perf metric: Don't compute unused events

For a metric like:

  EVENT1 if #smt_on else EVENT2

currently EVENT1 and EVENT2 will be measured and then when the metric is
reported EVENT1 or EVENT2 will be printed depending on the value from
smt_on() during the expr parsing. Computing both events is unnecessary and
can lead to multiplexing as discussed in this thread:

  https://lore.kernel.org/lkml/20201110100346.2527031-1-irogers@google.com/

If the input is constant to certain operators like:

 IDS1 if CONST else IDS2

then the result will be either IDS1 or IDS2 depending on CONST (which
may be evaluated from an entire expression), and so IDS1 or IDS2 may
be discarded avoiding events from being programmed.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: John Garry <john.garry@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandeep Dasgupta <sdasgup@google.com>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20210923074616.674826-13-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

2 files changed, 34 insertions, 7 deletions

diff --git a/tools/perf/tests/expr.c b/tools/perf/tests/expr.c
index 1c881bea7fca..287989321d2a 100644
--- a/tools/perf/tests/expr.c
+++ b/tools/perf/tests/expr.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "util/debug.h"
 #include "util/expr.h"
+#include "util/smt.h"
 #include "tests.h"
 #include <stdlib.h>
 #include <string.h>
@@ -132,6 +133,16 @@ int test__expr(struct test *t __maybe_unused, int subtest __maybe_unused)
 	TEST_ASSERT_VAL("find ids", hashmap__find(ctx->ids, "EVENT2,param=3/",
 						    (void **)&val_ptr));
 
+	/* Only EVENT1 or EVENT2 need be measured depending on the value of smt_on. */
+	expr__ctx_clear(ctx);
+	TEST_ASSERT_VAL("find ids",
+			expr__find_ids("EVENT1 if #smt_on else EVENT2",
+				NULL, ctx, 0) == 0);
+	TEST_ASSERT_VAL("find ids", hashmap__size(ctx->ids) == 1);
+	TEST_ASSERT_VAL("find ids", hashmap__find(ctx->ids,
+						  smt_on() ? "EVENT1" : "EVENT2",
+						  (void **)&val_ptr));
+
 	expr__ctx_free(ctx);
 
 	return 0;
diff --git a/tools/perf/util/expr.y b/tools/perf/util/expr.y
index 5a295e385914..5b878f044f22 100644
--- a/tools/perf/util/expr.y
+++ b/tools/perf/util/expr.y
@@ -123,14 +123,30 @@ start: if_expr
 
 if_expr: expr IF expr ELSE expr
 {
-	if (!compute_ids) {
-		$$.ids = NULL;
-		if (fpclassify($3.val) == FP_ZERO) {
-			$$.val = $5.val;
-		} else {
-			$$.val = $1.val;
-		}
+	if (fpclassify($3.val) == FP_ZERO) {
+		/*
+		 * The IF expression evaluated to 0 so treat as false, take the
+		 * ELSE and discard everything else.
+		 */
+		$$.val = $5.val;
+		$$.ids = $5.ids;
+		ids__free($1.ids);
+		ids__free($3.ids);
+	} else if (!compute_ids || is_const($3.val)) {
+		/*
+		 * If ids aren't computed then treat the expression as true. If
+		 * ids are being computed and the IF expr is a non-zero
+		 * constant, then also evaluate the true case.
+		 */
+		$$.val = $1.val;
+		$$.ids = $1.ids;
+		ids__free($3.ids);
+		ids__free($5.ids);
 	} else {
+		/*
+		 * Value is either the LHS or RHS and we need the IF expression
+		 * to compute it.
+		 */
 		$$ = union_expr($1, union_expr($3, $5));
 	}
 }