Simplify some of the startup code for SubStringStub::Generate.

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 8b0779bcf26fb41621d0b9e606f966f30326eb90..8c6d17e563c3be2d0d098e7b381f28783b9ba135 100644
--- a/src/arm/code-stubs-arm.cc
+++ b/src/arm/code-stubs-arm.cc
@@ -6007,23 +6007,28 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize + kSmiShiftSize == 1);
 
-  // I.e., arithmetic shift right by one un-smi-tags.
-  __ mov(r2, Operand(r2, ASR, 1), SetCC);
-  __ mov(r3, Operand(r3, 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.
+  // Arithmetic shift right by one un-smi-tags. In this case we rotate right
+  // instead because we bail out on non-smi values: ROR and ASR are equivalent
+  // for smis but they set the flags in a way that's easier to optimize.
+  __ mov(r2, Operand(r2, ROR, 1), SetCC);
+  __ mov(r3, Operand(r3, ROR, 1), SetCC, cc);
+  // If either to or from had the smi tag bit set, then C is set now, and N
+  // has the same value: we rotated by 1, so the bottom bit is now the top bit.
   // We want to bailout to runtime here if From is negative.  In that case, the
   // next instruction is not executed and we fall through to bailing out to
-  // runtime.  pl is the opposite of mi.
-  // Both r2 and r3 are untagged integers.
-  __ sub(r2, r2, Operand(r3), SetCC, pl);
-  __ b(mi, &runtime);  // Fail if from > to.
+  // runtime.
+  // Executed if both r2 and r3 are untagged integers.
+  __ sub(r2, r2, Operand(r3), SetCC, cc);
+  // One of the above un-smis or the above SUB could have set N==1.
+  __ b(mi, &runtime);  // Either "from" or "to" is not an smi, or from > to.
 
   // Make sure first argument is a string.
   __ ldr(r0, MemOperand(sp, kStringOffset));
   STATIC_ASSERT(kSmiTag == 0);
-  __ JumpIfSmi(r0, &runtime);
-  Condition is_string = masm->IsObjectStringType(r0, r1);
+  // Do a JumpIfSmi, but fold its jump into the subsequent string test.
+  __ tst(r0, Operand(kSmiTagMask));
+  Condition is_string = masm->IsObjectStringType(r0, r1, ne);
+  ASSERT(is_string == eq);
   __ b(NegateCondition(is_string), &runtime);
 
   // Short-cut for the case of trivial substring.