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

Currently emscripten is make assumptions about that memory layout,
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:

emscripten-core/emscripten#18057

Differential Revision: https://reviews.llvm.org/D135910

Author: sbc100

lld/test/wasm/export-all.s

@@ -34,15 +34,21 @@ foo:
 # 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

@@ -80,18 +80,24 @@ _start:
 # 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

lld/wasm/Driver.cpp

@@ -732,6 +732,8 @@ static void createOptionalSymbols() {
     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");

lld/wasm/Symbols.cpp

@@ -85,6 +85,8 @@ 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;

lld/wasm/Symbols.h

@@ -516,10 +516,13 @@ struct WasmSym {
   // 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

lld/wasm/Writer.cpp

@@ -242,12 +242,16 @@ void Writer::layoutMemory() {
     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));
   };