[PATCH] D58250: [AIX][CMake] Changes for building on AIX with XL and GCC

[Hubert Tong via Phabricator via llvm-commits]

hubert.reinterpretcast added inline comments.


================
Comment at: llvm/cmake/modules/HandleLLVMOptions.cmake:138
+if(${CMAKE_SYSTEM_NAME} MATCHES "AIX")
+  append("-Wl,-bbigtoc"
+    CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
----------------
hubert.reinterpretcast wrote:
> sfertile wrote:
> > I'm interested in why you choose bigtoc linker option over using large code model? From the little I understand about the bigtoc option it seems that large code model would be the preferable solution when expecting to have a toc that is a lot bigger then 64K.
> `-fPIC` is already specified. Even if the code is generated using the large code model, TOC overflow is a hard error for the linker unless if `-bbigtoc` is specified. On linking, the XL compiler specifies `-bbigtoc` to the linker in response `-qpic=large`.
> 
> You can see this for yourself if you compile the following with `-q64 -qpic=large` and link with only `-q64`:
> ```
> #define x2A( BASE )  BASE ## a, BASE ## b
> #define x2B( BASE )  x2A(BASE ## a), x2A(BASE ## b)
> #define x2C( BASE )  x2B(BASE ## a), x2B(BASE ## b)
> #define x2D( BASE )  x2C(BASE ## a), x2C(BASE ## b)
> #define x2E( BASE )  x2D(BASE ## a), x2D(BASE ## b)
> #define x2F( BASE )  x2E(BASE ## a), x2E(BASE ## b)
> #define x2G( BASE )  x2F(BASE ## a), x2F(BASE ## b)
> #define x2H( BASE )  x2G(BASE ## a), x2G(BASE ## b)
> #define x2I( BASE )  x2H(BASE ## a), x2H(BASE ## b)
> #define x2J( BASE )  x2I(BASE ## a), x2I(BASE ## b)
> #define x2K( BASE )  x2J(BASE ## a), x2J(BASE ## b)
> 
> #define x2KA( BASE )  x2K(BASE ## a), x2K(BASE ## b)
> #define x2KB( BASE )  x2KA(BASE ## a), x2KA(BASE ## b)
> int x2KB( x );
> 
> int main(void) { x2KB( ++x ); }
> ```
Okay, it seems `-fPIC` does not enable the large code model for GCC on AIX (but does for the new XL compilers). `-mcmodel=large` should be added for GCC; however, the linker option remains necessary.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D58250/new/

https://reviews.llvm.org/D58250