Hi,
Carl Love is working on a patch to add missing flavors of the
vec_xst_be intrinsic and test cases to cover all flavors. He ran
into a latent bug in swap optimization that this patch addresses.
Swap optimization operates on the principle that a computation can
have swaps removed if all permuting loads are accompanied by a swap,
all permuting stores are accompanied by a swap, and the remaining
vector computations are lane-insensitive or easy to adjust if lanes
are swapped across doublewords.
A new problem that arises with vec_xl_be and vec_xst_be is that the
same swap may accompany both a load and a store, so that removing
that swap changes the semantics of the program. Suppose we have a
vec_xl from *(a+b) followed by a vec_xst_be to *(c+d). The code at
expand time then looks like:
lxvd2x x,a,b
xxswapd x,x
stxvd2x x,c,d
The first two instructions are generated by vec_xl, while the last
is generated by vec_xst_be. Swap optimization removes the xxswapd
because this sequence satisfies the rules, but now we have the same
result as if the vec_xst_be were actually a vec_xst.
To avoid this, this patch marks a computation as unoptimizable if
it contains a swap that is both fed by a permuting load and feeds
into a permuting store.
Bootstrapped and tested on powerpc64le-unknown-linux-gnu for POWER8
with no regressions. Carl has verified this fixes the related
problems in his test cases under development. Is this okay for
trunk?
Thanks,
Bill
2017-12-19 Bill Schmidt <wschm...@linux.vnet.ibm.com>
* config/rs6000/rs6000-p8swap.c (swap_feeds_both_load_and_store):
New function.
(rs6000_analyze_swaps): Mark a web unoptimizable if it contains a
swap associated with both a load and a store.
Index: gcc/config/rs6000/rs6000-p8swap.c
===================================================================
--- gcc/config/rs6000/rs6000-p8swap.c (revision 255801)
+++ gcc/config/rs6000/rs6000-p8swap.c (working copy)
@@ -327,6 +327,38 @@ insn_is_swap_p (rtx insn)
return 1;
}
+/* Return 1 iff UID, known to reference a swap, is both fed by a load
+ and a feeder of a store. */
+static unsigned int
+swap_feeds_both_load_and_store (swap_web_entry *insn_entry)
+{
+ rtx insn = insn_entry->insn;
+ struct df_insn_info *insn_info = DF_INSN_INFO_GET (insn);
+ df_ref def, use;
+ struct df_link *link = 0;
+ rtx_insn *load = 0, *store = 0;
+ unsigned int fed_by_load = 0;
+ unsigned int feeds_store = 0;
+
+ FOR_EACH_INSN_INFO_USE (use, insn_info)
+ {
+ link = DF_REF_CHAIN (use);
+ load = DF_REF_INSN (link->ref);
+ if (insn_is_load_p (load) && insn_is_swap_p (load))
+ fed_by_load = 1;
+ }
+
+ FOR_EACH_INSN_INFO_DEF (def, insn_info)
+ {
+ link = DF_REF_CHAIN (def);
+ store = DF_REF_INSN (link->ref);
+ if (insn_is_store_p (store) && insn_is_swap_p (store))
+ feeds_store = 1;
+ }
+
+ return fed_by_load & feeds_store;
+}
+
/* Return TRUE if insn is a swap fed by a load from the constant pool. */
static bool
const_load_sequence_p (swap_web_entry *insn_entry, rtx insn)
@@ -2029,6 +2061,14 @@ rs6000_analyze_swaps (function *fun)
&& !insn_entry[i].is_swap && !insn_entry[i].is_swappable)
root->web_not_optimizable = 1;
+ /* If we have a swap that is both fed by a permuting load
+ and a feeder of a permuting store, then the optimization
+ isn't appropriate. (Consider vec_xl followed by vec_xst_be.) */
+ else if (insn_entry[i].is_swap && !insn_entry[i].is_load
+ && !insn_entry[i].is_store
+ && swap_feeds_both_load_and_store (&insn_entry[i]))
+ root->web_not_optimizable = 1;
+
/* If we have permuting loads or stores that are not accompanied
by a register swap, the optimization isn't appropriate. */
else if (insn_entry[i].is_load && insn_entry[i].is_swap)
