+linuxppc list
Le 17/06/2025 à 08:32, Eric Biggers a écrit :
On Tue, Jun 17, 2025 at 07:22:57AM +0200, Christophe Leroy wrote:
Le 17/06/2025 à 02:57, Ian Rogers a écrit :
Type punning is necessary for get/put unaligned but the use of a
packed struct violates strict aliasing rules, requiring
-fno-strict-aliasing to be passed to the C compiler. Switch to using
memcpy so that -fno-strict-aliasing isn't necessary.
VDSO build fails with this patch:
VDSO32L arch/powerpc/kernel/vdso/vdso32.so.dbg
arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not
supported
make[2]: *** [arch/powerpc/kernel/vdso/Makefile:79:
arch/powerpc/kernel/vdso/vdso32.so.dbg] Error 1
Behind the relocation issue, calling memcpy() for a single 4-bytes word
kills performance.
memcpy() does normally do the right thing for unaligned accesses of 1, 2, 4, or
8-byte values. The snag here seems to be that the VDSO is built with
-fno-builtin (and -ffreestanding which implies -fno-builtin). That causes the
compiler to no longer optimize out the calls to memcpy(). If __builtin_memcpy()
is used instead of memcpy(), it does work and generates the same code as before.
Yes right, changing memcpy() to __builtin_memcpy() seems to work, the
exact same code is generated.
Christophe
