https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83262
Bug ID: 83262
Summary: SELECT CASE slower than IF/ELSE
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
Created attachment 42781
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42781&action=edit
Test case with timings
The following test case by Tran Quoc Viet from comp.lang.fortran
run with the arguments
$ ./a.out 100000000 200000000 300000000
compiled with -O3 shows that the Fortran's select case statement
is slower by a factor of 1.33 vs. an if/else form.
Number of input arguments: 3
GOTO: n =100000000, ss =-0.1480800E+10, time = 0.347 (s)
SELECT CASE: n =100000000, ss =-0.1480800E+10, time = 0.199 (s)
IF-Goto: n =100000000, ss =-0.1480800E+10, time = 0.151 (s)
IF-noGoto: n =100000000, ss =-0.1480800E+10, time = 0.151 (s)
IF-ELSE: n =100000000, ss =-0.1480800E+10, time = 0.151 (s)
GOTO: n =200000000, ss =-0.2961600E+10, time = 0.702 (s)
SELECT CASE: n =200000000, ss =-0.2961600E+10, time = 0.401 (s)
IF-Goto: n =200000000, ss =-0.2961600E+10, time = 0.301 (s)
IF-noGoto: n =200000000, ss =-0.2961600E+10, time = 0.301 (s)
IF-ELSE: n =200000000, ss =-0.2961600E+10, time = 0.301 (s)
GOTO: n =300000000, ss =-0.4442400E+10, time = 1.054 (s)
SELECT CASE: n =300000000, ss =-0.4442400E+10, time = 0.602 (s)
IF-Goto: n =300000000, ss =-0.4442400E+10, time = 0.451 (s)
IF-noGoto: n =300000000, ss =-0.4442400E+10, time = 0.452 (s)
IF-ELSE: n =300000000, ss =-0.4442400E+10, time = 0.451 (s)
GOTO costs totally 2.103 (s)
SELECT CASE costs totally 1.202 (s)
IF-Goto costs totally 0.903 (s)
IF-noGoto costs totally 0.903 (s)
IF-ELSE costs totally 0.903 (s)
The Fortran FE translates the SELECT CASE into a switch.
