RE: [PATCH 3/3] RISC-V: memcpy() align dest when misaligned access is prohibited

I believe there is a config parameter to newlib, which you can use if your toolchain does not support the conventional macros.
I do not know them off by hard, but maybe this pointer helps you already.

> -----Original Message-----
> From: Sebastian Huber <[email protected]>
> Sent: Monday, 30 June 2025 11:41
> To: ma mandourr <[email protected]>
> Cc: newlib <[email protected]>; Christian Herber (OSS)
> <[email protected]>
> Subject: Re: [PATCH 3/3] RISC-V: memcpy() align dest when misaligned access is
> prohibited
> 
> Hello Mahmoud Abumandour,
> 
> I use Newlib on a Microchip PolarFire SoC which doesn't support misaligned
> access. I get exceptions (mcause is 4 or 6) with the current Newlib. I use the
> following machine options with GCC 13:
> 
> -march=rv64imac -mabi=lp64 -mcmodel=medany -mstrict-align
> 
> It seems that neither __riscv_misaligned_slow nor __riscv_misaligned_avoid is
> defined by GCC 13, so the workaround is not activated. This define is only
> available in GCC 14 and later.
> 
> Would it make sense to change this check to
> 
> #if !defined(__riscv_misaligned_fast)
> 
> instead of
> 
> #if defined(__riscv_misaligned_slow) || defined(__riscv_misaligned_avoid)
> 
> ?
> 
> ----- Am 27. Apr 2025 um 13:41 schrieb ma mandourr [email protected]:
> 
> > From: Mahmoud Abumandour <[email protected]>
> >
> > Add a code path for when source and dest are differently aligned.
> >
> > If misaligned access is slow or prohibited, and the alignments of the
> > source and destination are different, we align the destination to do
> > XLEN stores. This uses only one aligned store for every four (or eight
> > for XLEN == 64) bytes of data.
> >
> > Reviewed-by: Christian Herber <[email protected]>
> > Signed-off-by: Mahmoud Abumandour <[email protected]>
> > ---
> > newlib/libc/machine/riscv/memcpy.c | 72 +++++++++++++++++++++++++-----
> > 1 file changed, 60 insertions(+), 12 deletions(-)
> >
> > diff --git a/newlib/libc/machine/riscv/memcpy.c
> > b/newlib/libc/machine/riscv/memcpy.c
> > index 427005efd..5d6b2f301 100644
> > --- a/newlib/libc/machine/riscv/memcpy.c
> > +++ b/newlib/libc/machine/riscv/memcpy.c
> > @@ -16,7 +16,6 @@
> >
> > #include "../../string/local.h"
> > #include "xlenint.h"
> > -#include <stdint.h>
> > #include <string.h>
> > #include <sys/asm.h>
> >
> > @@ -31,6 +30,30 @@ __libc_memcpy_bytewise (unsigned char *dst, const
> > unsigned char *src,
> >     *dst++ = *src++;
> > }
> >
> > +#if defined(__riscv_misaligned_slow) ||
> > +defined(__riscv_misaligned_avoid) static uintxlen_t __libc_load_xlen
> > +(const void *src) {
> > +  const unsigned char *p = (const unsigned char *)src;
> > +  uintxlen_t ret = 0;
> > +  unsigned char b0 = *p++;
> > +  unsigned char b1 = *p++;
> > +  unsigned char b2 = *p++;
> > +  unsigned char b3 = *p++;
> > +  ret = (uintxlen_t)b0 | ((uintxlen_t)b1 << 8) | ((uintxlen_t)b2 << 16)
> > +        | ((uintxlen_t)b3 << 24);
> > +#if __riscv_xlen == 64
> > +  unsigned char b4 = *p++;
> > +  unsigned char b5 = *p++;
> > +  unsigned char b6 = *p++;
> > +  unsigned char b7 = *p++;
> > +  ret |= ((uintxlen_t)b4 << 32) | ((uintxlen_t)b5 << 40)
> > +         | ((uintxlen_t)b6 << 48) | ((uintxlen_t)b7 << 56); #endif
> > +  return ret;
> > +}
> > +#endif
> > +
> > void *
> > __inhibit_loop_to_libcall
> > memcpy (void *__restrict aa, const void *__restrict bb, size_t n) @@
> > -39,23 +62,51 @@ memcpy (void *__restrict aa, const void *__restrict
> > bb, size_t n)
> >   const unsigned char *b = (const unsigned char *)bb;
> >   unsigned char *end = a + n;
> >   uintptr_t msk = SZREG - 1;
> > -#if __riscv_misaligned_slow || __riscv_misaligned_fast
> >   if (n < SZREG)
> > -#else
> > -  if (unlikely ((((uintptr_t)a & msk) != ((uintptr_t)b & msk)) || n <
> > SZREG)) -#endif
> >     {
> >       if (__builtin_expect (a < end, 1))
> >         __libc_memcpy_bytewise (a, b, n);
> >       return aa;
> >     }
> >
> > +/*
> > + * If misaligned access is slow or prohibited, and the alignments of
> > +the source
> > + * and destination are different, we align the destination to do XLEN stores.
> > + * This uses only one aligned store for every four (or eight for XLEN
> > +== 64)
> > + * bytes of data.
> > + */
> > +#if defined(__riscv_misaligned_slow) ||
> > +defined(__riscv_misaligned_avoid)
> > +  if (unlikely ((((uintptr_t)a & msk) != ((uintptr_t)b & msk))))
> > +    {
> > +      size_t dst_pad = (uintptr_t)a & msk;
> > +      dst_pad = (SZREG - dst_pad) & msk;
> > +      __libc_memcpy_bytewise (a, b, dst_pad);
> > +      a += dst_pad;
> > +      b += dst_pad;
> > +
> > +      uintxlen_t *la = (uintxlen_t *)a;
> > +      const unsigned char *cb = (const unsigned char *)b;
> > +      uintxlen_t *lend = (uintxlen_t *)((uintptr_t)end & ~msk);
> > +
> > +      while (la < lend)
> > +        {
> > +          *la++ = __libc_load_xlen (cb);
> > +          cb += SZREG;
> > +        }
> > +      a = (unsigned char *)la;
> > +      b = (const unsigned char *)cb;
> > +      if (unlikely (a < end))
> > +        __libc_memcpy_bytewise (a, b, end - a);
> > +      return aa;
> > +    }
> > +#endif
> > +
> >   if (unlikely (((uintptr_t)a & msk) != 0))
> >     {
> > -      size_t rem = SZREG - ((uintptr_t)a & msk);
> > -      __libc_memcpy_bytewise (a, b, rem);
> > -      a += rem;
> > -      b += rem;
> > +      size_t pad = SZREG - ((uintptr_t)a & msk);
> > +      __libc_memcpy_bytewise (a, b, pad);
> > +      a += pad;
> > +      b += pad;
> >     }
> >
> >   uintxlen_t *la = (uintxlen_t *)a;
> > @@ -87,9 +138,6 @@ memcpy (void *__restrict aa, const void *__restrict
> > bb, size_t n)
> >         }
> >     }
> >
> > -  while (la < lend)
> > -      *la++ = *lb++;
> > -
> >   a = (unsigned char *)la;
> >   b = (const unsigned char *)lb;
> >   if (unlikely (a < end))
> > --
> > 2.43.0
> 
> --
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