On Mon, Apr 11, 2022 at 10:08:15PM +0100, Sergei Trofimovich wrote: > Current linux-5.17.1 on fresh gcc-12 fails to build with errors like: > > ERROR: modpost: "__popcountdi2" [drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko] undefined! > ERROR: modpost: "__popcountdi2" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined! > > It is occasionally seen by others on previous gcc versions as well: > > https://lkml.org/lkml/2021/7/11/261 > https://lkml.org/lkml/2018/10/24/403 > > '__popcountdi2' are inserted by gcc for code like the following > from 'drivers/gpu/drm/amd/display/modules/hdcp/hdcp1_execution.c': > > static inline enum mod_hdcp_status validate_bksv(struct mod_hdcp *hdcp) > { > uint64_t n = 0; > uint8_t count = 0; > u8 bksv[sizeof(n)] = { }; > > memcpy(bksv, hdcp->auth.msg.hdcp1.bksv, sizeof(hdcp->auth.msg.hdcp1.bksv)); > n = *(uint64_t *)bksv; > > /* Here gcc inserts 'count = __builtin_popcount(n);' */ > while (n) { > count++; > n &= (n - 1); > } > > return (count == 20) ? MOD_HDCP_STATUS_SUCCESS : > MOD_HDCP_STATUS_HDCP1_INVALID_BKSV; > } > > Note that gcc can insert it regardless of -mno-* options. Just FYI, this has been added in GCC 9 for https://gcc.gnu.org/PR82479 If the kernel implements its own __popcount?i2, it can perhaps with runtime patching use hw instructions when those are available and fallback to the generic version. Jakub
