[lld][WebAssembly] Add symbols marking start/end of stack region

[sbc100]

Summary

Currently emscripten is make assumptions about that memory layout and assuming
the stack is between __data_end and __heap_base:

https://github.com/emscripten-core/emscripten/blob/af961ad5c4c278ec510f0b7f7d522a95ee5a90f8/system/lib/compiler-rt/stack_limits.S#L42-L61

With this change we can be more precise:

https://github.com/emscripten-core/emscripten/pull/18057

[sunfish]

LGTM!

lld/test/wasm/export-all.s

 # CHECK-NEXT:         Kind:            FUNCTION
 # CHECK-NEXT:         Index:           1
 # CHECK-NEXT:       - Name:            __dso_handle
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           1
 # CHECK-NEXT:       - Name:            __data_end
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           2
-# CHECK-NEXT:       - Name:            __global_base
+# CHECK-NEXT:       - Name:            __stack_low
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           3
-# CHECK-NEXT:       - Name:            __heap_base
+# CHECK-NEXT:       - Name:            __stack_high
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           4
-# CHECK-NEXT:       - Name:            __memory_base
+# CHECK-NEXT:       - Name:            __global_base
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           5
-# CHECK-NEXT:       - Name:            __table_base
+# CHECK-NEXT:       - Name:            __heap_base
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           6
+# CHECK-NEXT:       - Name:            __memory_base
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         Index:           7
+# CHECK-NEXT:       - Name:            __table_base
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         Index:           8

lld/test/wasm/mutable-global-exports.s

 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           1
 # CHECK-ALL-NEXT:      - Name:            __dso_handle
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           2
 # CHECK-ALL-NEXT:      - Name:            __data_end
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           3
-# CHECK-ALL-NEXT:      - Name:            __global_base
+# CHECK-ALL-NEXT:      - Name:            __stack_low
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           4
-# CHECK-ALL-NEXT:      - Name:            __heap_base
+# CHECK-ALL-NEXT:      - Name:            __stack_high
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           5
-# CHECK-ALL-NEXT:      - Name:            __memory_base
+# CHECK-ALL-NEXT:      - Name:            __global_base
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           6
-# CHECK-ALL-NEXT:      - Name:            __table_base
+# CHECK-ALL-NEXT:      - Name:            __heap_base
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           7
+# CHECK-ALL-NEXT:      - Name:            __memory_base
+# CHECK-ALL-NEXT:        Kind:            GLOBAL
+# CHECK-ALL-NEXT:        Index:           8
+# CHECK-ALL-NEXT:      - Name:            __table_base
+# CHECK-ALL-NEXT:        Kind:            GLOBAL
+# CHECK-ALL-NEXT:        Index:           9
 # CHECK-ALL-NEXT:  - Type:            CODE
 
 # CHECK-ALL:         Name:            target_features
 # CHECK-ALL-NEXT:    Features:
 # CHECK-ALL-NEXT:      - Prefix:          USED
 # CHECK-ALL-NEXT:        Name:            atomics
 # CHECK-ALL-NEXT:      - Prefix:          USED
 # CHECK-ALL-NEXT:        Name:            mutable-globals
 # CHECK-ALL-NEXT: ...
 

lld/wasm/Driver.cpp

if (config->relocatable)
     return;
 
   WasmSym::dsoHandle = symtab->addOptionalDataSymbol("__dso_handle");
 
   if (!config->shared)
     WasmSym::dataEnd = symtab->addOptionalDataSymbol("__data_end");
 
   if (!config->isPic) {
+    WasmSym::stackLow = symtab->addOptionalDataSymbol("__stack_low");
+    WasmSym::stackHigh = symtab->addOptionalDataSymbol("__stack_high");
     WasmSym::globalBase = symtab->addOptionalDataSymbol("__global_base");
     WasmSym::heapBase = symtab->addOptionalDataSymbol("__heap_base");
     WasmSym::definedMemoryBase = symtab->addOptionalDataSymbol("__memory_base");
     WasmSym::definedTableBase = symtab->addOptionalDataSymbol("__table_base");
     if (config->is64.value_or(false))
       WasmSym::definedTableBase32 =
           symtab->addOptionalDataSymbol("__table_base32");
   }

lld/wasm/Symbols.h

 };
 
 // linker-generated symbols
 struct WasmSym {
   // __global_base
   // Symbol marking the start of the global section.
   static DefinedData *globalBase;
 
-  // __stack_pointer
-  // Global that holds the address of the top of the explicit value stack in
-  // linear memory.
+  // __stack_pointer/__stack_low/__stack_high
+  // Global that holds current value of stack pointer and data symbols marking
+  // the start and end of the stack region.  stackPointer is initialized to
+  // stackHigh and grows downwards towards stackLow
   static GlobalSymbol *stackPointer;
+  static DefinedData *stackLow;
+  static DefinedData *stackHigh;
 
   // __tls_base
   // Global that holds the address of the base of the current thread's
   // TLS block.
   static GlobalSymbol *tlsBase;
 
   // __tls_size
   // Symbol whose value is the size of the TLS block.

lld/wasm/Symbols.cpp

 DefinedFunction *WasmSym::initTLS;
 DefinedFunction *WasmSym::startFunction;
 DefinedData *WasmSym::dsoHandle;
 DefinedData *WasmSym::dataEnd;
 DefinedData *WasmSym::globalBase;
 DefinedData *WasmSym::heapBase;
 DefinedData *WasmSym::initMemoryFlag;
 GlobalSymbol *WasmSym::stackPointer;
+DefinedData *WasmSym::stackLow;
+DefinedData *WasmSym::stackHigh;
 GlobalSymbol *WasmSym::tlsBase;
 GlobalSymbol *WasmSym::tlsSize;
 GlobalSymbol *WasmSym::tlsAlign;
 UndefinedGlobal *WasmSym::tableBase;
 DefinedData *WasmSym::definedTableBase;
 UndefinedGlobal *WasmSym::tableBase32;
 DefinedData *WasmSym::definedTableBase32;
 UndefinedGlobal *WasmSym::memoryBase;

lld/wasm/Writer.cpp

 // static data loads and stores requires larger offsets.
 void Writer::layoutMemory() {
   uint64_t memoryPtr = 0;
 
   auto placeStack = [&]() {
     if (config->relocatable || config->isPic)
       return;
     memoryPtr = alignTo(memoryPtr, stackAlignment);
+    if (WasmSym::stackLow)
+      WasmSym::stackLow->setVA(memoryPtr);
     if (config->zStackSize != alignTo(config->zStackSize, stackAlignment))
       error("stack size must be " + Twine(stackAlignment) + "-byte aligned");
     log("mem: stack size  = " + Twine(config->zStackSize));
     log("mem: stack base  = " + Twine(memoryPtr));
     memoryPtr += config->zStackSize;
     setGlobalPtr(cast<DefinedGlobal>(WasmSym::stackPointer), memoryPtr);
+    if (WasmSym::stackHigh)
+      WasmSym::stackHigh->setVA(memoryPtr);
     log("mem: stack top   = " + Twine(memoryPtr));
   };
 
   if (config->stackFirst) {
     placeStack();
   } else {
     memoryPtr = config->globalBase;
     log("mem: global base = " + Twine(config->globalBase));