http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49857
Summary: Put constant switch-tables into flash
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Target: avr
Created attachment 24837
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24837
C source with constant switch statement
For constant switches like
int sw_2 (char x)
{
switch(x)
{
case '0': return -1;
case '1': return 2;
case '2': return 3;
case '3': return 5;
case '4': return 7;
case '5': return 11;
case '6': return 13;
case '7': return 17;
case '8': return 19;
case '9': return 23;
case 'a':return 29;
case 'A':return 29;
}
return -1;
}
avr-gcc 4.6.1 generates a lookup table (-Os -mmcu=atmega8 -S -fverbose-asm)
sw_2:
subi r24,lo8(-(-49)) ; csui.0,
cpi r24,lo8(49) ; csui.0,
brsh .L3 ; ,
mov r30,r24 ; tmp50, csui.0
ldi r31,lo8(0) ; ,
subi r30,lo8(-(CSWTCH.1)) ; tmp50,
sbci r31,hi8(-(CSWTCH.1)) ; tmp50,
ld r24,Z ; tmp51, CSWTCH.1
clr r25 ; D.1929
sbrc r24,7 ; D.1929
com r25 ; D.1929
ret
.L3:
ldi r24,lo8(-1) ; D.1929,
ldi r25,hi8(-1) ; D.1929,
ret
.size sw_2, .-sw_2
.data
.type CSWTCH.1, @object
.size CSWTCH.1, 49
CSWTCH.1:
.byte 2
.byte 3
.byte 5
.byte 7
.byte 11
.byte 13
.byte 17
.byte 19
.byte 23
.byte -1
.byte -1
.byte -1
...
CSWTCH.1 is put in .data (.rodata would not be better) so that the lookup table
ends up in RAM.
The table is constant and could go into flash memory, i.e. .progmem.data.
===================================
avr-gcc -v -Os -fverbose-asm -mmcu=atmega8 -W -Wall -S foo.c # dp
-mmcu=atmega128 foo.c -save-temps -Os -c
Using built-in specs.
COLLECT_GCC=e:\WinAVR\4.6.1\bin\avr-gcc.exe
COLLECT_LTO_WRAPPER=e:/winavr/4.6.1/bin/../libexec/gcc/avr/4.6.1/lto-wrapper.exe
Target: avr
Configured with: ../../gcc.gnu.org/gcc-4_6-branch/configure --target=avr
--prefix=/local/gnu/install/gcc-4.6-mingw32 --host=i586-mingw32
--build=i686-linux-gnu --enable-languages=c,c++ --disable-nls --disable-shared
--with-dwarf2
Thread model: single
gcc version 4.6.1 20110620 (prerelease) (GCC)
GNU C (GCC) version 4.6.1 20110620 (prerelease) (avr)
compiled by GNU C version 3.3.1 (mingw special 20030804-1), GMP version
4.3.2, MPFR version 2.4.2, MPC version 0.8.2
GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32702
