Arm64: Ensure that csp is always aligned to 16 byte values even if jssp is not.

src/arm64/macro-assembler-arm64.cc

Index: src/arm64/macro-assembler-arm64.cc
diff --git a/src/arm64/macro-assembler-arm64.cc b/src/arm64/macro-assembler-arm64.cc
index 0b2954e18f54ee9b78eeb5b6ff3a517e62b5876e..1c59128dd4ca92b83ed0d55ec141521d92873e61 100644
--- a/src/arm64/macro-assembler-arm64.cc
+++ b/src/arm64/macro-assembler-arm64.cc
@@ -800,13 +800,6 @@ void MacroAssembler::Pop(const CPURegister& dst0, const CPURegister& dst1,
 
   PrepareForPop(count, size);
   PopHelper(count, size, dst0, dst1, dst2, dst3);
-
-  if (!csp.Is(StackPointer()) && emit_debug_code()) {
-    // It is safe to leave csp where it is when unwinding the JavaScript stack,
-    // but if we keep it matching StackPointer, the simulator can detect memory
-    // accesses in the now-free part of the stack.
-    Mov(csp, StackPointer());

[jbramley, 2014/05/01 15:17:10]

It would be better to leave this here (or replace it with the new
SyncSystemStackPointer). Calling SyncSystemStackPointer in PrepareForPop works,
but it makes the check less strict than it could be. (I'd like this check to be
as strict as possible because bugs in this area would be hard to reproduce.)

[rmcilroy, 2014/05/01 18:29:11]

I see what you mean, but actually, looking at it again I actually think
PrepareForPop should be called after the PopHelper - it doesn't actually prepare
anything it just asserts that the stack is in a valid state and I think this
makes more sense to do after the PopHelper has been run.  I've made a change to
call it PushPreamble and PopPostamble and moved the calls about as appropriate. 
Wdyt?

-  }
 }
 
 
@@ -895,13 +888,6 @@ void MacroAssembler::PopCPURegList(CPURegList registers) {
     int count = count_before - registers.Count();
     PopHelper(count, size, dst0, dst1, dst2, dst3);
   }
-
-  if (!csp.Is(StackPointer()) && emit_debug_code()) {
-    // It is safe to leave csp where it is when unwinding the JavaScript stack,
-    // but if we keep it matching StackPointer, the simulator can detect memory
-    // accesses in the now-free part of the stack.
-    Mov(csp, StackPointer());
-  }
 }
 
 
@@ -1104,6 +1090,11 @@ void MacroAssembler::PrepareForPop(Operand total_size) {
 
     // Don't check access size for non-immediate sizes. It's difficult to do
     // well, and it will be caught by hardware (or the simulator) anyway.
+  } else if (emit_debug_code()) {
+    // It is safe to leave csp where it is when unwinding the JavaScript stack,
+    // but if we keep it matching StackPointer, the simulator can detect memory
+    // accesses in the now-free part of the stack.
+    SyncSystemStackPointer();
   }
 }
 
@@ -1200,12 +1191,12 @@ void MacroAssembler::PopCalleeSavedRegisters() {
 
 void MacroAssembler::AssertStackConsistency() {
   if (emit_debug_code()) {
-    if (csp.Is(StackPointer())) {
-      // We can't check the alignment of csp without using a scratch register
-      // (or clobbering the flags), but the processor (or simulator) will abort
-      // if it is not properly aligned during a load.
-      ldr(xzr, MemOperand(csp, 0));
-    } else if (FLAG_enable_slow_asserts) {
+    // Always check the alignment of csp since it could have been bumped even if
+    // it is not the stack pointer. We can't check the alignment of csp without
+    // using a scratch register (or clobbering the flags), but the processor
+    // (or simulator) will abort if it is not properly aligned during a load.
+    ldr(xzr, MemOperand(csp, 0));
+    if (FLAG_enable_slow_asserts && !csp.Is(StackPointer())) {
       Label ok;
       // Check that csp <= StackPointer(), preserving all registers and NZCV.
       sub(StackPointer(), csp, StackPointer());