PPC64: Adjust simulator stack safety margin.

This fixes a failure in test/mjsunit/regress/regress-crbug-491062.js.

R=dstence@us.ibm.com, michael_dawson@ca.ibm.com, svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1174693002

Cr-Commit-Position: refs/heads/master@{#28924}

src/flag-definitions.h

@@ -728,7 +728,7 @@ DEFINE_INT(sim_stack_alignment, 8,
            "Stack alingment in bytes in simulator (4 or 8, 8 is default)")
 #endif
 DEFINE_INT(sim_stack_size, 2 * MB / KB,
-           "Stack size of the ARM64 and MIPS64 simulator "
+           "Stack size of the ARM64, MIPS64 and PPC64 simulator "
            "in kBytes (default is 2 MB)")
 DEFINE_BOOL(log_regs_modified, true,
             "When logging register values, only print modified registers.")

src/ppc/simulator-ppc.cc

@@ -794,10 +794,11 @@ Simulator::Simulator(Isolate* isolate) : isolate_(isolate) {
 // Set up simulator support first. Some of this information is needed to
 // setup the architecture state.
 #if V8_TARGET_ARCH_PPC64
-  size_t stack_size = 2 * 1024 * 1024;  // allocate 2MB for stack
+  size_t stack_size = FLAG_sim_stack_size * KB;
 #else
-  size_t stack_size = 1 * 1024 * 1024;  // allocate 1MB for stack
+  size_t stack_size = MB;  // allocate 1MB for stack
 #endif
+  stack_size += 2 * stack_protection_size_;
   stack_ = reinterpret_cast<char*>(malloc(stack_size));
   pc_modified_ = false;
   icount_ = 0;
@@ -823,7 +824,8 @@ Simulator::Simulator(Isolate* isolate) : isolate_(isolate) {
   // The sp is initialized to point to the bottom (high address) of the
   // allocated stack area. To be safe in potential stack underflows we leave
   // some buffer below.
-  registers_[sp] = reinterpret_cast<intptr_t>(stack_) + stack_size - 64;
+  registers_[sp] =
+      reinterpret_cast<intptr_t>(stack_) + stack_size - stack_protection_size_;
   InitializeCoverage();
 
   last_debugger_input_ = NULL;
@@ -1107,9 +1109,8 @@ void Simulator::WriteDW(intptr_t addr, int64_t value) {
 
 // Returns the limit of the stack area to enable checking for stack overflows.
 uintptr_t Simulator::StackLimit() const {
-  // Leave a safety margin of 1024 bytes to prevent overrunning the stack when
-  // pushing values.
-  return reinterpret_cast<uintptr_t>(stack_) + 1024;
+  // Leave a safety margin to prevent overrunning the stack when pushing values.
+  return reinterpret_cast<uintptr_t>(stack_) + stack_protection_size_;
 }
 
 

src/ppc/simulator-ppc.h

@@ -351,6 +351,7 @@ class Simulator {
 
   // Simulator support.
   char* stack_;
+  static const size_t stack_protection_size_ = 256 * kPointerSize;
   bool pc_modified_;
   int icount_;