Re: GCC bug?
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian
Hi Ian, the code is as intended:
It's during ISR prologue where we determine whether RAMPZ has to be
saved / restored. Conditions are that:
1) RAMPZ exists and
2) Z register is used.
The call to emit_push_sfr() to push RAMPZ has argument clr_p =
HAVE_RAMPD (sic!) because we have two cases when RAMPZ might occur:
A) The hardware has RAMPZ but none of the other RAMP* registers. This
is the case on devices with more than 64 KiB of flash and with RAM less
than 64 KiB. RAMPZ is only used to augment ELPM. The rule is that any
insn may use and clobber RAMPZ as needed *without* the requirement to
restore the previous value.
B) The hardware has more than 64 KiB of RAM and all RAMP registers are
present. The convention is that all RAMP* registers contain a value of
zero. If an insn clobbers RAMP*, it has to restore its value.
Consequently, upon entering an ISR, some RAMPZ might be non-zero, but
the compiler expects all RAMP registers to contain zero.
As both cases A) and B) have RAMPZ, AVR_HAVE_RAMPZ cannot be used to
discriminate these cases, but AVR_HAVE_RAMPD *does* discriminate.
There are similar use cases of AVR_HAVE_RAMPD in the startup code or
after using ELPM, for example at the end of avr.cc::avr_out_lpm():
if (xop[4] == xstring_e && AVR_HAVE_RAMPD)
{
/* Reset RAMPZ to 0 so that EBI devices don't read garbage from
RAM. */
xop[0] = zero_reg_rtx;
avr_asm_len ("out %i6,%0", xop, plen, 1);
}
xop[6] is RAMPZ, and xop[4] == xstring_e if ELPM was used.
Johann
Re: GCC bug?
Ah, this becomes (somewhat) clearer now. And then not...
Following it through, it seems that its possible to generate two
slightly differing prologues;
ignoring other registers, and assuming a tmp reg (r0), you get:
move treg, rampd
push treg
move rampd, 0
...
move treg, rampz
push treg
if AVR_HAVE_RAMP{D,Z}
but this:
mov treg, rampz
push treg
mov rampz, 0
if only AVR_HAVE_RAMPZ is set,
with an extra clear of the z register that weasnt there before.
So on AVRs with both RAMPD and RAMPZ, RAMPD is cleared, but RAMPZ is not.
I presume then that the compiler treats RAMPZ specially on those parts,
never assuming it to be zero.
...but then why doesn't that apply on parts with RAMPZ, but without
RAMPD (most AVRs?) I dont see anything in the gcc-avr ABI docs that
states it treats RAMPZ differently on different AVRs...
Its also counter to the comment a few lines above that reads:
/* Push and clear RAMPD/X/Y/Z if present and low-part register is used.
??? There are no dwarf2 columns reserved for RAMPD/X/Y/Z. */
which seems to imply that RAMPZ is to be zeroed unconditionally.
-Ian
On 17/12/2021 22:01, Bruce D. Lightner wrote:
> Ian,
>
> Trying again... :-)
>
> No, the "last one" should NOT be AVR_HAVE_RAMPZ.
>
> The Z register and the RAMPD register are used together to address
> memory in an AVR part which supports RAMPD.
>
> Some later AVR micrcontrollers have a "Z" register (i.e., AVR_HAVE_RAMPZ
> is set). Newer, more capable versions of those same AVR parts can ALSO
> have a "RAMPD" register (i.e., AVR_HAVE_RAMPD is set). If the AVR part
> has a "RAMPD" register then it will ALWAYS have a "Z" register.
>
> The code you referenced changes the behavior of "emit_push_sfr()"
> depending upon whether the AVR part has both registers or only the Z
> register.
>
> If there is no Z register then there NEVER is a RAMPD register. In that
> case neither AVR_HAVE_RAMPZ or AVR_HAVE_RAMPD will be set and
> "emit_push_sfr()" will never be called in the context you referenced.
>
> Best regards,
>
> Bruce
>
>
> On 12/17/2021 1:33 PM, Ian Molton wrote:
>> Ok, I don't want to sound rude, but did anyone actually *read* what I wrote?
>>
>> I'll highlight the parts of lines in question this time, perhaps that
>> will help...
>>
>> if (AVR_HAVE_RAMPZ
>> ^^
>>&& TEST_HARD_REG_BIT (set, REG_Z)
>>&& TEST_HARD_REG_BIT (set, REG_Z + 1))
>> {
>>emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
>> ^^
>> treg);
>> }
>>
>> OVER HERE ->--^
>>
>>
>> Either I'm missing something here, or someone needs to explain to me why
>> a test for AVR_HAVE_RAMPZ preceeds the use of AVR_HAVE_RAMPD in the code.
>>
>> Shouldn't that last one also be RAMPZ ?
>>
>> If not, why?
>>
>> -Ian
>>
>>
>> On 17/12/2021 16:53, Bruce D. Lightner wrote:
>>> Ian,
>>>
>>> I apologize if I was being obtuse! :-)
>>>
>>> The AVR_HAVE_RAMPD #define tells the routine "emit_push_sfr()" to save
>>> the RAMPD register, but only if the third parameter to the call is
>>> non-zero. The constant AVR_HAVE_RAMPD will be non-zero if the AVR
>>> micro-architecture supports a RAMPD register.
>>>
>>> That's why you see the code you reported. Much older versions of
>>> avr-gcc have "emit_push_sfr()" source code with one less parameter
>>> (i.e., no AVR_HAVE_RAMPD reference) because the RAMPD "paging" register
>>> is a relatively "new" AVR feature.
>>>
>>> Best regards,
>>>
>>> Bruce
>>>
>>>
>>> On 12/17/2021 5:39 AM, Ian Molton wrote:
I think you misunderstood me;
I know this is where the compiler will save RAMPZ where needed.
So why is it referencing RAMP *D* ?
On 17/12/2021 00:50, Bruce D. Lightner wrote:
> Ian,
>
> Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
> #define indicates that the AVR chip in question supports the "RAMP"
> paging register, described as follows:
>
> *RAMPD*
> Register concatenated with the Z-register enabling direct addressing
> of the whole data space on MCUs
> with more than 64KB data space.
>
> Eventually the originally tiny AVR chips' addressable memory got so big
> that we needed a "paging" register. (That's a repeating theme with
> every 8-bit/16-bit microcontroller family.)
>
> So the "RAMPD" register needs to be saved along with the X, Y and Z
> "special function" registers, if it is present.
>
> Best regards,
>
> Bruce
>
>
> On 12/16/2021 3:46 PM, Ian Molton wrote:
>> Browsing the GCC source, I found this in gcc/config/avr/avr.c
>>
Re: GCC bug?
Ian,
Trying again... :-)
No, the "last one" should NOT be AVR_HAVE_RAMPZ.
The Z register and the RAMPD register are used together to address
memory in an AVR part which supports RAMPD.
Some later AVR micrcontrollers have a "Z" register (i.e., AVR_HAVE_RAMPZ
is set). Newer, more capable versions of those same AVR parts can ALSO
have a "RAMPD" register (i.e., AVR_HAVE_RAMPD is set). If the AVR part
has a "RAMPD" register then it will ALWAYS have a "Z" register.
The code you referenced changes the behavior of "emit_push_sfr()"
depending upon whether the AVR part has both registers or only the Z
register.
If there is no Z register then there NEVER is a RAMPD register. In that
case neither AVR_HAVE_RAMPZ or AVR_HAVE_RAMPD will be set and
"emit_push_sfr()" will never be called in the context you referenced.
Best regards,
Bruce
On 12/17/2021 1:33 PM, Ian Molton wrote:
Ok, I don't want to sound rude, but did anyone actually *read* what I wrote?
I'll highlight the parts of lines in question this time, perhaps that
will help...
if (AVR_HAVE_RAMPZ
^^
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
^^
treg);
}
OVER HERE ->--^
Either I'm missing something here, or someone needs to explain to me why
a test for AVR_HAVE_RAMPZ preceeds the use of AVR_HAVE_RAMPD in the code.
Shouldn't that last one also be RAMPZ ?
If not, why?
-Ian
On 17/12/2021 16:53, Bruce D. Lightner wrote:
Ian,
I apologize if I was being obtuse! :-)
The AVR_HAVE_RAMPD #define tells the routine "emit_push_sfr()" to save
the RAMPD register, but only if the third parameter to the call is
non-zero. The constant AVR_HAVE_RAMPD will be non-zero if the AVR
micro-architecture supports a RAMPD register.
That's why you see the code you reported. Much older versions of
avr-gcc have "emit_push_sfr()" source code with one less parameter
(i.e., no AVR_HAVE_RAMPD reference) because the RAMPD "paging" register
is a relatively "new" AVR feature.
Best regards,
Bruce
On 12/17/2021 5:39 AM, Ian Molton wrote:
I think you misunderstood me;
I know this is where the compiler will save RAMPZ where needed.
So why is it referencing RAMP *D* ?
On 17/12/2021 00:50, Bruce D. Lightner wrote:
Ian,
Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
#define indicates that the AVR chip in question supports the "RAMP"
paging register, described as follows:
*RAMPD*
Register concatenated with the Z-register enabling direct addressing
of the whole data space on MCUs
with more than 64KB data space.
Eventually the originally tiny AVR chips' addressable memory got so big
that we needed a "paging" register. (That's a repeating theme with
every 8-bit/16-bit microcontroller family.)
So the "RAMPD" register needs to be saved along with the X, Y and Z
"special function" registers, if it is present.
Best regards,
Bruce
On 12/16/2021 3:46 PM, Ian Molton wrote:
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian
--
*Bruce D. Lightner*
*Lightner Engineering*
La Jolla, California 92037-3044
Email: light...@lightner.net
URL: http://www.lightner.net/lightner/bruce/
Re: GCC bug?
Ok, I don't want to sound rude, but did anyone actually *read* what I wrote?
I'll highlight the parts of lines in question this time, perhaps that
will help...
if (AVR_HAVE_RAMPZ
^^
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
^^
treg);
}
OVER HERE ->--^
Either I'm missing something here, or someone needs to explain to me why
a test for AVR_HAVE_RAMPZ preceeds the use of AVR_HAVE_RAMPD in the code.
Shouldn't that last one also be RAMPZ ?
If not, why?
-Ian
On 17/12/2021 16:53, Bruce D. Lightner wrote:
> Ian,
>
> I apologize if I was being obtuse! :-)
>
> The AVR_HAVE_RAMPD #define tells the routine "emit_push_sfr()" to save
> the RAMPD register, but only if the third parameter to the call is
> non-zero. The constant AVR_HAVE_RAMPD will be non-zero if the AVR
> micro-architecture supports a RAMPD register.
>
> That's why you see the code you reported. Much older versions of
> avr-gcc have "emit_push_sfr()" source code with one less parameter
> (i.e., no AVR_HAVE_RAMPD reference) because the RAMPD "paging" register
> is a relatively "new" AVR feature.
>
> Best regards,
>
> Bruce
>
>
> On 12/17/2021 5:39 AM, Ian Molton wrote:
>> I think you misunderstood me;
>>
>> I know this is where the compiler will save RAMPZ where needed.
>>
>> So why is it referencing RAMP *D* ?
>>
>> On 17/12/2021 00:50, Bruce D. Lightner wrote:
>>> Ian,
>>>
>>> Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
>>> #define indicates that the AVR chip in question supports the "RAMP"
>>> paging register, described as follows:
>>>
>>> *RAMPD*
>>> Register concatenated with the Z-register enabling direct addressing
>>> of the whole data space on MCUs
>>> with more than 64KB data space.
>>>
>>> Eventually the originally tiny AVR chips' addressable memory got so big
>>> that we needed a "paging" register. (That's a repeating theme with
>>> every 8-bit/16-bit microcontroller family.)
>>>
>>> So the "RAMPD" register needs to be saved along with the X, Y and Z
>>> "special function" registers, if it is present.
>>>
>>> Best regards,
>>>
>>> Bruce
>>>
>>>
>>> On 12/16/2021 3:46 PM, Ian Molton wrote:
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian
>>>
>
> --
> *Bruce D. Lightner*
> *Lightner Engineering*
> La Jolla, CA
> Email: light...@lightner.net
> URL: http://www.lightner.net/lightner/bruce/
Re: GCC bug?
Ian,
I apologize if I was being obtuse! :-)
The AVR_HAVE_RAMPD #define tells the routine "emit_push_sfr()" to save
the RAMPD register, but only if the third parameter to the call is
non-zero. The constant AVR_HAVE_RAMPD will be non-zero if the AVR
micro-architecture supports a RAMPD register.
That's why you see the code you reported. Much older versions of
avr-gcc have "emit_push_sfr()" source code with one less parameter
(i.e., no AVR_HAVE_RAMPD reference) because the RAMPD "paging" register
is a relatively "new" AVR feature.
Best regards,
Bruce
On 12/17/2021 5:39 AM, Ian Molton wrote:
I think you misunderstood me;
I know this is where the compiler will save RAMPZ where needed.
So why is it referencing RAMP *D* ?
On 17/12/2021 00:50, Bruce D. Lightner wrote:
Ian,
Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
#define indicates that the AVR chip in question supports the "RAMP"
paging register, described as follows:
*RAMPD*
Register concatenated with the Z-register enabling direct addressing
of the whole data space on MCUs
with more than 64KB data space.
Eventually the originally tiny AVR chips' addressable memory got so big
that we needed a "paging" register. (That's a repeating theme with
every 8-bit/16-bit microcontroller family.)
So the "RAMPD" register needs to be saved along with the X, Y and Z
"special function" registers, if it is present.
Best regards,
Bruce
On 12/16/2021 3:46 PM, Ian Molton wrote:
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian
--
*Bruce D. Lightner*
*Lightner Engineering*
La Jolla, CA
Email: light...@lightner.net
URL: http://www.lightner.net/lightner/bruce/
Re: GCC bug?
Maybe one day even a problem in 32bit architectures.
On Thu, Dec 16, 2021 at 7:50 PM Bruce D. Lightner
wrote:
> Ian,
>
> Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
> #define indicates that the AVR chip in question supports the "RAMP" paging
> register, described as follows:
>
>*RAMPD*
>Register concatenated with the Z-register enabling direct addressing of
> the whole data space on MCUs
>with more than 64KB data space.
>
> Eventually the originally tiny AVR chips' addressable memory got so big
> that we needed a "paging" register. (That's a repeating theme with every
> 8-bit/16-bit microcontroller family.)
>
> So the "RAMPD" register needs to be saved along with the X, Y and Z
> "special function" registers, if it is present.
>
> Best regards,
>
> Bruce
>
> --
> On 12/16/2021 3:46 PM, Ian Molton wrote:
>
> Browsing the GCC source, I found this in gcc/config/avr/avr.c
>
> if (AVR_HAVE_RAMPZ
> && TEST_HARD_REG_BIT (set, REG_Z)
> && TEST_HARD_REG_BIT (set, REG_Z + 1))
> {
> emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
> treg);
> }
>
>
> I wont pretend to fully understand this part of the compiler, but that
> AVR_HAVE_RAMPD looks shady to me?
>
> Anyone with deeper knowledge want to have a look?
>
> -Ian
>
>
>
>
> --
> *Bruce D. Lightner*
> *Lightner Engineering*
> 8551 La Jolla Shores Drive
> La Jolla, California 92037-3044
> Mobile/SMS: +1-858-228-7579
> Voice: +1-858-551-0770 ext. 2
> Email: light...@lightner.net
> URL: http://www.lightner.net/lightner/bruce/
>
> *CONFIDENTIALITY NOTICE: This e-mail and any attachments may contain
> confidential information intended solely for the use of the addressee. If
> the reader of this message is not the intended recipient, any distribution,
> copying, or use of this e-mail or its attachments is prohibited. If you
> received this message in error, please notify the sender immediately by
> e-mail and delete this message and any copies. Thank you.*
>
Re: GCC bug?
Ian,
Just from a purely AVR architecture point of view, the AVR_HAVE_RAMPD
#define indicates that the AVR chip in question supports the "RAMP"
paging register, described as follows:
*RAMPD*
Register concatenated with the Z-register enabling direct addressing
of the whole data space on MCUs
with more than 64KB data space.
Eventually the originally tiny AVR chips' addressable memory got so big
that we needed a "paging" register. (That's a repeating theme with
every 8-bit/16-bit microcontroller family.)
So the "RAMPD" register needs to be saved along with the X, Y and Z
"special function" registers, if it is present.
Best regards,
Bruce
On 12/16/2021 3:46 PM, Ian Molton wrote:
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian
--
*Bruce D. Lightner*
*Lightner Engineering*
8551 La Jolla Shores Drive
La Jolla, California 92037-3044
Mobile/SMS: +1-858-228-7579
Voice: +1-858-551-0770 ext. 2
Email: light...@lightner.net
URL: http://www.lightner.net/lightner/bruce/
/CONFIDENTIALITY NOTICE: This e-mail and any attachments may contain
confidential information intended solely for the use of the addressee.
If the reader of this message is not the intended recipient, any
distribution, copying, or use of this e-mail or its attachments is
prohibited. If you received this message in error, please notify the
sender immediately by e-mail and delete this message and any copies.
Thank you./
