[libc] investigate the usage of `-fno-builtin-foo` for foo.c rather than -ffreestanding

[nickdesaulniers]

cmake/modules/LLVMLibCCompileOptionRules.cmake currently blankets -ffreestanding (for CPU targets) for TUs to avoid creating infinite recursion via things like loop idiom recognition turning the implementation of memcpy into memcpy. I think I'd like to remove -ffreestanding and instead move to building strlen with -fno-builtin-strlen, etc, for all symbols, and remove -ffreestanding. Clang supports a function attribute for this, but gcc does not, so command line flags it is.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96628

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

cmake/modules/LLVMLibCCompileOptionRules.cmake currently blankets `-ffreestanding` (for CPU targets) for TUs to avoid creating infinite recursion via things like loop idiom recognition turning the implementation of memcpy into memcpy. I think I'd like to remove `-ffreestanding` and instead move to building strlen with `-fno-builtin-strlen`, etc, for all symbols, and remove `-ffreestanding`. Clang supports a function attribute for this, but gcc does not, so command line flags it is.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96628

[nickdesaulniers]

There's also the related discussion: https://discourse.llvm.org/t/rfc-libc-ffreestanding-fno-builtin/75883

[jdoerfert]

I think this would solve our libm GPU performance issues. @jhuber6