OP_PHI's target can interfer with it's own source swhen defined
in the same basic block. Such interference can create problem
like the 'swap problem' (which only exist if the phi-node are
'processed' sequentially if they're processed in parallel such
problems don't exist) when phi-nodes are destructed. To avoid
such problems OP_PHI are rewritten as OP_COPY.
if an OP_PHI and it's OP_PHISOURCE are in different basic blocks
no such interference is possible and the copy is not needed.
This patch detect such situation and eliminate these unneeded copies.
Note: during unSSA we're removing the OP_PHI & OP_PHISOURCE
but we need to use the def-use chains between them. We must
thus not use kill_instruction() in OP_PHI (this would break
def-use chains and leave stray OP_PHISOURCE), it's enough
to set their bb to NULL.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
---
unssa.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/unssa.c b/unssa.c
index 2660e94c..50782352 100644
--- a/unssa.c
+++ b/unssa.c
@@ -30,6 +30,32 @@
#include <assert.h>
+static int simplify_phi_node(struct instruction *phi, pseudo_t tmp)
+{
+ pseudo_t target = phi->target;
+ struct pseudo_user *pu;
+ pseudo_t src;
+
+ // verify if this phi can be simplified
+ FOR_EACH_PTR(phi->phi_list, src) {
+ struct instruction *def = src->def;
+
+ if (!def)
+ continue;
+ if (def->bb == phi->bb)
+ return 0;
+ } END_FOR_EACH_PTR(src);
+
+ // no need to make a copy of this one
+ // -> replace the target pseudo by the tmp
+ FOR_EACH_PTR(target->users, pu) {
+ use_pseudo(pu->insn, tmp, pu->userp);
+ } END_FOR_EACH_PTR(pu);
+
+ phi->bb = NULL;
+ return 1;
+}
+
static void replace_phi_node(struct instruction *phi)
{
pseudo_t tmp;
@@ -40,6 +66,9 @@ static void replace_phi_node(struct instruction *phi)
tmp->ident = phi->target->ident;
tmp->def = NULL; // defined by all the phisrc
+ // can we avoid to make of copy?
+ simplify_phi_node(phi, tmp);
+
// rewrite all it's phi_src to copy to a new tmp
FOR_EACH_PTR(phi->phi_list, p) {
struct instruction *def = p->def;
@@ -53,6 +82,9 @@ static void replace_phi_node(struct instruction *phi)
def->target = tmp;
} END_FOR_EACH_PTR(p);
+ if (!phi->bb)
+ return;
+
// rewrite the phi node:
// phi %rt, ...
// to:
--
2.10.2
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