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This PR registers the writeout and reset functions for `gcov` for all modules in the PGO runtime, instead of registering them using global constructors in each module. The change is made for AIX only, but the same mechanism works on Linux on Power. When registering such functions using global constructors in each module without `-ffunction-sections`, the AIX linker cannot garbage collect unused undefined symbols, because such symbols are grouped in the same section as the `__sinit` symbol. Keeping such undefined symbols causes link errors (see test case https://github.com/llvm/llvm-project/pull/108570/files#diff-500a7e1ba871e1b6b61b523700d5e30987900002add306e1b5e4972cf6d5a4f1R1 for this scenario). This PR implements the initialization in the runtime, hence avoiding introducing `__sinit` into each module. The implementation adds a new global variable `__llvm_covinit_functions` to each module. This new global variable contains the function pointers to the `Writeout` and `Reset` functions. `__llvm_covinit_functions`'s section is the named section `__llvm_covinit`. The linker will aggregate all the `__llvm_covinit` sections from each module to form one single named section in the final binary. The pair of functions ``` const __llvm_gcov_init_func_struct *__llvm_profile_begin_covinit(); const __llvm_gcov_init_func_struct *__llvm_profile_end_covinit(); ``` are implemented to return the start and end address of this named section in the final binary, and they are used in function ``` __llvm_profile_gcov_initialize() ``` (which is a constructor function in the runtime) so the runtime knows the addresses of all the `Writeout` and `Reset` functions from all the modules. One noticeable implementation detail relevant to AIX is that to preserve the `__llvm_covinit` from the linker's garbage collection, a `.ref` pseudo instruction is inserted into them, referring to the section that contains the `__llvm_gcov_ctr` variables, which are used in the instrumented code. The `__llvm_gcov_ctr` variables did not belong to named sections before, but this PR added them to the `__llvm_gcov_ctr_section` named section, so we can add a `.ref` pseudo instruction that refers to them in the `__llvm_covinit` section.
Compiler-RT ================================ This directory and its subdirectories contain source code for the compiler support routines. Compiler-RT is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. ================================