ARM: Implement fast code for ModI.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 5baed365c0b3922ad98623329a86737c1c71d0d9..99cec39d78ddfdb8a7b6c177b74b19ce821e5bfa 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -1038,6 +1038,25 @@ void LCodeGen::DoModI(LModI* instr) {
     __ bind(&ok);
   }
 
+  // Try a few common cases before using the generic stub.

[Søren Thygesen Gjesse, 2011/01/18 15:58:38]

Maybe extend this comment a bit (it took me some time to find out that you where
subtracting left 0 - 2 times from right).

[Karl Klose, 2011/01/20 02:12:12]

Done.

+  const int kUnfolds = 3;
+  __ mov(scratch, left);
+  for (int i = 0; i < kUnfolds; i++) {
+    // Check if the left hand side is less or equal.
+    if (i == 0) __ cmp(scratch, right);
+    __ mov(result, scratch, LeaveCC, le);
+    __ b(le, &done);
+    if (i < kUnfolds - 1) __ sub(scratch, left, right);

[Søren Thygesen Gjesse, 2011/01/18 15:58:38]

Shouldn't this be

__ sub(scratch, scratch, right, SetCC)

? Or maybe not, but when le is set will scratch not be negative (for positive
left/right)?

[Karl Klose, 2011/01/20 02:12:12]

Thanks for the catch.
On 2011/01/18 15:58:38, Søren Gjesse wrote:

+  }
+
+  // Check for power of two on the right hand side.

[Søren Thygesen Gjesse, 2011/01/18 15:58:38]

I think the code we have in GenericBinaryOpStub does this in fewer instructions.
It works on smis but should work for int32 as well.

  // Check for two positive smis.
  __ orr(smi_test_reg, lhs, Operand(rhs));
  __ tst(smi_test_reg, Operand(0x80000000u | kSmiTagMask));
  __ b(ne, &slow);
  // Check that rhs is a power of two and not zero.
  Register mask_bits = r3;
  __ sub(scratch, rhs, Operand(1), SetCC);
  __ b(mi, &slow);
  __ and_(mask_bits, rhs, Operand(scratch), SetCC);
  __ b(ne, &not_power_of_2);
  // Calculate power of two modulus.
  __ and_(result, lhs, Operand(scratch));

[Karl Klose, 2011/01/20 02:12:12]

Done.

+  const int kMaxPowerOfTwo = 32;
+  for (int i = 2; i <= kMaxPowerOfTwo; i <<= 1) {
+    __ cmp(right, Operand(i));
+    __ and_(result, left, Operand(i-1), LeaveCC, eq);
+    __ b(&done, eq);
+  }
+
   // Call the generic stub. The numbers in r0 and r1 have
   // to be tagged to Smis. If that is not possible, deoptimize.
   DeferredModI* deferred = new DeferredModI(this, instr);
@@ -1049,7 +1068,7 @@ void LCodeGen::DoModI(LModI* instr) {
 
   // If the result in r0 is a Smi, untag it, else deoptimize.
   __ BranchOnNotSmi(result, &deoptimize);
-  __ mov(result, Operand(result, ASR, 1));
+  __ SmiUntag(result);
 
   __ b(al, &done);
   __ bind(&deoptimize);