| Summary: | avoid goto table to reduce code size when optimized for size | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Carrot <carrot> |
| Component: | middle-end | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | NEW --- | ||
| Severity: | enhancement | CC: | ramana |
| Priority: | P3 | Keywords: | missed-optimization |
| Version: | 4.6.0 | ||
| Target Milestone: | --- | ||
| Host: | linux | Target: | arm-linux-androideabi |
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | 2011-02-01 01:19:27 | |
| Attachments: | modified testcase | ||
This is partly rtl-optimizers around the way in which we expand switch tables and partly in the target. |
Created attachment 23040 [details] modified testcase When I compiled the infback.c from zlib 1.2.5 with options -march=armv7-a -mthumb -Os, gcc 4.6 generates following code for a large switch statement: subs r3, r3, #11 cmp r3, #18 bhi .L16 tbh [pc, r3, lsl #1] .L23: .2byte (.L17-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L18-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L154-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L20-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L16-.L23)/2 .2byte (.L21-.L23)/2 .2byte (.L121-.L23)/2 .L17: GCC generates a goto table for 19 cases. The table and the instructions which manipulate it occupies 19*2 + 10 = 48 bytes. Actually most of the targets in the table are same. There are only 6 targets other than .L16. So if we generate a sequence of cmp & br instructions, we need only 6 cmp&br and one br to default, that's only 4*6+2=26 bytes. When I randomly modified the source code, gcc sometimes generate the absolute address in the goto table, double the table size, make result worse. The modified source code is attached.