+  // Shorter than original string's length: an actual substring.
 
-  Label result_longer_than_two;
-  // Check for special case of two character ASCII string, in which case
-  // we do a lookup in the symbol table first.
-  __ cmp(r2, Operand(2));
-  __ b(gt, &result_longer_than_two);
-  __ b(lt, &runtime);
-
-  __ JumpIfInstanceTypeIsNotSequentialAscii(r1, r1, &runtime);
-
-  // Get the two characters forming the sub string.
-  __ add(r0, r0, Operand(r3));
-  __ ldrb(r3, FieldMemOperand(r0, SeqAsciiString::kHeaderSize));
-  __ ldrb(r4, FieldMemOperand(r0, SeqAsciiString::kHeaderSize + 1));
-
-  // Try to lookup two character string in symbol table.
-  Label make_two_character_string;
-  StringHelper::GenerateTwoCharacterSymbolTableProbe(
-      masm, r3, r4, r1, r5, r6, r7, r9, &make_two_character_string);
-  __ jmp(&return_r0);
-
-  // r2: result string length.
-  // r3: two characters combined into halfword in little endian byte order.
-  __ bind(&make_two_character_string);
-  __ AllocateAsciiString(r0, r2, r4, r5, r9, &runtime);
-  __ strh(r3, FieldMemOperand(r0, SeqAsciiString::kHeaderSize));
-  __ jmp(&return_r0);
-
-  __ bind(&result_longer_than_two);
   // Deal with different string types: update the index if necessary
   // and put the underlying string into r5.
   // r0: original string