Subzero. Wasm. Implement sbrk and correctly do bounds checks.

Cleans up and generally improves memory handling in
WASM. WasmTranslator now outputs the number of pages requested so the
runtime can do correct bounds checks. The runtime also initializes the
stack pointer correctly (stored at address 1024), so we no longer have
to deal with negative pointers. This allows bounds checks to be done
with a single comparison against the size of the heap. Because of
this, we now support non-power-of-two heap sizes.

Sbrk is implemented by having the runtime keep track of the current
heap break and incrementing it as necessary. The heap break is
initialized to the start of the first page beyond any initialized data
in the WASM heap.

These changes allow us to pass the complete set of torture tests that
are passing on the Wasm waterfall.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4369
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1913153003 .

runtime/wasm-runtime.cpp

diff --git a/runtime/wasm-runtime.cpp b/runtime/wasm-runtime.cpp
index 609da7a..d867509 100644
--- a/runtime/wasm-runtime.cpp
+++ b/runtime/wasm-runtime.cpp
@@ -12,13 +12,27 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cassert>
 #include <cmath>
+// TODO (eholk): change to cstdint
+#include <stdint.h>
+
+namespace {
+uint32_t HeapBreak;
+
+// TODO (eholk): make all of these constexpr.
+const uint32_t PageSizeLog2 = 16;
+const uint32_t PageSize = 1 << PageSizeLog2; // 64KB
+const uint32_t StackPtrLoc = 1024;           // defined by emscripten
+
+uint32_t pageNum(uint32_t Index) { return Index >> PageSizeLog2; }
+} // end of anonymous namespace
 
 namespace env {
 double floor(double X) { return std::floor(X); }
 
 float floor(float X) { return std::floor(X); }
-}
+} // end of namespace env
 
 // TODO (eholk): move the C parts outside and use C++ name mangling.
 extern "C" {
@@ -33,6 +47,8 @@
 #include <unistd.h>
 
 extern char WASM_MEMORY[];
+extern uint32_t WASM_DATA_SIZE;
+extern uint32_t WASM_NUM_PAGES;
 
 void env$$abort() {
   fprintf(stderr, "Aborting...\n");
@@ -53,7 +69,11 @@
     abort();                                                                   \
   }
 
-UNIMPLEMENTED(sbrk)
+int32_t env$$sbrk(int32_t Increment) {
+  HeapBreak += Increment;
+  return HeapBreak;
+}
+
 UNIMPLEMENTED(setjmp)
 UNIMPLEMENTED(longjmp)
 UNIMPLEMENTED(__assert_fail)
@@ -83,18 +103,16 @@
 UNIMPLEMENTED(__unlock)
 UNIMPLEMENTED(__syscall6)   // sys_close
 UNIMPLEMENTED(__syscall140) // sys_llseek
-UNIMPLEMENTED(__syscall192) // sys_mmap?
 UNIMPLEMENTED(__unordtf2)
 UNIMPLEMENTED(__fixunstfsi)
 UNIMPLEMENTED(__floatunsitf)
 UNIMPLEMENTED(__extenddftf2)
 
-void *wasmPtr(int Index) {
-  // TODO (eholk): get the mask from the WASM file.
-  const int MASK = 0xffffff;
-  Index &= MASK;
-
-  return WASM_MEMORY + Index;
+void *wasmPtr(uint32_t Index) {
+  if (pageNum(Index) < WASM_NUM_PAGES) {
+    return WASM_MEMORY + Index;
+  }
+  abort();
 }
 
 extern int __szwasm_main(int, const char **);
@@ -102,7 +120,15 @@
 #define WASM_REF(Type, Index) ((Type *)wasmPtr(Index))
 #define WASM_DEREF(Type, Index) (*WASM_REF(Type, Index))
 
-int main(int argc, const char **argv) { return __szwasm_main(argc, argv); }
+int main(int argc, const char **argv) {
+  // Initialize the break to the nearest page boundary after the data segment
+  HeapBreak = (WASM_DATA_SIZE + PageSize - 1) & ~(PageSize - 1);
+
+  // Initialize the stack pointer.
+  WASM_DEREF(int32_t, StackPtrLoc) = WASM_NUM_PAGES << PageSizeLog2;
+
+  return __szwasm_main(argc, argv);
+}
 
 int env$$abs(int a) { return abs(a); }
 
@@ -168,4 +194,14 @@
   }
   return Count;
 }
+
+/// sys_mmap_pgoff
+int env$$__syscall192(int Which, int VarArgs) {
+  (void)Which;
+  (void)VarArgs;
+
+  // TODO (eholk): figure out how to implement this.
+
+  return -ENOMEM;
+}
 } // end of extern "C"