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

[Sean Fertile via Phabricator via llvm-commits]

sfertile 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:
> 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.
I had a look at the example. You are right, the code XL generates does need the link-option. The code generated for main uses the large code model with R_TOCU/R_TOCL relocations, but the compiler inserts a bunch of code that still uses small code model R_TOC relocations (namely `._start`, `._thread_init`, and `._C_runtime_startup`). The code gcc generates  for the example links just fine without the extra option as they **only** use large code model relocations.  We need to add `-mcmodel=large` for gcc, and only use `-bbigtoc` for XL.


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

https://reviews.llvm.org/D58250