[vm] Simplify VirtualMemory::Truncate

Callers always want to try to unmap the trailing pages, and the
OS-specific FreeSubSegment helper routine can never fail. Simplify
code accordingly.

Change-Id: I32e4bc72f626e15032d73326bee76a4a2ef71025
Reviewed-on: https://dart-review.googlesource.com/c/91146
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Zach Anderson <zra@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/virtual_memory.cc

@@ -14,14 +14,12 @@ bool VirtualMemory::InSamePage(uword address0, uword address1) {
           Utils::RoundDown(address1, PageSize()));
 }
 
-void VirtualMemory::Truncate(intptr_t new_size, bool try_unmap) {
-  ASSERT((new_size & (PageSize() - 1)) == 0);
+void VirtualMemory::Truncate(intptr_t new_size) {
+  ASSERT(Utils::IsAligned(new_size, PageSize()));
   ASSERT(new_size <= size());
-  if (try_unmap &&
-      (reserved_.size() ==
-       region_.size()) && /* Don't create holes in reservation. */
-      FreeSubSegment(reinterpret_cast<void*>(start() + new_size),
-                     size() - new_size)) {
+  if (reserved_.size() == region_.size()) { // Don't create holes in reservation.
+    FreeSubSegment(reinterpret_cast<void*>(start() + new_size),
+                   size() - new_size);
     reserved_.set_size(new_size);
   }
   region_.Subregion(region_, 0, new_size);

runtime/vm/virtual_memory.h

@@ -57,10 +57,8 @@ class VirtualMemory {
 
   static bool InSamePage(uword address0, uword address1);
 
-  // Truncate this virtual memory segment. If try_unmap is false, the
-  // memory beyond the new end is still accessible, but will be returned
-  // upon destruction.
-  void Truncate(intptr_t new_size, bool try_unmap = true);
+  // Truncate this virtual memory segment.
+  void Truncate(intptr_t new_size);
 
   // False for a part of a snapshot added directly to the Dart heap, which
   // belongs to the embedder and must not be deallocated or have its
@@ -72,7 +70,7 @@ class VirtualMemory {
  private:
   // Free a sub segment. On operating systems that support it this
   // can give back the virtual memory to the system. Returns true on success.
-  static bool FreeSubSegment(void* address, intptr_t size);
+  static void FreeSubSegment(void* address, intptr_t size);
 
   // This constructor is only used internally when reserving new virtual spaces.
   // It does not reserve any virtual address space on its own.

runtime/vm/virtual_memory_fuchsia.cc

@@ -126,11 +126,10 @@ VirtualMemory::~VirtualMemory() {
   }
 }
 
-bool VirtualMemory::FreeSubSegment(void* address, intptr_t size) {
+void VirtualMemory::FreeSubSegment(void* address, intptr_t size) {
   const uword start = reinterpret_cast<uword>(address);
   unmap(zx_vmar_root_self(), start, start + size);
   LOG_INFO("zx_vmar_unmap(%p, %lx) success\n", address, size);
-  return true;
 }
 
 void VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {

runtime/vm/virtual_memory_posix.cc

@@ -76,11 +76,10 @@ VirtualMemory::~VirtualMemory() {
   }
 }
 
-bool VirtualMemory::FreeSubSegment(void* address,
+void VirtualMemory::FreeSubSegment(void* address,
                                    intptr_t size) {
   const uword start = reinterpret_cast<uword>(address);
   unmap(start, start + size);
-  return true;
 }
 
 void VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {

runtime/vm/virtual_memory_test.cc

@@ -77,25 +77,13 @@ VM_UNIT_TEST_CASE(FreeVirtualMemory) {
   for (intptr_t i = 0; i < kIterations; ++i) {
     VirtualMemory* vm =
         VirtualMemory::Allocate(kVirtualMemoryBlockSize, false, NULL);
-    vm->Truncate(kVirtualMemoryBlockSize / 2, true);
+    vm->Truncate(kVirtualMemoryBlockSize / 2);
     delete vm;
   }
   for (intptr_t i = 0; i < kIterations; ++i) {
     VirtualMemory* vm =
         VirtualMemory::Allocate(kVirtualMemoryBlockSize, true, NULL);
-    vm->Truncate(kVirtualMemoryBlockSize / 2, false);
-    delete vm;
-  }
-  for (intptr_t i = 0; i < kIterations; ++i) {
-    VirtualMemory* vm =
-        VirtualMemory::Allocate(kVirtualMemoryBlockSize, true, NULL);
-    vm->Truncate(0, true);
-    delete vm;
-  }
-  for (intptr_t i = 0; i < kIterations; ++i) {
-    VirtualMemory* vm =
-        VirtualMemory::Allocate(kVirtualMemoryBlockSize, false, NULL);
-    vm->Truncate(0, false);
+    vm->Truncate(0);
     delete vm;
   }
 }

runtime/vm/virtual_memory_win.cc

@@ -62,12 +62,11 @@ VirtualMemory::~VirtualMemory() {
   }
 }
 
-bool VirtualMemory::FreeSubSegment(void* address,
+void VirtualMemory::FreeSubSegment(void* address,
                                    intptr_t size) {
   if (VirtualFree(address, size, MEM_DECOMMIT) == 0) {
     FATAL1("VirtualFree failed: Error code %d\n", GetLastError());
   }
-  return true;
 }
 
 void VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {