On Wed, Feb 02, 2022 at 06:19:46AM +0100, Thomas Huth wrote:
> On 01/02/2022 20.01, Andrew Jones wrote:
> > When compiling with an ancient compiler (gcc-4.8.5-36.el7_6.2.aarch64)
> > the build fails with
> >
> > lib/libcflat.a(alloc.o): In function `mult_overflow':
> > /home/drjones/kvm-unit-tests/lib/alloc.c:19: undefined reference to `__multi3'
> >
> > According to kernel commit fb8722735f50 ("arm64: support __int128 on
> > gcc 5+") GCC5+ will not emit __multi3 for __int128 multiplication,
> > so let's just fallback to the non-__int128 overflow check when we
> > use gcc versions older than 5.
> >
> > Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx>
> > ---
> > lib/alloc.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/alloc.c b/lib/alloc.c
> > index f4266f5d064e..70228aa32c6c 100644
> > --- a/lib/alloc.c
> > +++ b/lib/alloc.c
> > @@ -1,6 +1,7 @@
> > #include "alloc.h"
> > #include "asm/page.h"
> > #include "bitops.h"
> > +#include <linux/compiler.h>
> > void *malloc(size_t size)
> > {
> > @@ -13,7 +14,7 @@ static bool mult_overflow(size_t a, size_t b)
> > /* 32 bit system, easy case: just use u64 */
> > return (u64)a * (u64)b >= (1ULL << 32);
> > #else
> > -#ifdef __SIZEOF_INT128__
> > +#if defined(__SIZEOF_INT128__) && (!defined(__aarch64__) || GCC_VERSION >= 50000)
> > /* if __int128 is available use it (like the u64 case above) */
> > unsigned __int128 res = a;
> > res *= b;
>
> I'd also be OK if we'd simply declare GCC compiler versions < 5 as
> unsupported ... but since it's an easy fix:
I'd like to continue supporting this particular old compiler, because I
only have one machine where I can run the unit tests with KVM AArch32,
and on that machine I still have an old RHEL (RHEL7-Alt) installed. If
it gets too difficult to support, then I'll try upgrading that machine's
RHEL version or at least its compiler version.
>
> Acked-by: Thomas Huth <thuth@xxxxxxxxxx>
>
Thanks, but I need a v2 since I just remembered clang. For v2, I see two
options:
1) Just change the line as below and hope we don't need to worry about
clang versions
#if defined(__SIZEOF_INT128__) && (!defined(__aarch64__) || defined(__clang__) || GCC_VERSION >= 50000)
2) Replace lib/alloc.c's mult_overflow() with lib/linux/compiler.h's
check_mul_overflow()
(2) seems like the better choice except for the fact that it lazily
doesn't implement a fallback for compilers which don't support the
builtins (and that's older than GCC7.1). Do we care? Of course I
could combine the two implementations if we really want that fallback
and the cleanup.
Thanks,
drew
