Implement __attribute__((noipa)) to make a function a black box for interprocedural optimizations

[comex]

Bugzilla Link	41474
Version	trunk
OS	All
CC	@hfinkel,@zygoloid

Extended Description

Currently, Clang will always optimize this to return 3:

static int inner(int a) { return a + 1; } int outer() { return inner(2); }

Notably, this happens even if inner is marked noinline, because of the Interprocedural Sparse Conditional Constant Propagation pass.

This is unhelpful for various use cases, including:

• testing the compiler's code generation
• defining a dummy function whose purpose is to be a place to break in a debugger

GCC has a function attribute noipa, whose effect is (to quote the manual):

Disable interprocedural optimizations between the function with this attribute and its callers, as if the body of the function is not available when optimizing callers and the callers are unavailable when optimizing the body.

It would be good to support this in LLVM/Clang as well.

[dwblaikie]

Some work/discussion around this did happen last year: https://discourse.llvm.org/t/revisiting-refining-the-definition-of-optnone-with-interprocedural-transformations/58095/11 https://reviews.llvm.org/D101011