nickdesaulniers added a comment. In D104475#2825795 <https://reviews.llvm.org/D104475#2825795>, @MaskRay wrote:
> In D104475#2825772 <https://reviews.llvm.org/D104475#2825772>, > @nickdesaulniers wrote: > >> In D104475#2825711 <https://reviews.llvm.org/D104475#2825711>, @MaskRay >> wrote: >> >>> So if we don't want to offer guarantee for IR/C/C++ attributes, we can >>> document that users may need to additionally specify >>> `__attribute__((noinline))` to suppress inlining. >> >> I don't generally like that approach: > > If a guarantee would cause inferior optimization or we cannot think of all > the complication/fallout initially, I'd prefer that we keep the initial > semantics narrow. The guarantee is more important than the optimization. > If we cannot make a promise, don't provide a promise. > No-suppressing inlining makes the attribute freely composable with other > attributes (https://lists.llvm.org/pipermail/llvm-dev/2021-April/150062.html ) IIUC, isn't @jdoerfert arguing that `optnone` and `noinline` being composable is _bad_? >> 1. it's not easy for developers to validate their call call chains to ensure >> that a caller with a restrictive function attribute doesn't have >> unrestricted callers inlined into the restricted caller. >> 2. that behavior opposes the principle of least surprise. > > Disagree. If the user wants a function not to be inlined, let them add > `__attribute__((noinline))`. > >> 3. We don't have a statement attribute for call sites to say "please don't >> inline this call" which would be fine grain. noinline is a course-grain >> hammer; what if we want to inline a callee into most callers but not this >> one caller that has such a restricted function attribute? > > > >> See also D91816 <https://reviews.llvm.org/D91816> where I took the >> conservative approach for `no_stack_protector` and simply chose not to >> perform such inline substitution on caller and callee function attribute >> mismatch. I find this behavior to be the least surprising, and the >> developer is provided with introspection as to why the compile did not >> perform such inlining via `-Rpass=inline` flag. > > I think D91816 <https://reviews.llvm.org/D91816> was an unfortunate case. > That would require the inliner to understand every possible attribute. Hyperbole. Only a few such function attributes would need such restrictions. > That was the practical approach back then because it is not easy for the > kernel to cope. > For new things we should strive for making the kernel do the right thing. > > For example, if a kernel function can sometimes be inlinable, sometimes not: > add a alias and add `__attribute__((noinline))` to that alias. Interesting idea, but `noinline` is straight up broken in LLVM: https://godbolt.org/z/MoE1a7c3v. The Linux kernel relies on `noinline` meaning don't perform inline substitution. That violates the principal of least surprise. When a developer uses `noinline`, `no_stack_protector`, or `no_profile`, they mean DO NOT inline, insert a stack protector, or insert profiling/coverage instrumentation EVER. Also debugging to find out that inlining was the cause of such violated expectations sucks. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D104475/new/ https://reviews.llvm.org/D104475 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
