Eric,
without this patch, the mips code for the memcpy test-case in the patch looks
like this. The loadbytes (lbu) and the storebytes (sb) are considered to alias,
so the scheduler cannot rearrange them in a more optimal order.
...
lui $2,%hi($LC0)
lbu $3,%lo($LC0)($2)
addiu $2,$2,%lo($LC0)
sb $3,0($4)
lbu $3,1($2)
nop
sb $3,1($4)
lbu $2,2($2)
j $31
sb $2,2($4)
...
With the patch, the loadbytes and the storebytes are considered not to alias
because the static const string src argument is marked as read-only, and the
scheduler rearranges them.
...
lui $3,%hi($LC0)
addiu $2,$3,%lo($LC0)
lbu $5,%lo($LC0)($3)
lbu $3,1($2)
lbu $2,2($2)
sb $5,0($4)
sb $3,1($4)
j $31
sb $2,2($4)
...
The patch makes sure that the src_mem computed here in expand_builtin_memcpy is
marked as MEM_READONLY_P:
...
src_mem = get_memory_rtx (src, len);
...
build and reg-tested on i686, arm, and mips.
OK for trunk?
Thanks,
- Tom
2011-10-10 Tom de Vries <[email protected]>
* gcc/emit-rtl.c (set_mem_attributes_minus_bitpos): Set MEM_READONLY_P
for static const strings.
* gcc/testsuite/gcc.dg/memcpy-3.c: New test.
Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c (revision 179662)
+++ gcc/emit-rtl.c (working copy)
@@ -1696,6 +1696,11 @@ set_mem_attributes_minus_bitpos (rtx ref
&& !TREE_THIS_VOLATILE (base))
MEM_READONLY_P (ref) = 1;
+ /* Mark static const strings readonly as well. */
+ if (base && TREE_CODE (base) == STRING_CST && TREE_READONLY (base)
+ && TREE_STATIC (base))
+ MEM_READONLY_P (ref) = 1;
+
/* If this expression uses it's parent's alias set, mark it such
that we won't change it. */
if (component_uses_parent_alias_set (t))
Index: gcc/testsuite/gcc.dg/memcpy-4.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/memcpy-4.c (revision 0)
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-rtl-expand" } */
+
+void
+f1 (char *p)
+{
+ __builtin_memcpy (p, "123", 3);
+}
+
+/* { dg-final { scan-rtl-dump-times "mem/s/u" 3 "expand" { target mips*-*-* } } } */
+/* { dg-final { cleanup-rtl-dump "expand" } } */
