linux-kernel - [PATCH v9 11/13] perf expr: Propagate constants for binary operations

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Date:   Thu, 23 Sep 2021 00:46:14 -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 11/13] perf expr: Propagate constants for binary operations

When we're computing ID values, if we have constant values then compute
the constant result. For example:
  1 + 2
Previously .val would be set to BOTTOM by union_expr, meaning that
all values are possible. With this change .val is set to 3.
Later changes will use the constant values to hopefully eliminate ID
values that don't need to be computed.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/perf/util/expr.y | 63 ++++++++++++++++++++++++++++++------------
 1 file changed, 45 insertions(+), 18 deletions(-)

diff --git a/tools/perf/util/expr.y b/tools/perf/util/expr.y
index 6aeead54760a..5a295e385914 100644
--- a/tools/perf/util/expr.y
+++ b/tools/perf/util/expr.y
@@ -68,6 +68,12 @@ static void expr_error(double *final_val __maybe_unused,
  */
 #define BOTTOM NAN
 
+/* During computing ids, does val represent a constant (non-BOTTOM) value? */
+static bool is_const(double val)
+{
+	return isfinite(val);
+}
+
 static struct ids union_expr(struct ids ids1, struct ids ids2)
 {
 	struct ids result = {
@@ -77,8 +83,15 @@ static struct ids union_expr(struct ids ids1, struct ids ids2)
 	return result;
 }
 
+/*
+ * If we're not computing ids or $1 and $3 are constants, compute the new
+ * constant value using OP. Its invariant that there are no ids.  If computing
+ * ids for non-constants union the set of IDs that must be computed.
+ */
 #define BINARY_LONG_OP(RESULT, OP, LHS, RHS)				\
-	if (!compute_ids) {						\
+	if (!compute_ids || (is_const(LHS.val) && is_const(RHS.val))) { \
+		assert(LHS.ids == NULL);				\
+		assert(RHS.ids == NULL);				\
 		RESULT.val = (long)LHS.val OP (long)RHS.val;		\
 		RESULT.ids = NULL;					\
 	} else {							\
@@ -86,7 +99,9 @@ static struct ids union_expr(struct ids ids1, struct ids ids2)
 	}
 
 #define BINARY_OP(RESULT, OP, LHS, RHS)					\
-	if (!compute_ids) {						\
+	if (!compute_ids || (is_const(LHS.val) && is_const(RHS.val))) { \
+		assert(LHS.ids == NULL);				\
+		assert(RHS.ids == NULL);				\
 		RESULT.val = LHS.val OP RHS.val;			\
 		RESULT.ids = NULL;					\
 	} else {							\
@@ -163,40 +178,52 @@ expr: NUMBER
 | expr '*' expr { BINARY_OP($$, *, $1, $3); }
 | expr '/' expr
 {
-	if (!compute_ids) {
-		if (fpclassify($3.val) == FP_ZERO) {
-			pr_debug("division by zero\n");
-			YYABORT;
-		}
+	if (fpclassify($3.val) == FP_ZERO) {
+		pr_debug("division by zero\n");
+		YYABORT;
+	} else if (!compute_ids || (is_const($1.val) && is_const($3.val))) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
 		$$.val = $1.val / $3.val;
 		$$.ids = NULL;
 	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
 		$$ = union_expr($1, $3);
 	}
 }
 | expr '%' expr
 {
-	if (!compute_ids) {
-		if (fpclassify($3.val) == FP_ZERO) {
-			pr_debug("division by zero\n");
-			YYABORT;
-		}
+	if (fpclassify($3.val) == FP_ZERO) {
+		pr_debug("division by zero\n");
+		YYABORT;
+	} else if (!compute_ids || (is_const($1.val) && is_const($3.val))) {
+		assert($1.ids == NULL);
+		assert($3.ids == NULL);
 		$$.val = (long)$1.val % (long)$3.val;
 		$$.ids = NULL;
 	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
 		$$ = union_expr($1, $3);
 	}
 }
 | D_RATIO '(' expr ',' expr ')'
 {
-	if (!compute_ids) {
+	if (fpclassify($5.val) == FP_ZERO) {
+		/*
+		 * Division by constant zero always yields zero and no events
+		 * are necessary.
+		 */
+		assert($5.ids == NULL);
+		$$.val = 0.0;
+		$$.ids = NULL;
+		ids__free($3.ids);
+	} else if (!compute_ids || (is_const($3.val) && is_const($5.val))) {
+		assert($3.ids == NULL);
+		assert($5.ids == NULL);
+		$$.val = $3.val / $5.val;
 		$$.ids = NULL;
-		if (fpclassify($5.val) == FP_ZERO) {
-			$$.val = 0.0;
-		} else {
-			$$.val = $3.val / $5.val;
-		}
 	} else {
+		/* LHS and/or RHS need computing from event IDs so union. */
 		$$ = union_expr($3, $5);
 	}
 }
-- 
2.33.0.464.g1972c5931b-goog