In try_to_simplify_bb(), simplification is attempted when a constant
phisrc whose corresponding phi-node control a conditional branch.
In this case, for the path between the phisrc and the conditional
branch, the following OP_PHI is sort of unneeded since the constant
determine which branch will be taken.
If this simplification can be made, it means that the OP_PHI
doesn't depend anymore on the constant phisrc. In fact the OP_PHI's bb
won't be anymore reachable from the constant phisrc's bb.
But currently, the OP_PHI usage is not adjusted and this leads to all
sort of oddities and causes to miss further simplifications.
The situation can be more concretly seen on test-linearize's output,
here an extract of Linux's kernel/futex.c:get_futex_key().
.L594:
...
br %r1294, .L651, .L656
.L651:
phisrc.32 %phi85 <- $1
br .L649
.L656:
...
and.64 %r1340 <- %r1339, $1
phisrc.32 %phi87 <- %r1340
phi.32 %r1343 <- %phi85, %phi87
br %r1343, .L649, .L648
The constant phisrc is %phi85 in .L651, the OP_PHI and the conditional
branch are .L656's last instructions (which have been packed with its
now unique parent which is sign of a problem). One can see that the
OP_PHi seems to depend on %phi85 (it's in its phi_list) but in fact
depends only on %phi87 (.L656 can't be reached from .L651 where %phi85
is defined since after the simplification .L651 directly branches to
one of the .L656's children, here .L649).
The fix consists in adjusting the OP_PHI's usage when the
simplification is made.
On the same code extract we can now see that the situation is more
"normal". In fact the OP_PHI have now been able to be optimized away
together, as well as .L651:
.L594:
...
br %r1294, .L649, .L656
.L656:
...
and.64 %r1340 <- %r1339, $1
br %r1340, .L649, .L648
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
---
flow.c | 2 ++
validation/kill-phi-ttsbb.c | 28 ++++++++++++++++++++++++++++
2 files changed, 30 insertions(+)
create mode 100644 validation/kill-phi-ttsbb.c
diff --git a/flow.c b/flow.c
index fbac9336c..088c21785 100644
--- a/flow.c
+++ b/flow.c
@@ -121,6 +121,8 @@ static int try_to_simplify_bb(struct basic_block *bb, struct instruction *first,
continue;
changed |= rewrite_branch(source, &br->bb_true, bb, target);
changed |= rewrite_branch(source, &br->bb_false, bb, target);
+ if (changed)
+ kill_use(THIS_ADDRESS(phi));
} END_FOR_EACH_PTR(phi);
return changed;
}
diff --git a/validation/kill-phi-ttsbb.c b/validation/kill-phi-ttsbb.c
new file mode 100644
index 000000000..178a65d19
--- /dev/null
+++ b/validation/kill-phi-ttsbb.c
@@ -0,0 +1,28 @@
+int def(void);
+void use(int);
+
+static int foo(int a, int b)
+{
+ int c;
+
+ if (a)
+ c = 1;
+ else
+ c = def();
+
+ if (c)
+ use(1);
+ else
+ use(0);
+}
+
+/*
+ * check-name: kill-phi-ttsbb
+ * check-description:
+ * Verify if OP_PHI usage is adjusted after successful try_to_simplify_bb()
+ * check-command: test-linearize $file
+ * check-output-ignore
+ *
+ * check-output-excludes: phi\\.
+ * check-output-excludes: phisrc\\.
+ */
--
2.11.0
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