Bdragon28 added a comment. In D79916#2279866 <https://reviews.llvm.org/D79916#2279866>, @jrtc27 wrote:
> In D79916#2279863 <https://reviews.llvm.org/D79916#2279863>, @Bdragon28 wrote: > >> In D79916#2279816 <https://reviews.llvm.org/D79916#2279816>, @jrtc27 wrote: >> >>> In D79916#2279812 <https://reviews.llvm.org/D79916#2279812>, @Bdragon28 >>> wrote: >>> >>>> In D79916#2279045 <https://reviews.llvm.org/D79916#2279045>, @jrtc27 wrote: >>>> >>>>> This has significantly regressed FreeBSD's performance with the new >>>>> version of Clang. It seems Clang does not inline functions at -O1, unlike >>>>> GCC, and since FreeBSD currently compiles its kernel with -O whenever >>>>> debug symbols are enabled[1] (which, of course, is almost always true), >>>>> this results in all its `static inline` helper functions not being >>>>> inlined at all, a pattern that is common in the kernel, used for things >>>>> like `get_curthread` and the atomics implementations. >>>>> >>>>> [1] This is a dubious decision made in r140400 in 2005 to provide "truer >>>>> debugger stack traces" (well, before then there was ping-ponging between >>>>> -O and -O2 based on concerns around correctness vs performance, but amd64 >>>>> is an exception that has always used -O2 since r127180 it seems). Given >>>>> that GCC will inline at -O, at least these days, the motivation seems to >>>>> no longer exist, and compiling a kernel at anything other than -O2 (or >>>>> maybe -O3) seems like a silly thing to do, but nevertheless it's what is >>>>> currently done. >>>>> >>>>> Cc: @dim @trasz >>>> >>>> This is actually SUCH a bad idea that a kernel built with -O will *not >>>> work at all* on 32 bit powerpc platforms (presumably due to allocating >>>> stack frames in the middle of assembly fragments in the memory management >>>> that are supposed to be inlined at all times.) I had to hack kern.pre.mk >>>> to rquest -O2 at all times just to get a functioning kernel. >>> >>> Well, -O0, -O1, -O2 and -O should all produce working kernels, and any >>> cases where they don't are compiler bugs (or kernel bugs if they rely on >>> UB) that should be fixed, not worked around by tweaking the compiler flags >>> in a fragile way until you get the behaviour relied on. Correctness and >>> performance are very different issues here. >> >> As an example: >> >> static __inline void >> mtsrin(vm_offset_t va, register_t value) >> { >> >> __asm __volatile ("mtsrin %0,%1; isync" :: "r"(value), "r"(va)); >> } >> >> This code is used in the mmu when bootstrapping the cpu like so: >> >> for (i = 0; i < 16; i++) >> mtsrin(i << ADDR_SR_SHFT, kernel_pmap->pm_sr[i]); >> powerpc_sync(); >> >> sdr = (u_int)moea_pteg_table | (moea_pteg_mask >> 10); >> __asm __volatile("mtsdr1 %0" :: "r"(sdr)); >> isync(); >> >> tlbia(); >> >> During the loop there, we are in the middle of programming the MMU segment >> registers in real mode, and is supposed to be doing all work out of >> registers. (and powerpc_sync() and isync() should be expanded to their >> single assembly instruction, not a function call. The whole point of calling >> those is that we are in an inconsistent hardware state and need to sync up >> before continuing execution) >> >> If there isn't a way to force inlining, we will have to change to using >> preprocessor macros in cpufunc.h. > > There is, it's called `__attribute__((always_inline))` and supported by both > GCC and Clang. But at -O0 you'll still have register allocation to deal with, > so really that code is just fundamentally broken and should not be written in > C. There is no way for you to guarantee stack spills are not used, it's way > out of scope for C. Is there a way to have always_inline and unused at the same time? I tried using always_inline and it caused warnings in things that used *parts* of cpufunc.h. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D79916/new/ https://reviews.llvm.org/D79916 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
