[clang] [compiler-rt] [llvm] [AIX] Implement the ifunc attribute. (PR #153049)

[Wael Yehia via llvm-commits]

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@@ -0,0 +1,22 @@
+typedef void* Ptr;
+typedef struct { Ptr addr, toc, env; } Descr;
+typedef struct { Descr* desc; Ptr (*resolver)(); } IFUNC_PAIR;
+
+// A zero-length entry in section "ifunc_sec" to satisfy the __start_ifunc_sec
+// and __stop_ifunc_sec references in this file, when no user code has any.
+__attribute__((section("ifunc_sec"))) static int dummy_ifunc_sec[0];
+
+extern IFUNC_PAIR __start_ifunc_sec, __stop_ifunc_sec;
+
+__attribute__((constructor))
+void __init_ifuncs() {
+  void *volatile ref = &dummy_ifunc_sec; // hack to keep dummy_ifunc_sec alive
+
+  // hack to prevent compiler from assuming __start_ifunc_sec and
+  // __stop_ifunc_sec occupy different addresses.
+  IFUNC_PAIR *volatile volatile_end = &__stop_ifunc_sec;
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w2yehia wrote:

I think the compiler assumes that two variable (non-alias) declarations cannot point to the same symbol, so the address of each variable is unique, so `&__start_ifunc_sec != &__stop_ifunc_sec` and so the for-loop can be transformed into a do-while loop (which executes at least once). So I used volatile to prevent the compiler making that assumption, and also this code forces a volatile load (of variable `volatile_end`) once rather than on every loop condition check.

https://github.com/llvm/llvm-project/pull/153049