[lld][WebAssembly] Add new __rodata_start/__rodata_end symbols

This is similar to etext/_etext in the ELF linker.  Its useful in
emscripten to know where the RO data data ends and the data begins (even
though the Wasm format itself has no concept of RO data).

See emscripten-core/emscripten#25939 (reply in thread)

Author: sbc100

lld/test/wasm/export-all.s

@@ -34,24 +34,30 @@ foo:
 # CHECK-NEXT:       - Name:            __data_end
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           2
-# CHECK-NEXT:       - Name:            __stack_low
+# CHECK-NEXT:       - Name:            __rodata_start
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           3
-# CHECK-NEXT:       - Name:            __stack_high
+# CHECK-NEXT:       - Name:            __rodata_end
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           4
-# CHECK-NEXT:       - Name:            __global_base
+# CHECK-NEXT:       - Name:            __stack_low
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           5
-# CHECK-NEXT:       - Name:            __heap_base
+# CHECK-NEXT:       - Name:            __stack_high
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           6
-# CHECK-NEXT:       - Name:            __heap_end
+# CHECK-NEXT:       - Name:            __global_base
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           7
-# CHECK-NEXT:       - Name:            __memory_base
+# CHECK-NEXT:       - Name:            __heap_base
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           8
-# CHECK-NEXT:       - Name:            __table_base
+# CHECK-NEXT:       - Name:            __heap_end
 # CHECK-NEXT:         Kind:            GLOBAL
 # CHECK-NEXT:         Index:           9
+# CHECK-NEXT:       - Name:            __memory_base
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         Index:           10
+# CHECK-NEXT:       - Name:            __table_base
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         Index:           11

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

@@ -91,30 +91,36 @@ _start:
 # CHECK-ALL-NEXT:      - Name:            __data_end
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           4
-# CHECK-ALL-NEXT:      - Name:            __stack_low
+# CHECK-ALL-NEXT:      - Name:            __rodata_start
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           5
-# CHECK-ALL-NEXT:      - Name:            __stack_high
+# CHECK-ALL-NEXT:      - Name:            __rodata_end
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           6
-# CHECK-ALL-NEXT:      - Name:            __global_base
+# CHECK-ALL-NEXT:      - Name:            __stack_low
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           7
-# CHECK-ALL-NEXT:      - Name:            __heap_base
+# CHECK-ALL-NEXT:      - Name:            __stack_high
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           8
-# CHECK-ALL-NEXT:      - Name:            __heap_end
+# CHECK-ALL-NEXT:      - Name:            __global_base
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           9
-# CHECK-ALL-NEXT:      - Name:            __memory_base
+# CHECK-ALL-NEXT:      - Name:            __heap_base
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           10
-# CHECK-ALL-NEXT:      - Name:            __table_base
+# CHECK-ALL-NEXT:      - Name:            __heap_end
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           11
-# CHECK-ALL-NEXT:      - Name:            __wasm_first_page_end
+# CHECK-ALL-NEXT:      - Name:            __memory_base
 # CHECK-ALL-NEXT:        Kind:            GLOBAL
 # CHECK-ALL-NEXT:        Index:           12
+# CHECK-ALL-NEXT:      - Name:            __table_base
+# CHECK-ALL-NEXT:        Kind:            GLOBAL
+# CHECK-ALL-NEXT:        Index:           13
+# CHECK-ALL-NEXT:      - Name:            __wasm_first_page_end
+# CHECK-ALL-NEXT:        Kind:            GLOBAL
+# CHECK-ALL-NEXT:        Index:           14
 # CHECK-ALL-NEXT:  - Type:            CODE
 
 # CHECK-ALL:         Name:            target_features

lld/wasm/Config.h

@@ -173,6 +173,11 @@ struct Ctx {
     // Symbol whose value is the alignment of the TLS block.
     GlobalSymbol *tlsAlign;
 
+    // __rodata_start/__rodata_end
+    // Symbols marking the start/end of readonly data
+    DefinedData *rodataStart;
+    DefinedData *rodataEnd;
+
     // __data_end
     // Symbol marking the end of the data and bss.
     DefinedData *dataEnd;

lld/wasm/Driver.cpp

@@ -988,8 +988,11 @@ static void createOptionalSymbols() {
 
   ctx.sym.dsoHandle = symtab->addOptionalDataSymbol("__dso_handle");
 
-  if (!ctx.arg.shared)
+  if (!ctx.arg.shared) {
     ctx.sym.dataEnd = symtab->addOptionalDataSymbol("__data_end");
+    ctx.sym.rodataStart = symtab->addOptionalDataSymbol("__rodata_start");
+    ctx.sym.rodataEnd = symtab->addOptionalDataSymbol("__rodata_end");
+  }
 
   if (!ctx.isPic) {
     ctx.sym.stackLow = symtab->addOptionalDataSymbol("__stack_low");

lld/wasm/Writer.cpp

@@ -401,7 +401,14 @@ void Writer::layoutMemory() {
       }
     }
 
+    if (ctx.sym.rodataStart && seg->name.starts_with(".rodata") && !ctx.sym.rodataStart->getVA())
+      ctx.sym.rodataStart->setVA(memoryPtr);
+
     memoryPtr += seg->size;
+
+    // Might get set more than once if segment merging is not enabled.
+    if (ctx.sym.rodataEnd && seg->name.starts_with(".rodata"))
+      ctx.sym.rodataEnd->setVA(memoryPtr);
   }
 
   // Make space for the memory initialization flag