[lld][WebAssembly] Don't allow --global-base to be specified in -sh…

…are/-pie or --relocatable modes

Add some checks around this combination of flags

Also, honor `--global-base` when specified in `--stack-first` mode
rather than ignoring it.  But error out if the specified base preseeds
the end of the stack.

Differential Revision: https://reviews.llvm.org/D136117

lld/test/wasm/global-base.test

@@ -1,5 +1,15 @@
 RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/start.s -o %t.o
 
+# Check for error on `--global-base` with `-shared` and `-pie`
+RUN: not wasm-ld --global-base=2048 --experimental-pic -shared -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=SHARED-ERROR
+RUN: not wasm-ld --global-base=2048 --experimental-pic -pie -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=SHARED-ERROR
+SHARED-ERROR: error: --global-base may not be used with -shared/-pie
+
+# Check for error on `--global-base` which is lower than that end of the stack
+# when `--stack-first` is used.
+RUN: not wasm-ld --global-base=2048 --stack-first -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=STACK-ERROR
+STACK-ERROR: error: --global-base cannot be less than stack size when --stack-first is used
+
 RUN: wasm-ld --export=__global_base --export=__data_end --allow-undefined -o %t.wasm %t.o
 RUN: obj2yaml %t.wasm | FileCheck %s  -check-prefix=CHECK-1024
 CHECK-1024:       - Type:            GLOBAL

lld/wasm/Driver.cpp

@@ -428,7 +428,7 @@ static void readConfigs(opt::InputArgList &args) {
   LLVM_DEBUG(errorHandler().verbose = true);
 
   config->initialMemory = args::getInteger(args, OPT_initial_memory, 0);
-  config->globalBase = args::getInteger(args, OPT_global_base, 1024);
+  config->globalBase = args::getInteger(args, OPT_global_base, 0);
   config->maxMemory = args::getInteger(args, OPT_max_memory, 0);
   config->zStackSize =
       args::getZOptionValue(args, OPT_z, "stack-size", WasmPageSize);
@@ -551,6 +551,8 @@ static void checkOptions(opt::InputArgList &args) {
       error("-r and -pie may not be used together");
     if (config->sharedMemory)
       error("-r and --shared-memory may not be used together");
+    if (config->globalBase)
+      error("-r and --global-base may not by used together");
   }
 
   // To begin to prepare for Module Linking-style shared libraries, start
@@ -578,6 +580,10 @@ static void checkOptions(opt::InputArgList &args) {
   if (config->bsymbolic && !config->shared) {
     warn("-Bsymbolic is only meaningful when combined with -shared");
   }
+
+  if (config->globalBase && config->isPic) {
+    error("--global-base may not be used with -shared/-pie");
+  }
 }
 
 // Force Sym to be entered in the output. Used for -u or equivalent.

lld/wasm/Writer.cpp

@@ -226,9 +226,9 @@ static void setGlobalPtr(DefinedGlobal *g, uint64_t memoryPtr) {
 // to each of the input data sections as well as the explicit stack region.
 // The default memory layout is as follows, from low to high.
 //
-//  - initialized data (starting at Config->globalBase)
+//  - initialized data (starting at config->globalBase)
 //  - BSS data (not currently implemented in llvm)
-//  - explicit stack (Config->ZStackSize)
+//  - explicit stack (config->ZStackSize)
 //  - heap start / unallocated
 //
 // The --stack-first option means that stack is placed before any static data.
@@ -257,11 +257,25 @@ void Writer::layoutMemory() {
 
   if (config->stackFirst) {
     placeStack();
+    if (config->globalBase) {
+      if (config->globalBase < memoryPtr) {
+        error("--global-base cannot be less than stack size when --stack-first is used");
+        return;
+      }
+      memoryPtr = config->globalBase;
+    }
   } else {
+    if (!config->globalBase && !config->relocatable && !config->isPic) {
+      // The default offset for static/global data, for when --global-base is
+      // not specified on the command line.  The precise value of 1024 is
+      // somewhat arbitrary, and pre-dates wasm-ld (Its the value that
+      // emscripten used prior to wasm-ld).
+      config->globalBase = 1024;
+    }
     memoryPtr = config->globalBase;
-    log("mem: global base = " + Twine(config->globalBase));
   }
 
+  log("mem: global base = " + Twine(memoryPtr));
   if (WasmSym::globalBase)
     WasmSym::globalBase->setVA(memoryPtr);
 
@@ -1523,9 +1537,6 @@ void Writer::createSyntheticSectionsPostLayout() {
 }
 
 void Writer::run() {
-  if (config->relocatable || config->isPic)
-    config->globalBase = 0;
-
   // For PIC code the table base is assigned dynamically by the loader.
   // For non-PIC, we start at 1 so that accessing table index 0 always traps.
   if (!config->isPic) {