Fix comparison of similarity to threshold in GIST trigram searches.

There was some very strange code here, dating to commit b525bf77, that
purported to work around an ancient gcc bug by forcing a float4 comparison
to be done as int instead.  Commit 5871b8848 broke that when it changed
one side of the comparison to "double" but left the comparison code alone.
Commit f576b17cd doubled down on the weirdness by introducing a "volatile"
marker, which had nothing to do with the actual problem.

Guess that the gcc bug, even if it's still present in the wild, was
triggered by comparison of float4's and can be avoided if we store the
result of cnt_sml() into a double before comparing to the double "nlimit".
This will at least work correctly on non-broken compilers, and it's way
more readable.

Per bug #14202 from Greg Navis.  Add a regression test based on his
example.

Report: <20160620115321.5792.10766@wrigleys.postgresql.org>

contrib/pg_trgm/expected/pg_trgm.out

@@ -3838,3 +3838,56 @@ select * from test2 where t ~ '  z foo';
    z foo bar
 (1 row)
 
+-- Check similarity threshold (bug #14202)
+CREATE TEMP TABLE restaurants (city text);
+INSERT INTO restaurants SELECT 'Warsaw' FROM generate_series(1, 10000);
+INSERT INTO restaurants SELECT 'Szczecin' FROM generate_series(1, 10000);
+CREATE INDEX ON restaurants USING gist(city gist_trgm_ops);
+-- Similarity of the two names (for reference).
+SELECT similarity('Szczecin', 'Warsaw');
+ similarity 
+------------
+          0
+(1 row)
+
+-- Should get only 'Warsaw' for either setting of set_limit.
+EXPLAIN (COSTS OFF)
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Unique
+   ->  Sort
+         Sort Key: city, (similarity(city, 'Warsaw'::text))
+         ->  Bitmap Heap Scan on restaurants
+               Recheck Cond: (city % 'Warsaw'::text)
+               ->  Bitmap Index Scan on restaurants_city_idx
+                     Index Cond: (city % 'Warsaw'::text)
+(7 rows)
+
+SELECT set_limit(0.3);
+ set_limit 
+-----------
+       0.3
+(1 row)
+
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';
+  city  | similarity | show_limit 
+--------+------------+------------
+ Warsaw |          1 |        0.3
+(1 row)
+
+SELECT set_limit(0.5);
+ set_limit 
+-----------
+       0.5
+(1 row)
+
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';
+  city  | similarity | show_limit 
+--------+------------+------------
+ Warsaw |          1 |        0.5
+(1 row)
+

contrib/pg_trgm/sql/pg_trgm.sql

@@ -113,3 +113,24 @@ select * from test2 where t ~ 'z foo bar';
 select * from test2 where t ~ ' z foo bar';
 select * from test2 where t ~ '  z foo bar';
 select * from test2 where t ~ '  z foo';
+
+-- Check similarity threshold (bug #14202)
+
+CREATE TEMP TABLE restaurants (city text);
+INSERT INTO restaurants SELECT 'Warsaw' FROM generate_series(1, 10000);
+INSERT INTO restaurants SELECT 'Szczecin' FROM generate_series(1, 10000);
+CREATE INDEX ON restaurants USING gist(city gist_trgm_ops);
+
+-- Similarity of the two names (for reference).
+SELECT similarity('Szczecin', 'Warsaw');
+
+-- Should get only 'Warsaw' for either setting of set_limit.
+EXPLAIN (COSTS OFF)
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';
+SELECT set_limit(0.3);
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';
+SELECT set_limit(0.5);
+SELECT DISTINCT city, similarity(city, 'Warsaw'), show_limit()
+  FROM restaurants WHERE city % 'Warsaw';

contrib/pg_trgm/trgm_gist.c

@@ -296,16 +296,9 @@ gtrgm_consistent(PG_FUNCTION_ARGS)
 
 			if (GIST_LEAF(entry))
 			{					/* all leafs contains orig trgm */
+				double		tmpsml = cnt_sml(qtrg, key, *recheck);
 
-				/*
-				 * Prevent gcc optimizing the tmpsml variable using volatile
-				 * keyword. Otherwise comparison of nlimit and tmpsml may give
-				 * wrong results.
-				 */
-				float4 volatile tmpsml = cnt_sml(qtrg, key, *recheck);
-
-				/* strange bug at freebsd 5.2.1 and gcc 3.3.3 */
-				res = (*(int *) &tmpsml == *(int *) &nlimit || tmpsml > nlimit);
+				res = (tmpsml >= nlimit);
 			}
 			else if (ISALLTRUE(key))
 			{					/* non-leaf contains signature */