[lld][WebAssembly] Add new __rodata_start/__rodata_end symbols #172102
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@llvm/pr-subscribers-lld @llvm/pr-subscribers-lld-wasm Author: Sam Clegg (sbc100) ChangesThis 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) Full diff: https://github.com/llvm/llvm-project/pull/172102.diff 3 Files Affected:
diff --git a/lld/wasm/Config.h b/lld/wasm/Config.h
index 9d903e0c799db..2c73dabb8c3a0 100644
--- a/lld/wasm/Config.h
+++ b/lld/wasm/Config.h
@@ -173,6 +173,10 @@ struct Ctx {
// Symbol whose value is the alignment of the TLS block.
GlobalSymbol *tlsAlign;
+ // __rodata_end
+ // Symbol marking the end of readonly data (similar to _etext under ELF)
+ DefinedData *rodataEnd;
+
// __data_end
// Symbol marking the end of the data and bss.
DefinedData *dataEnd;
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
index 97e50783985a8..293ebe63fc5e0 100644
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -988,8 +988,10 @@ 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.rodataEnd = symtab->addOptionalDataSymbol("__rodata_end");
+ }
if (!ctx.isPic) {
ctx.sym.stackLow = symtab->addOptionalDataSymbol("__stack_low");
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 9a5b56fc52e2f..9c8b484033834 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -400,8 +400,10 @@ void Writer::layoutMemory() {
setGlobalPtr(tlsBase, memoryPtr);
}
}
-
memoryPtr += seg->size;
+ // Might get set more than once if segment merging is not enabled.
+ if (ctx.sym.rodataEnd)
+ ctx.sym.rodataEnd->setVA(memoryPtr);
}
// Make space for the memory initialization flag
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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)
| DefinedData *rodataStart; | ||
| DefinedData *rodataEnd; | ||
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| // __data_end |
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Any particular reason why __data_start doesn't exist?
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Just historical need I believe.
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| // __rodata_start/__rodata_end | ||
| // Symbols marking the start/end of readonly data | ||
| DefinedData *rodataStart; |
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Kind of unfortunate that we have __stack_low, __heap_base, and __rodata_start. But I guess it's too late to fix the inconsistency.
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Hmm.. that is kind of sad/absurd I agree :)
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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)