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diff --git a/chapter25/README.md b/chapter25/README.md index 8e51c557..bbe0c50e 100644 --- a/chapter25/README.md +++ b/chapter25/README.md @@ -52,6 +52,24 @@ these addresses through the in-memory linker. Thread-local variables and C macro need accessor functions: the system library exposes `libc.errno()` through a small native runtime accessor. +Native function pointers still occupy four bytes. A native library can live +above that address range, so the AMD64 ELF writer gives each address-taken +native function a small entry in Simple's low-address code. The entry tail-jumps +through the native symbol's PLT entry; arguments and the return address pass +through unchanged. Direct calls still use the native symbol directly. + +Static class fields and addresses computed in code both refer to that entry. +Its stable weak symbol, `__simple_native$<name>`, lets separately compiled +objects agree on one pointer when linked into an executable. The class image +uses a four-byte relocation to this defined entry, instead of a four-byte +relocation to a dynamic native symbol in writable data, which GNU ld rejects. +Constant field Stores can still fold away. The entry is a Simple function +identity; it need not equal the raw address obtained by native code. + +`StaticDataTest.testNativeFunctionIdentity` reads two separately compiled class +images before their initializers run, compares their stored and computed +addresses, and calls the native function. It checks both object-link orders. + ## Object files contain ideal IR The ELF writer stores native code and a `.simple` section containing canonical |