ARM: Fast path for compare against constant.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
===================================================================
--- src/ia32/lithium-codegen-ia32.cc	(revision 9243)
+++ src/ia32/lithium-codegen-ia32.cc	(working copy)
@@ -317,6 +317,12 @@
 }
 
 
+double LCodeGen::ToDouble(LConstantOperand* op) const {
+  Handle<Object> value = chunk_->LookupLiteral(op);
+  return value->Number();
+}
+
+
 Immediate LCodeGen::ToImmediate(LOperand* op) {
   LConstantOperand* const_op = LConstantOperand::cast(op);
   Handle<Object> literal = chunk_->LookupLiteral(const_op);
@@ -1527,32 +1533,40 @@
 }
 
 
-void LCodeGen::EmitCmpI(LOperand* left, LOperand* right) {
-  if (right->IsConstantOperand()) {
-    __ cmp(ToOperand(left), ToImmediate(right));
-  } else {
-    __ cmp(ToRegister(left), ToOperand(right));
-  }
-}
-
-
 void LCodeGen::DoCmpIDAndBranch(LCmpIDAndBranch* instr) {
   LOperand* left = instr->InputAt(0);
   LOperand* right = instr->InputAt(1);
   int false_block = chunk_->LookupDestination(instr->false_block_id());
   int true_block = chunk_->LookupDestination(instr->true_block_id());
+  Condition cc = TokenToCondition(instr->op(), instr->is_double());
 
-  if (instr->is_double()) {
-    // Don't base result on EFLAGS when a NaN is involved. Instead
-    // jump to the false block.
-    __ ucomisd(ToDoubleRegister(left), ToDoubleRegister(right));
-    __ j(parity_even, chunk_->GetAssemblyLabel(false_block));
+  if (left->IsConstantOperand() && right->IsConstantOperand()) {
+    // We can statically evaluate the comparison.
+    double left_val = ToDouble(LConstantOperand::cast(left));
+    double right_val = ToDouble(LConstantOperand::cast(right));
+    int next_block =
+      EvalComparison(instr->op(), left_val, right_val) ? true_block
+                                                       : false_block;
+    EmitGoto(next_block);
   } else {
-    EmitCmpI(left, right);
+    if (instr->is_double()) {
+      // Don't base result on EFLAGS when a NaN is involved. Instead
+      // jump to the false block.
+      __ ucomisd(ToDoubleRegister(left), ToDoubleRegister(right));
+      __ j(parity_even, chunk_->GetAssemblyLabel(false_block));
+    } else {
+      if (right->IsConstantOperand()) {
+        __ cmp(ToOperand(left), ToImmediate(right));

[William Hesse, 2011/10/20 10:28:40]

If the immediate value is 0, this can also be test(left, left) on ia32 and x64. 
You should try this on ARM, if possible.  I'll test it on ia32 and x64.  Also,
this can be a ToRegister(left)  (fixed).

[Alexandre, 2011/10/20 10:59:49]

I forgot to change it to tst for Intel.
I intentionally used a cmp as it is better on ARM:
- the immediate is encoded in the instruction.
- cmp sets all the flags, whereas tst doesn't, resulting in more complex
handling for the pipeline.

On 2011/10/20 10:28:40, William Hesse wrote:

+      } else if (left->IsConstantOperand()) {
+        __ cmp(ToOperand(right), ToImmediate(left));
+        // We transposed the operands. Reverse the condition.
+        cc = ReverseCondition(cc);
+      } else {
+        __ cmp(ToRegister(left), ToOperand(right));
+      }
+    }
+    EmitBranch(true_block, false_block, cc);
   }
-
-  Condition cc = TokenToCondition(instr->op(), instr->is_double());
-  EmitBranch(true_block, false_block, cc);
 }