[lld][WebAssembly] Fix memory size in dylink section for -pie exectuables

[sbc100]

Summary

This field to represents the amount of static data needed by
an dynamic library or executable it should not include things
like heap or stack areas, which in the case of -pie are
not determined until runtime (e.g. __stack_pointer is imported).

[kripken]

lld/wasm/Writer.cpp

299

memoryPtr - dataStart also appears on line 296, maybe worth a lccal variable? (I'm not sure of LLVM conventions there.)

[sbc100]

• feedback

lld/test/wasm/pie.ll

 ; RUN: llc -relocation-model=pic -mattr=+mutable-globals -filetype=obj %s -o %t.o
-; RUN: wasm-ld --no-gc-sections --allow-undefined -pie -o %t.wasm %t.o
+; RUN: wasm-ld --no-gc-sections --allow-undefined --experimental-pic -pie -o %t.wasm %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target triple = "wasm32-unknown-emscripten"
 
 @data = global i32 2, align 4
 @data_external = external global i32
 @indirect_func = local_unnamed_addr global i32 ()* @foo, align 4
 

 entry:
   ret i32** @data_addr_external
 }
 
 define void @_start() {
   ret void
 }
 
+;      CHECK: Sections:
+; CHECK-NEXT:   - Type:            CUSTOM
+; CHECK-NEXT:     Name:            dylink
+; CHECK-NEXT:     MemorySize:      16
+; CHECK-NEXT:     MemoryAlignment: 2
+; CHECK-NEXT:     TableSize:       1
+; CHECK-NEXT:     TableAlignment:  0
+; CHECK-NEXT:     Needed:          []
+
 ; CHECK:        - Type:            IMPORT
 ; CHECK-NEXT:     Imports:
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __indirect_function_table
 ; CHECK-NEXT:         Kind:            TABLE
 ; CHECK-NEXT:         Table:
 ; CHECK-NEXT:           Index:           0
 ; CHECK-NEXT:           ElemType:        FUNCREF

lld/wasm/Writer.cpp

if (WasmSym::initMemoryFlag) {
     memoryPtr += 4;
   }
 
   if (WasmSym::dataEnd)
     WasmSym::dataEnd->setVirtualAddress(memoryPtr);
 
   log("mem: static data = " + Twine(memoryPtr - dataStart));
 
-  if (config->shared) {
-    out.dylinkSec->memSize = memoryPtr;
+  if (config->isPic) {
+    out.dylinkSec->memSize = memoryPtr - dataStart;
     return;
   }
 
   if (!config->stackFirst)
     placeStack();
 
   // Set `__heap_base` to directly follow the end of the stack or global data.
   // The fact that this comes last means that a malloc/brk implementation
if (config->initialMemory != alignTo(config->initialMemory, WasmPageSize))
       error("initial memory must be " + Twine(WasmPageSize) + "-byte aligned");
     if (memoryPtr > config->initialMemory)
       error("initial memory too small, " + Twine(memoryPtr) + " bytes needed");
     if (config->initialMemory > maxMemorySetting)
       error("initial memory too large, cannot be greater than " +
             Twine(maxMemorySetting));
     memoryPtr = config->initialMemory;
   }
-  out.dylinkSec->memSize = memoryPtr;
   out.memorySec->numMemoryPages =
       alignTo(memoryPtr, WasmPageSize) / WasmPageSize;
   log("mem: total pages = " + Twine(out.memorySec->numMemoryPages));
 
   // Check max if explicitly supplied or required by shared memory
   if (config->maxMemory != 0 || config->sharedMemory) {
     if (config->maxMemory != alignTo(config->maxMemory, WasmPageSize))
       error("maximum memory must be " + Twine(WasmPageSize) + "-byte aligned");