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

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Date:   Thu, 23 Sep 2021 00:46:16 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Paul Clarke <pc@...ibm.com>, kajoljain <kjain@...ux.ibm.com>,
        linux-perf-users@...r.kernel.org
Cc:     Stephane Eranian <eranian@...gle.com>,
        Sandeep Dasgupta <sdasgup@...gle.com>,
        Ian Rogers <irogers@...gle.com>
Subject: [PATCH v9 13/13] perf metric: Avoid events for an 'if' constant result

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@...gle.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