[lkml]   [2019]   [Aug]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: objtool warning "uses BP as a scratch register" with clang-9
On Thu, Aug 29, 2019 at 8:30 PM Linus Torvalds
<> wrote:
> On Thu, Aug 29, 2019 at 10:35 AM Josh Poimboeuf <> wrote:
> So:
> - we do want "memcpy()" to become "__builtin_memcpy()" which can then
> be optimized to either individual inlined assignments _or_ to an
> out-of-line call to memcpy().
> - we do *not* want individual assignments to be randomly turned into
> memset/memcpy(), because of various different reasons (including
> function tracing, but also store tearing, yadda yadda)
> Conceptually, "-ffreestanding" is definitely what a kernel needs, but
> it has been *too* big of a hammer and disables real code generation,
> iirc.

I just tried passing just "-fno-builtin-memcpy -fno-builtin-memset".
This avoids going all the way to -ffreestanding and prevents the insertion
of unwanted memcpy and memset calls, but unfortunately (and
unsurprisingly) it also prevents the optimization of trivial memset calls.

On the other hand, I could not produce any trivial case like this without

clang seems to behave similarly to gcc here, it will produce
calls to memset or memcpy when setting a lot of adjacent
members (17 for x86-clang, 29 for arm64 gcc), but not for two
or three of them. x86 gcc appears to always use string instructions
over memset().

Maybe we can just pass -fno-builtin-memcpy -fno-builtin-memset
for clang when CONFIG_KASAN is set and hope for the best?


 \ /
  Last update: 2019-08-29 22:22    [W:0.136 / U:6.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site