[PATCH v9 13/13] perf metric: Avoid events for an 'if' constant result

Ian Rogers
2021-09-23T07:47:56Z

For a metric like:
CONST if expr else CONST

if the values of CONST are identical then expr doesn't need evaluating,
and events, in order to compute a result.

Signed-off-by: Ian Rogers <irogers@google.com>
---
tools/perf/tests/expr.c | 7 +++++++
tools/perf/util/expr.y | 10 ++++++++++
2 files changed, 17 insertions(+)

diff --git a/tools/perf/tests/expr.c b/tools/perf/tests/expr.c
index 287989321d2a..f1d8411fce12 100644
--- a/tools/perf/tests/expr.c
+++ b/tools/perf/tests/expr.c
@@ -143,6 +143,13 @@ int test__expr(struct test *t __maybe_unused, int subtest __maybe_unused)
smt_on() ? "EVENT1" : "EVENT2",
(void **)&val_ptr));

+ /* The expression is a constant 1.0 without needing to evaluate EVENT1. */
+ expr__ctx_clear(ctx);
+ TEST_ASSERT_VAL("find ids",
+ expr__find_ids("1.0 if EVENT1 > 100.0 else 1.0",
+ NULL, ctx, 0) == 0);
+ TEST_ASSERT_VAL("find ids", hashmap__size(ctx->ids) == 0);
+
expr__ctx_free(ctx);

return 0;
diff --git a/tools/perf/util/expr.y b/tools/perf/util/expr.y
index 5b878f044f22..ba7d3b667fcb 100644
--- a/tools/perf/util/expr.y
+++ b/tools/perf/util/expr.y
@@ -142,6 +142,16 @@ if_expr: expr IF expr ELSE expr
$$.ids = $1.ids;
ids__free($3.ids);
ids__free($5.ids);
+ } else if ($1.val == $5.val) {
+ /*
+ * LHS == RHS, so both are an identical constant. No need to
+ * evaluate any events.
+ */
+ $$.val = $1.val;
+ $$.ids = NULL;
+ ids__free($1.ids);
+ ids__free($3.ids);
+ ids__free($5.ids);
} else {
/*
* Value is either the LHS or RHS and we need the IF expression
--
2.33.0.464.g1972c5931b-goog