On Thu, Oct 19, 2023 at 01:33:05PM +0900, Sergey Senozhatsky wrote: > On (23/10/18 15:20), Mike Kravetz wrote: > > > I think you need to initialize h, otherwise what value is passed to > > > prep_and_add_bootmem_folios if the loop is not run because the list is > > > empty. The compiler sees `h` is only given a value in the loop, so > > > the loop must be run. That's obviously hazardous, but the compiler > > > assumes there's no UB. At least that's my limited understanding > > > looking at the IR diff Nathan got me in > > > https://github.com/ClangBuiltLinux/linux/issues/1946. > > > > Thanks for looking closer at this Nick and Nathan! > > > > I think you are saying the compiler is running the loop because it wants > > to initialize h before passing the value to another function. It does > > this even if the explicit loop entry condition is false. Is that correct? > > The loop is getting promoted to "infinite" loop, there is no > &pos->member != (head) condition check in the generated code > at all (at least on my machine). > > I wish we could at least get the "possibly uninitialized variable" > warning from the compiler in this case, which we'd translate to > "hold my beer, I'm going to try one thing". GCC would warn about this under -Wmaybe-uninitialized but it has been disabled in a normal build for the past three years, see commit 78a5255ffb6a ("Stop the ad-hoc games with -Wno-maybe-initialized"). In function 'gather_bootmem_prealloc', inlined from 'hugetlb_init' at mm/hugetlb.c:4299:2: mm/hugetlb.c:3203:9: warning: 'h' may be used uninitialized [-Wmaybe-uninitialized] 3203 | prep_and_add_allocated_folios(h, &folio_list); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ mm/hugetlb.c: In function 'hugetlb_init': mm/hugetlb.c:3166:24: note: 'h' was declared here 3166 | struct hstate *h, *prev_h = NULL; | ^ Clang's -Wconditional-uninitialized would have flagged it too but it suffers from the same problems as -Wmaybe-uninitialized. mm/hugetlb.c:3203:32: warning: variable 'h' may be uninitialized when used here [-Wconditional-uninitialized] 3203 | prep_and_add_allocated_folios(h, &folio_list); | ^ mm/hugetlb.c:3166:18: note: initialize the variable 'h' to silence this warning 3166 | struct hstate *h, *prev_h = NULL; | ^ | = NULL I know clang has some handling for loops in -Wsometimes-uninitialized, I wonder why that does not trigger here... Cheers, Nathan
