[PATCH] D61634: [clang/llvm] Allow efficient implementation of libc's memory functions in C/C++

[Eli Friedman via Phabricator via cfe-commits]

efriedma added a comment.

I would be careful about trying to over-generalize here.  There are a few different related bits of functionality which seem to be interesting, given the discussion in the llvm-dev thread, here, and in related patches:

1. The ability to specify -fno-builtin* on a per-function level, using function attributes.
2. Improved optimization when -fno-builtin-memcpy is specified.
3. The ability to avoid calls to memcpy for certain C constructs which would naturally be lowered to a memcpy call, like struct assignment of large structs, or explicit calls to __builtin_memcpy().  Maybe also some generalization of this involving other libc/libm/compiler-rt calls.
4. The ability to force the compiler to generate "rep; movs" on x86 without inline asm.

It's not clear to me that all of this should be tied together.  In particular, I'm not sure -fno-builtin-memcpy should imply the compiler never generates a call to memcpy().  On recent x86 chips, you might be able to get away with unconditionally using "rep movs", but generally an efficient memcpy for more than a few bytes is a lot longer than one instruction, and is not something reasonable for the compiler to synthesize inline.

If we're adding new IR attributes here, we should also consider the interaction with LTO.


Repository:
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  https://reviews.llvm.org/D61634/new/

https://reviews.llvm.org/D61634