http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51425
Bug #: 51425
Summary: [4.7 Regression] Compiler fails to produce SBIS/SBIC
instructions
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Target: avr
Versions <= 4.6 of avr-gcc produce the SBIC/SBIC (Skip one instruction if Bit
in I/O is Clear/Set), but 4.7 fails to synthesize these instructions for bit
numbers 0..6.
Test program:
#define PORTB (*((unsigned char volatile*) 0x38))
extern char c;
void skip_1 (void)
{
if (PORTB & 0x20) c = 0;
}
void skip_2 (void)
{
if (!(PORTB & 0x20)) c = 0;
}
Compiled with
$ avr-gcc -Os -S -dp -mmcu=atmega128 -fdump-rtl-combine-details
Output with 4.7:
skip_1:
in r24,56-0x20 ; 6 *movqi/4 [length = 1]
sbrs r24,5 ; 9 *sbrx_branchqi [length = 2]
rjmp .L1
sts c,__zero_reg__ ; 11 *movqi/3 [length = 2]
.L1:
ret ; 28 return [length = 1]
skip_2:
in r24,56-0x20 ; 6 *movqi/4 [length = 1]
sbrc r24,5 ; 9 *sbrx_branchqi [length = 2]
rjmp .L6
sts c,__zero_reg__ ; 11 *movqi/3 [length = 2]
.L6:
ret ; 20 return [length = 1]
Output with 4.6.2:
skip_1:
sbis 56-0x20,5 ; 10 *sbix_branch [length = 2]
rjmp .L1
sts c,__zero_reg__ ; 12 *movqi/3 [length = 2]
.L1:
ret ; 22 return [length = 1]
skip_2:
sbic 56-0x20,5 ; 10 *sbix_branch [length = 2]
rjmp .L3
sts c,__zero_reg__ ; 12 *movqi/3 [length = 2]
.L3:
ret ; 20 return [length = 1]
*sbix_branch is a combine pattern that is matched by 4.6:
Trying 6 -> 10:
Successfully matched this instruction:
(set (pc)
(if_then_else (eq (zero_extract:HI (mem/v:QI (const_int 56 [0x38]) [0
MEM[(volatile unsigned char *)56B]+0 S1 A8])
(const_int 1 [0x1])
(const_int 5 [0x5]))
(const_int 0 [0]))
(label_ref:HI 15)
(pc)))
deferring deletion of insn with uid = 6.
modifying insn i3 10 pc={(zero_extract([0x38],0x1,0x5)==0)?L15:pc}
REG_BR_PROB: 0xf3c
deferring rescan insn with uid = 10.
but rejected by 4.7:
Trying 6 -> 9:
Failed to match this instruction:
(set (pc)
(if_then_else (eq (zero_extract:QI (mem/v:QI (const_int 56 [0x38]) [0
MEM[(volatile unsigned char *)56B]+0 S1 A8])
(const_int 1 [0x1])
(const_int 5 [0x5]))
(const_int 0 [0]))
(label_ref:HI 14)
(pc)))
4.7 tries zero_extract:QI whereas 4.6 tried zero_extract:HI so that using QIHI
mode iterator in *sbix_branch/*sbix_branch_tmp instead of HI maybe does the
trick.
