[lld] [WebAssembly] Fix __init_tls_base global in coop threading (PR #208597)

[Sam Clegg via llvm-commits]

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@@ -1416,13 +1416,32 @@ void Writer::createInitMemoryFunction() {
         // When we initialize the TLS segment we also set the TLS base.
         // This allows the runtime to use this static copy of the TLS data
         // for the first/main thread.
+        //
+        // Note that for `--cooperative-threading` this additionally configures
+        // the `__init_tls_base` global which is the initial TLS value that can
+        // be used for all new component model tasks. For non-PIC builds this
+        // global's statically known value is now calculated, so it's updated
+        // here. For PIC builds the result of the address computation above is
+        // what's stored into the global.
+        //
+        // Finally, note that a temporary local is used here to ensure that the
+        // result of the addition above can be reused a number of times.
         if (ctx.arg.isMultithreaded() && s->isTLS()) {
           if (ctx.isPic) {
             // Cache the result of the addition in the TLS address local
             writeU8(os, WASM_OPCODE_LOCAL_TEE, "local.tee");
             writeUleb128(os, tlsAddressLocal, "tls address local");
+            if (ctx.arg.libcallThreadContext) {
+              writeU8(os, WASM_OPCODE_GLOBAL_SET, "GLOBAL_SET");
+              writeUleb128(os, ctx.sym.tlsBase->getGlobalIndex(),
+                           "__init_tls_base");
+              writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
+              writeUleb128(os, tlsAddressLocal, "tls address local");
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sbc100 wrote:

I guess it doesn't really matter if the `local.get` comes before or after the `GLOBAL_SET`?   Maybe slightly more maintainable if `local.get` so its very explicit that `tlsAddressLocal` is being assigned to the global?

Also, should it be `global.set` rather then `GLOBAL_SET` to match the other lines here?

https://github.com/llvm/llvm-project/pull/208597