On Thu, May 14, 2020 at 3:04 PM Gustavo A. R. Silva <[email protected]> wrote: > > On Thu, May 14, 2020 at 12:06:48PM -0700, Ian Rogers wrote: > > On Thu, May 14, 2020 at 8:01 AM Gustavo A. R. Silva > > <[email protected]> wrote: > > > > > > On Thu, May 14, 2020 at 10:10:30AM -0300, Arnaldo Carvalho de Melo wrote: > > > > Em Wed, May 13, 2020 at 06:47:38PM -0500, Gustavo A. R. Silva escreveu: > > > > > Fix the following build failure generated with command > > > > > $ make CC=clang HOSTCC=clang -C tools/ perf: > > > > > > > > > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized > > > > > type 'struct branch_stack' not at the end of a struct or class is a > > > > > GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end] > > > > > struct branch_stack br_stack; > > > > > ^ > > > > > 1 error generated. > > > > > > > > > > Fix this by reordering the members of struct br. > > > > > > > > Yeah, I noticed that as far back as with ubuntu 16.04's clang: > > > > > > > > clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final) > > > > > > > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized > > > > type 'struct branch_stack' not at the end of a struct or class is a GNU > > > > extension [-Werror,-Wgnu-variable-sized-type-not-at-end] > > > > struct branch_stack br_stack; > > > > ^ > > > > 1 error generated. > > > > > > > > > > > > Will fold this with the bug introducing the problem to avoid bisection > > > > problems. > > > > > > > > > > I agree. Also, the commit hash of the "Fixes" tag only applies to the > > > perf/core branch and, I guess that might create confusion. > > > > > > So while this fixes the warning I believe it breaks the intent of the code. > > > > tools/perf/util/branch.h: > > struct branch_stack { > > u64 nr; > > u64 hw_idx; > > struct branch_entry entries[]; > > }; > > > > tools/perf/util/intel-pt.c: > > struct { > > struct branch_stack br_stack; > > struct branch_entry entries[LBRS_MAX]; > > } br; > > > > The array in br is trying to extend branch_stack's entries array. You > > might have to do something like: > > > > alignas(alignof(branch_stack)) char storage[sizeof(branch_stack) + > > sizeof(branch_entry) * LBRS_MAX]; > > struct branch_stack *br = &storage; > > > > malloc/free may be nicer on the eyeballs. > > > > Yep, I'd go for zalloc/free. There are a couple of places where dynamic > memory is being allocated for struct branch_stack: > > tools/perf/util/cs-etm.c-256- if (etm->synth_opts.last_branch) { > tools/perf/util/cs-etm.c:257: size_t sz = sizeof(struct > branch_stack); > tools/perf/util/cs-etm.c-258- > tools/perf/util/cs-etm.c-259- sz += etm->synth_opts.last_branch_sz * > tools/perf/util/cs-etm.c-260- sizeof(struct branch_entry); > tools/perf/util/cs-etm.c-261- tidq->last_branch = zalloc(sz); > > tools/perf/util/thread-stack.c-148- if (br_stack_sz) { > tools/perf/util/thread-stack.c:149: size_t sz = sizeof(struct > branch_stack); > tools/perf/util/thread-stack.c-150- > tools/perf/util/thread-stack.c-151- sz += br_stack_sz * > sizeof(struct branch_entry); > tools/perf/util/thread-stack.c-152- ts->br_stack_rb = zalloc(sz); > > there is even function intel_pt_alloc_br_stack(). > > Just out of curiosity, why the need of such a hack in this case (the > on-stack extension of branch_stack's entries array)?
My guess would be that the lbr size is an architectural constant and so avoiding malloc/free in what could be a hot loop was desirable. As this is part of a larger patch set, is this the only place this problem has been encountered? Perhaps a macro could perform the complicated stack allocation I suggested. It may be nice to save cycles if code this pattern is widespread and the code hot. Thanks, Ian > Thanks > -- > Gustavo >
