https://bugs.llvm.org/show_bug.cgi?id=41474
Bug ID: 41474
Summary: Implement __attribute__((noipa)) to make a function a
black box for interprocedural optimizations
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: com...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org,
neeil...@live.com, richard-l...@metafoo.co.uk
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.
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