[PATCH kvm-unit-tests] arm64: Fix compiling with ancient compiler

[PATCH kvm-unit-tests] arm64: Fix compiling with ancient compiler

[Andrew Jones]

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@redhat.com>
---
 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;
-- 
2.34.1

Re: [PATCH kvm-unit-tests] arm64: Fix compiling with ancient compiler

[Thomas Huth]

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@redhat.com>
> ---
>   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:

Acked-by: Thomas Huth <thuth@redhat.com>

Re: [PATCH kvm-unit-tests] arm64: Fix compiling with ancient compiler

[Andrew Jones]

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@redhat.com>
> > ---
> >   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@redhat.com>
> 

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