Wouter van Gulik wrote:
David Brown schreef:
Wouter van Gulik wrote:
David Brown schreef:
Weddington, Eric wrote:
-----Original Message----- From:
avr-libc-dev-bounces+eric.weddington=atmel....@nongnu.org
[mailto:avr-libc-dev-bounces+eric.weddington=atmel....@nongnu. org]
On Behalf Of Dmitry K. Sent: Sunday, November 22, 2009 12:21 AM To:
avr-libc-dev@nongnu.org Subject: Re: [avr-libc-dev]
eeprom_read_byte and clr ret_hi
eeprom_read_byte returns a uint8_t. Why does it clear r25?
eerd_byte.S: clr ret_hi
Does the AVR ABI require that r25 be zeroed in a function
returning a
single byte? If not, this instruction could be removed.
This is a misty point. Look an example:
unsigned char foo1 (unsigned char *p) { return *p; }
extern unsigned char ext2 (void); int foo2 (void) { return ext2() +
1; }
Old Avr-gcc (3.3 - 4.2) are clear R25 in both cases: foo1() and
foo2(). The new Avr-gcc (4.3.3 and 4.4.2) are not clear R25 in
foo1().
Note, the function return value is present only in expression. So
it is promoted to integer. So it would be better to clear R25 in
foo1() only (at one place).
I agree that that is the way it should be.
I'm a little confused by this - I hope this is not implying a return
to avr-gcc 4.2 R25 clearing? With avr-gcc 4.2, foo1() above would
clear R25 even though it is returning an 8-bit value. This has
always been a waste of time and space - caller functions make no use
of the cleared R25, and thus clear it again themselves (such as in
foo2() after calling ext2()). With avr-gcc 4.3, the extra clear R25
instructions are omitted for functions returning 8-bit values. This
is the way it should be (unless the C standards disagree...), IMHO.
But it looks a little like you want foo1() to clear R25 here?
Incidentally, avr-gcc 4.2.2 actually produces better code for foo2()
than avr-gcc 4.3.2.
With 4.2.2, the code is:
foo2:
call ext2
ldi R25, lo8(0)
adiw r24,1
ret
With 4.3.2 (and 4.3.0), we get:
foo2:
call ext2
mov r18, r24
ldi r19, lo8(0)
subi r18, lo8(-(1))
sbci r19, hi8(-(1))
movw r24, r18
ret
Is this regression is news to you, I can take it up in the main
avr-gcc mailing list and/or a missed optimisation bug report.
This is "normal" GCC behaviour, caused by internal promotion to int
types. Having the clr r25 would have saved the ldi r19. Altough these
type of missed optimization are known for long, it (apparently) is
difficult to fix them. IIRC the main problem is the carry bit
propagation. I still don't now why exactly that is such a big
problem, but then again I am no gcc expert.
I understand about int promotion, and how it's a PITA to remove all
the extra code that the avr backend has to put in because the gcc
frontend has promoted the 8-bit data to 16-bit ints. However, avr-gcc
4.2 /does/ do a clr R25 in the function returning an 8-bit result, and
then it clears it /again/ after calling it (as in foo2 above). That
is definitely a waste, and it has been removed in 4.3+.
The problem I have with the 4.3.2 code above is not the zeroing of the
upper byte - that's standard int promotion, and is required for
correct code. The problem is that the result of ext2() is first moved
into a new register pair and then promoted to an int - an unnecessary
move, which forces the subi/sbci pair instead of subw, and requires an
additional move before the ret instruction. In more complex code, the
wasted register resources may also be relevant.
You're right, I was not grasping the real problem. However using
-fno-split-wide-types makes 4.3.2 behave like 4.2.2. Split wide types
seems to be the problem here.
You are correct - I hadn't thought of that. However,
-fno-split-wide-types is a workaround, rather than a solution. Ideally,
the good code should be produced regardless of that flag, since
split-wide-types is enabled implicitly by all -Ox flags. The
split-wide-types is also useful to improve some code sequences, such as
when you have 32-bit data but only want to look at part of it.
mvh.,
David
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