gcc inserts __builtin_popcount, causes 'modpost: "__popcountdi2" ... amdgpu.ko] undefined'

[Jakub Jelinek]

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