http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47373
Summary: avoid goto table to reduce code size when optimized
for size
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
Host: linux
Target: arm-linux-androideabi
Created attachment 23040
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23040
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.
