Allow MD backend to prevent the optimization of a bcopy() or memmove() of

size 1 (the size being known at compile-time) into an inline mempcpy()
expansion, which will in turn expand into a byte load and store operation.

This expansion loses precious address alignment information at some point
(because everybody knows that you can read a byte from any address, right?),
and this loses bigtime on strict alignment platforms which lack the ability
to accesse bytes directly, such as alpha (unless compiling with -mbwx and
runnning on a BWX-capable cpu).

An example of such call with size 1 is lib/libkvm/kvm.c line 780.

1 files changed, 13 insertions, 4 deletions

diff --git a/gnu/gcc/gcc/builtins.c b/gnu/gcc/gcc/builtins.c
index f6c00447aa6..7a927d2d6da 100644
--- a/gnu/gcc/gcc/builtins.c
+++ b/gnu/gcc/gcc/builtins.c
@@ -3060,10 +3060,19 @@ expand_builtin_memmove (tree arglist, tree type, rtx target,
 	 it is ok to use memcpy as well.  */
       if (integer_onep (len))
 	{
-	  rtx ret = expand_builtin_mempcpy (arglist, type, target, mode,
-					    /*endp=*/0);
-	  if (ret)
-	    return ret;
+#if defined(SUBWORD_ACCESS_P)
+	  if (SUBWORD_ACCESS_P
+	      || (src_align >= BIGGEST_ALIGNMENT
+		  && dest_align >= BIGGEST_ALIGNMENT))
+	    {
+#endif
+	      rtx ret = expand_builtin_mempcpy (arglist, type, target, mode,
+						/*endp=*/0);
+	      if (ret)
+		return ret;
+#if defined(SUBWORD_ACCESS_P)
+	    }
+#endif
 	}
 
       /* Otherwise, call the normal function.  */