Assembler changes for enabling GrowHeap in Wasm

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index a7fb0dc742f27c3d184846e4538f49d89550c1ed..6c3370d3041a4dc89d4ef5d5c846500176e60abe 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -1114,6 +1114,17 @@ void MacroAssembler::Set(Register dst, int64_t x) {
   }
 }
 
+void MacroAssembler::Set(Register dst, int64_t x, RelocInfo::Mode rmode) {
+  if (x == 0) {
+    xorl(dst, dst);
+  } else if (is_uint32(x)) {
+    movl(dst, Immediate(static_cast<int32_t>(x), rmode));

[Yang, 2016/03/08 05:41:50]

Can you elaborate on where this is being used? I see at least two problems with
this:
- If x is 0, then no reloc info is emitted. There will be no way to use reloc
info to find this place again, and even if we can find it, there is no way to
replace the xorl with something other than setting dst to 0. I already noted
this previously.
- If x is below 32 bits, there will be no way to set it to a larger value
anymore.
All of this looks like premature optimization, and you even had to introduce a
new Immediate constructor for this.

[gdeepti1, 2016/03/09 04:01:43]

The specific case this is being used for is when in AssembleMove in the x64 code
generator when source is a constant with RelocInfo::Mode WASM_MEMORY_REFERENCE.
As the immediate in this case should be a pointer constant within the bounds of
the wasm memory buffer, it should realistically never be 0. 
 - If x is 0, I agree it is non intuitive to silently drop relocation mode that
is being passed in. Fixed to remove special case. 
 - For the specific case of wasm memory references I'm assuming it would be
possible to update to a larger value even if the current value is below 32 bits
on an update. Changed to use movq. For cases where relocation information is
anything else, this function mirrors the Set(Register dst, int64_t x) above this
function and is modified only to record relocation mode. 

The new Immediate constructor is used to be able to pass in relocation mode from
the compiler to the assembler, other ways to pass it to the assembler and record
relocation mode seemed messy to me.  

+  } else if (is_int32(x)) {
+    movq(dst, Immediate(static_cast<int32_t>(x), rmode));
+  } else {
+    movq(dst, x, rmode);
+  }
+}
 
 void MacroAssembler::Set(const Operand& dst, intptr_t x) {
   if (kPointerSize == kInt64Size) {