// David Li
For switch statement with very large case value range, but the number of actual
cases are not large, gcc generated code seems suboptimal: in some cases, huge
jump table is generated (with lots of duplicated entries), while in other
cases, huge concatenated ifs are generated (the number of compares for a hit
may be more than 3 even with binary search scheme). For those cases, mixed mode
expansion may be preferred -- where both compares and jump tables are used.
// Example 1: huge jump table (> 300 entries)
int g;
int foo(int k)
{
switch (k)
{
case 20: g += 10; break;
case 18: g += 11; break;
case 16: g += 12; break;
case 14: g += 13; break;
case 12: g += 14; break;
case 10: g += 15; break;
case 8: g += 3; break;
case 4: g += 2; break;
case 2: g -= 1; break;
case 0: g += 1; break;
case 120: g += 10; break;
case 118: g += 11; break;
case 116: g += 12; break;
case 114: g += 13; break;
case 112: g += 14; break;
case 110: g += 15; break;
case 119: g += 3; break;
case 121: g += 2; break;
case 122: g -= 1; break;
case 123: g += 1; break;
case 220: g += 10; break;
case 218: g += 11; break;
case 216: g += 12; break;
case 214: g += 13; break;
case 212: g += 14; break;
case 210: g += 15; break;
case 324: g += 10; break;
case 323: g += 10; break;
case 322: g += 10; break;
case 321: g += 10; break;
case 320: g += 10; break;
case 318: g += 11; break;
case 316: g += 12; break;
case 314: g += 13; break;
case 312: g += 14; break;
case 310: g += 15; break;
default: break;
}
return g;
}
Example 2: No jump table used:
int g;
int foo(int k)
{
switch (k)
{
case 20: g += 10; break;
case 18: g += 11; break;
case 16: g += 12; break;
case 14: g += 13; break;
case 12: g += 14; break;
case 10: g += 15; break;
case 8: g += 3; break;
case 4: g += 2; break;
case 2: g -= 1; break;
case 0: g += 1; break;
case 120: g += 10; break;
case 118: g += 11; break;
case 116: g += 12; break;
case 114: g += 13; break;
case 112: g += 14; break;
case 110: g += 15; break;
case 119: g += 3; break;
case 121: g += 2; break;
case 122: g -= 1; break;
case 123: g += 1; break;
case 220: g += 10; break;
case 218: g += 11; break;
case 216: g += 12; break;
case 214: g += 13; break;
case 212: g += 14; break;
case 210: g += 15; break;
case 324: g += 10; break;
case 323: g += 10; break;
case 322: g += 10; break;
case 321: g += 10; break;
case 320: g += 10; break;
case 318: g += 11; break;
case 316: g += 12; break;
case 314: g += 13; break;
case 312: g += 14; break;
case 310: g += 15; break;
case 1324: g += 10; break;
case 1323: g += 10; break;
case 1322: g += 10; break;
case 1321: g += 10; break;
case 1320: g += 10; break;
case 1318: g += 11; break;
default: break;
}
return g;
}
--
Summary: Switch expansion Enh
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: xinliangli at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35362
