[v8-dev] ARM: Improve Lithium register constraints.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 415b32b87a7b841efb09f4af6d7d6b8edfadc884..ebfc45cfd424b60f0ee4f49ca436d015ea37c47b 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -163,6 +163,13 @@ namespace internal {
 #error Unknown target architecture endiannes
 #endif
 
+#if V8_TARGET_ARCH_ARM
+// On ARM, for some arithmetic instructions, the macro assembler defines
+// XXXMemOperand macro instructions which are able to load the right operand
+// from memory.
+#define V8_BETTER_OPERAND_ON_RIGHT

[Benedikt Meurer, 2013/08/20 11:55:34]

I don't like this #define. Why not switch the order for the XXXMemOperand macro
instructions and use the existing logic?

[vincent.belliard.fr, 2013/08/22 12:35:48]

Once again, the code has been made for IA32 where operands only used once are
better at left because operations clobber the left value.
With ARM, we have 3 operands operations so there is no advantage to put used
once operands on the left.

The XXXMemOperand functions could have been defined with the MemOperand on the
left instead of on the right. However, it would have been not consistent with
other macro assembler functions.

The choice of left operand is arbitrary (but this is good for IA32). We only put
this define to be able to make the better choice for a CPU.

It was difficult to find a good name for the define (and we can't use the
ARCH_ARM define directly in hydrogen-instructions.h). If you have some
suggestions we can change the name.

[Benedikt Meurer, 2013/08/23 07:41:02]

I still don't see why this macro is needed at all. As you say, ARM has 3 operand
instructions, so it doesn't matter and we could as well have the better operand
on the left there as well. What am I missing? 

+#endif
+
 // Support for alternative bool type. This is only enabled if the code is
 // compiled with USE_MYBOOL defined. This catches some nasty type bugs.
 // For instance, 'bool b = "false";' results in b == true! This is a hidden