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* [Bug c/43088] New: [avr] Suspect optimizer missed code in gcc 4.4.3..
@ 2010-02-16 6:03 uhmgawa at third-harmonic dot com
2010-09-13 5:58 ` [Bug target/43088] " abnikant dot singh at atmel dot com
` (3 more replies)
0 siblings, 4 replies; 7+ messages in thread
From: uhmgawa at third-harmonic dot com @ 2010-02-16 6:03 UTC (permalink / raw)
To: gcc-bugs
I just built a recent toolchain (avr-gcc 4.4.3, binutils
2.20) in hopes of eliminating what appeared to be
a few artifacts apparently escaping optimization even
with -Os relative to a 4.0.2 version. Referencing the dump
below, use of r25 is superfluous as it is cleared at 0x14c,
ANDed with zero at 0x150, and then participates in an
additionally unneeded word wide test operation at 0x152:
00000142 <foo>:
void foo(void)
{
static unsigned char count;
if (++count & 0x3f)
142: 80 91 01 01 lds r24, 0x0101
146: 8f 5f subi r24, 0xFF ; 255
148: 80 93 01 01 sts 0x0101, r24
14c: 90 e0 ldi r25, 0x00 ; 0
14e: 8f 73 andi r24, 0x3F ; 63
150: 90 70 andi r25, 0x00 ; 0
150: 90 70 andi r25, 0x00 ; 0
152: 00 97 sbiw r24, 0x00 ; 0
154: 11 f0 breq .+4 ; 0x15a <foo+0x18>
PORTC &= ~0x1;
156: 40 98 cbi 0x08, 0 ; 8
158: 08 95 ret
else
PORTC |= 0x1;
15a: 40 9a sbi 0x08, 0 ; 8
15c: 08 95 ret
}
This is slightly different (although appearing to derive
from the same code generation logic) than I'd seen with a
4.0.2 toolchain where the first two operations above are
followed by an OR of r25 into r24 as a word width test:
0000013c <foo>:
void foo(void)
{
static unsigned char count;
if (++count & 0x3f)
13c: 80 91 00 01 lds r24, 0x0100
140: 8f 5f subi r24, 0xFF ; 255
142: 80 93 00 01 sts 0x0100, r24
146: 99 27 eor r25, r25
148: 8f 73 andi r24, 0x3F ; 63
14a: 90 70 andi r25, 0x00 ; 0
14c: 89 2b or r24, r25
14e: 11 f0 breq .+4 ; 0x154 <foo+0x18>
PORTC &= ~0x1;
150: 40 98 cbi 0x08, 0 ; 8
152: 08 95 ret
else
PORTC |= 0x1;
154: 40 9a sbi 0x08, 0 ; 8
156: 08 95 ret
}
Both examples above were compiled with:
"-mmcu=atmega48 -nostdlib -Os -funsigned-char"
FWIW -O2 and -O3 give the same results. It seems to be
an artifact of a scalar widening operation during code
generation although can be quietly eliminated post-code
generation given the char width type.
I've seen some similar bug reports but not specifically
related to a bitwise-and operation where more than one
set bit exists in the constant operand. Note if the
constant '3' is replaced with an 'unsigned char'
variable of the same value, the expected minimal
code sequence results (sans the fetch of the added
variable from memory).
-----------------------------------------------------------
Per requested bug report boilerplate:
/home/john/avr/gnu/toolbin//bin/avr-gcc -v -save-temps -c -o rt1.o -c
-mmcu=atmega48 -nostdlib -Os -funsigned-char -D__BUILD_GAS__
-DDATE=\"100215200656\" -g rt1.c
Using built-in specs.
Target: avr
Configured with: ../configure --prefix=/home/john/avr/gnu/toolbin --target=avr
--enable-languages=c,c++ --disable-nls --disable-libssp --with-dwarf2
Thread model: single
gcc version 4.4.3 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-o' 'rt1.o' '-c' '-mmcu=atmega48'
'-nostdlib' '-Os' '-funsigned-char' '-D__BUILD_GAS__' '-DDATE="100215200656"'
'-g'
/home/john/avr/gnu/toolbin/libexec/gcc/avr/4.4.3/cc1 -E -quiet -v -imultilib
avr4 -D__BUILD_GAS__ -DDATE="100215200656" rt1.c -mmcu=atmega48 -funsigned-char
-g -fworking-directory -Os -fpch-preprocess -o rt1.i
ignoring nonexistent directory
"/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/sys-include"
#include "..." search starts here:
#include <...> search starts here:
/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/include
/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/include-fixed
/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-o' 'rt1.o' '-c' '-mmcu=atmega48'
'-nostdlib' '-Os' '-funsigned-char' '-D__BUILD_GAS__' '-DDATE="100215200656"'
'-g'
/home/john/avr/gnu/toolbin/libexec/gcc/avr/4.4.3/cc1 -fpreprocessed rt1.i
-quiet -dumpbase rt1.c -mmcu=atmega48 -auxbase-strip rt1.o -g -Os -version
-funsigned-char -o rt1.s
GNU C (GCC) version 4.4.3 (avr)
compiled by GNU C version 4.4.2 20091222 (Red Hat 4.4.2-20), GMP
version 4.3.1, MPFR version 2.4.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 2cb821fe223eaff7cfe636a2c880b1cc
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-o' 'rt1.o' '-c' '-mmcu=atmega48'
'-nostdlib' '-Os' '-funsigned-char' '-D__BUILD_GAS__' '-DDATE="100215200656"'
'-g'
/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/bin/as
-mmcu=atmega48 -o rt1.o rt1.s
COMPILER_PATH=/home/john/avr/gnu/toolbin/libexec/gcc/avr/4.4.3/:/home/john/avr/gnu/toolbin/libexec/gcc/avr/4.4.3/:/home/john/avr/gnu/toolbin/libexec/gcc/avr/:/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/:/home/john/avr/gnu/toolbin/lib/gcc/avr/:/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/bin/
LIBRARY_PATH=/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/avr4/:/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/lib/avr4/:/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/:/home/john/avr/gnu/toolbin/lib/gcc/avr/4.4.3/../../../../avr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-o' 'rt1.o' '-c' '-mmcu=atmega48'
'-nostdlib' '-Os' '-funsigned-char' '-D__BUILD_GAS__' '-DDATE="100215200656"'
'-g'
--
Summary: [avr] Suspect optimizer missed code in gcc 4.4.3..
Product: gcc
Version: 4.4.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: uhmgawa at third-harmonic dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43088
^ permalink raw reply [flat|nested] 7+ messages in thread[parent not found: <bug-43088-4@http.gcc.gnu.org/bugzilla/>]
* [Bug target/43088] [avr] Suspect optimizer missed code in gcc 4.4.3..
[not found] <bug-43088-4@http.gcc.gnu.org/bugzilla/>
@ 2011-06-10 20:01 ` gjl at gcc dot gnu.org
2011-07-02 21:08 ` gjl at gcc dot gnu.org
1 sibling, 0 replies; 7+ messages in thread
From: gjl at gcc dot gnu.org @ 2011-06-10 20:01 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43088
Georg-Johann Lay <gjl at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
CC| |gjl at gcc dot gnu.org
Known to fail| |
--- Comment #5 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-06-10 19:59:47 UTC ---
Funnc artifact. It occurs only if all low-bits are set, i.e. wich constants
0x3f, 0x1f, 0xf, ... So persumably a middle and pass makes some strange
assumption.
(In reply to comment #1)
> This bug is confirmed. andhi3/andsi3 causing this problem. conditional checks
> in andhi3 and andsi3 need to compare with zero instead of 0xff [etc].
> i.e. in andhi3 we need to replace
> (mask & 0x00ff) != 0xff by (mask & 0x00ff) != 0.
>
> Similarly other checks in andhi3 and andsi3 need to be replaced.
Can you explain?
ANDding on a 8-bit part can only be omitted if the mask is 0xff. Otherwise, the
AND has to be performed, even if the result is known to be always zero.
^ permalink raw reply [flat|nested] 7+ messages in thread* [Bug target/43088] [avr] Suspect optimizer missed code in gcc 4.4.3..
[not found] <bug-43088-4@http.gcc.gnu.org/bugzilla/>
2011-06-10 20:01 ` gjl at gcc dot gnu.org
@ 2011-07-02 21:08 ` gjl at gcc dot gnu.org
1 sibling, 0 replies; 7+ messages in thread
From: gjl at gcc dot gnu.org @ 2011-07-02 21:08 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43088
--- Comment #6 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-07-02 21:07:50 UTC ---
Created attachment 24659
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24659
C source file
Compile with -mmcu=atmega168 -Os.
This file shows the flaw in function foo_3f that compares against 0x3f in
comparison to foo_3e that compares against 0x3e.
If the comparison is against 0x3e the code is fine.
The difference happens in pass combine. For 0x3f, combine tryes ZERO_EXTRACT:
Trying 8, 9 -> 10:
Failed to match this instruction:
(parallel [
(set (cc0)
(compare (zero_extract:HI (reg:QI 43 [ count.3 ])
(const_int 6 [0x6])
(const_int 0 [0]))
(const_int 0 [0])))
(clobber (scratch:QI))
])
Failed to match this instruction:
(set (cc0)
(compare (zero_extract:HI (reg:QI 43 [ count.3 ])
(const_int 6 [0x6])
(const_int 0 [0]))
(const_int 0 [0])))
Failed to match this instruction:
(set (reg:HI 52)
(zero_extract:HI (reg:QI 43 [ count.3 ])
(const_int 6 [0x6])
(const_int 0 [0])))
.............................................
But for 0x3e it tryes AND, which matches (presumably combine-split):
Trying 8, 9 -> 10:
Failed to match this instruction:
(parallel [
(set (cc0)
(compare (and:QI (reg:QI 43 [ count.1 ])
(const_int 62 [0x3e]))
(const_int 0 [0])))
(clobber (scratch:QI))
])
Failed to match this instruction:
(set (cc0)
(compare (and:QI (reg:QI 43 [ count.1 ])
(const_int 62 [0x3e]))
(const_int 0 [0])))
Successfully matched this instruction:
(set (reg:QI 52)
(and:QI (reg:QI 43 [ count.1 ])
(const_int 62 [0x3e])))
Successfully matched this instruction:
(set (cc0)
(compare (reg:QI 52)
(const_int 0 [0])))
deferring deletion of insn with uid = 8.
...
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2011-07-02 21:08 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2010-02-16 6:03 [Bug c/43088] New: [avr] Suspect optimizer missed code in gcc 4.4.3 uhmgawa at third-harmonic dot com
2010-09-13 5:58 ` [Bug target/43088] " abnikant dot singh at atmel dot com
2010-09-20 2:30 ` eric dot weddington at atmel dot com
2010-09-20 4:24 ` abnikant dot singh at atmel dot com
2010-09-20 12:59 ` eric dot weddington at atmel dot com
[not found] <bug-43088-4@http.gcc.gnu.org/bugzilla/>
2011-06-10 20:01 ` gjl at gcc dot gnu.org
2011-07-02 21:08 ` gjl at gcc dot gnu.org
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